Modem-HOWTO
David S.Lawyer <mailto:
[email protected]>
v0.12, December 2000
Help with selecting, connecting, configuring, trouble-shooting, and
understanding modems for a PC. See Serial-HOWTO for multiport serial
boards.
______________________________________________________________________
Table of Contents
1. Introduction
1.1 DSL, Cable, and ISDN Modems in other HOWTOs
1.2 Also not covered: PCMCIA Modems, PPP
1.3 Copyright, Disclaimer, Trademarks, & Credits
1.3.1 Copyright
1.3.2 Disclaimer
1.3.3 Trademarks.
1.3.4 Credits
1.4 Contacting the Author
1.5 New Versions of this HOWTO
1.6 New in this Version
1.7 What is a Modem ?
1.8 Quick Install
1.8.1 External Modem Install
1.8.2 Internal Modems (Both ISA and PCI)
1.8.3 ISA Modems: What IOs and IRQs may be used?
1.8.4 PCI Modems: What IOs and IRQs have been set?
1.8.5 Both PCI and ISA: Use setserial to tell the driver
1.8.6 Use MS Windows to set the BIOS (A last resort method)
1.8.7 All Modems
2. Modems for a Linux PC
2.1 External vs. Internal
2.2 External Modems
2.2.1 PnP External Modems
2.2.2 Cabling & Installation
2.2.3 What the Lights (LED's) Mean (for some modems)
2.3 Internal Modems
2.4 Software-based Modems (winmodems)
2.4.1 Introduction software modems (winmodems)
2.4.2 Linmodems
2.4.3 Software-based modem types
2.4.4 Is this modem a software modem?
2.4.5 Should I get a software modem?
2.5 PCI Modems
2.6 Which Internal Modems might not work with Linux
2.6.1 MWave and DSP Modems
2.6.2 Rockwell (RPI) Drivers
3. Modem Pools, Digital Modems
3.1 Analog Modem Pools, Multiport Modem Cards
3.2 Digital Modems
4. Serial Port and Modem Basics
4.1 Modem Converts Digital to Analog (and conversely)
4.2 What is a Serial Port ?
4.2.1 Intro to Serial
4.2.2 Pins and Wires
4.2.3 Internal Modem Contains Serial Port
4.3 IO Address & IRQ
4.4 Names: ttyS0, ttyS1, etc.
4.5 Interrupts
4.6 Data Compression (by the Modem)
4.7 Error Correction
4.8 Data Flow (Speeds)
4.9 Flow Control
4.9.1 Example of Flow Control
4.9.2 Hardware vs. Software Flow Control
4.9.3 Symptoms of No Flow Control
4.9.4 Modem-to-Modem Flow Control
4.10 Data Flow Path; Buffers
4.11 Modem Commands
4.12 Serial Driver Module
5. Configuring Overview
6. Configuring the Serial Port
6.1 PCI Bus Support Underway
6.2 Configuring Overview
6.3 Common mistakes made re low-level configuring
6.4 I/O Address & IRQ: Boot-time messages
6.5 What is the current IO address and IRQ of my Serial Port ?
6.5.1 What does the device driver think?
6.5.2 What is set in my serial port hardware ?
6.5.3 What is set in my PnP serial port hardware ?
6.6 Choosing Serial IRQs
6.6.1 IRQ 0 is not an IRQ
6.6.2 Interrupt sharing and Kernels 2.2+
6.6.3 What IRQs to choose?
6.7 Choosing Addresses --Video card conflict with ttyS3
6.8 Set IO Address & IRQ in the hardware (mostly for PnP)
6.8.1 Using a PnP BIOS to I0-IRQ Configure
6.9 Giving the IRQ and IO Address to Setserial
6.10 Other Configuring
6.10.1 Configuring Hardware Flow Control (RTS/CTS)
7. Modem Configuration (excluding serial port)
7.1 Finding Your Modem
7.2 AT Commands
7.3 Init Strings: Saving and Recalling
7.3.1 Where is my "init string" so I can modify it ?
7.4 Other Modem Commands
8. Serial Port Devices /dev/ttyS2, etc.
8.1 Serial Port Device Names & Numbers
8.2 Link ttySN to /dev/modem ?
8.3 The cua Device
9. Interesting Programs You Should Know About
9.1 What is setserial ?
9.1.1 Introduction
9.1.2 Probing
9.1.3 Boot-time Configuration
9.1.4 Configuration Scripts/Files
9.1.5 Edit a script (required prior to version 2.15)
9.1.6 New configuration method using /etc/serial.conf
9.1.7 IRQs
9.2 What is isapnp ?
9.3 What is wvdialconf ?
9.4 What is stty ?
10. Trying Out Your Modem (Dialing Out)
10.1 Are You Ready to Dial Out ?
10.2 Dialing Out with Minicom
10.3 Dialing Out with Kermit
11. Dial-In
11.1 Overview
11.2 Getty
11.2.1 About mgetty
11.2.2 About uugetty
11.2.3 About getty_em
11.2.4 About agetty and mingetty
11.3 What Happens when Someone Dials In ?
11.4 Why Manual Answer is Best
11.5 Callback
11.6 Voice Mail
12. Uugetty for Dial-In (from the old Serial-HOWTO)
12.1 Installing getty_ps
12.2 Setting up uugetty
12.2.1 Modern Modems
12.2.2 Old slow modems
12.2.3 Login Banner
12.3 Customizing uugetty
13. What Speed Should I Use with My Modem?
13.1 Speed and Data Compression
13.2 Where do I Set Speed ?
13.3 Can't Set a High Enough Speed
13.3.1 How speed is set in hardware: the divisor and baud_base
13.3.2 Work-arounds for setting speed
13.3.3 Crystal frequency is not baud_base
13.4 Speed Table
14. Communications Programs And Utilities
14.1 Minicom vs. Kermit
14.2 List of Communication Software
14.2.1 Least Popular Dialout
14.2.2 Most Popular Dialout
14.2.3 Fax
14.2.4 Voicemail Software
14.2.5 Dial-in (uses getty)
14.2.6 Other
14.3 SLiRP and term
15. Troubleshooting
15.1 My Modem is Physically There but Can't be Found
15.1.1 No response to AT
15.2 "Modem is busy"
15.3 I can't get near 56k on my 56k modem
15.4 Uploading (downloading) files is broken/slow
15.5 For Dial-in I Keep Getting "line NNN of inittab invalid"
15.6 I Keep Getting: ``Id "S3" respawning too fast: disabled for 5 minutes''
15.7 My Modem is Hosed after Someone Hangs Up, or uugetty doesn't respawn
15.8 uugetty Still Doesn't Work
15.9 (The following subsections are in both the Serial and Modem HOWTOs)
15.10 My Serial Port is Physically There but Can't be Found
15.11 Extremely Slow: Text appears on the screen slowly after long delays
15.12 Somewhat Slow: I expected it to be a few times faster
15.13 The Startup Screen Show Wrong IRQs for the Serial Ports.
15.14 "Cannot open /dev/ttyS?: Permission denied"
15.15 "Operation not supported by device" for ttyS?
15.16 "Cannot create lockfile. Sorry"
15.17 "Device /dev/ttyS? is locked."
15.18 "/dev/tty? Device or resource busy"
15.19 "Input/output error" from setserial or stty
15.20 Overrun errors on serial port
15.21 Modem doesn't pick up incoming calls
15.22 Troubleshooting Tools
16. Flash Upgrades
17. Other Sources of Information
17.1 Misc
17.2 Books
17.3 HOWTOs
17.4 Usenet newsgroups
17.5 Web Sites
18. Appendix A: How Analog Modems Work (technical) (unfinished)
18.1 Modulation Details
18.1.1 Intro to Modulation
18.1.2 Frequency Modulation
18.1.3 Amplitude Modulation
18.1.4 Phase Modulation
18.1.5 Combination Modulation
18.2 56k Modems (v.90)
18.3 Full Duplex on One Circuit
18.4 Echo Cancellation
19. Appendix B: Digital Modem Signal Processing (not done)
20. Appendix C: "baud" vs. "bps"
20.1 A simple example
20.2 Real examples
21. Appendix D: Terminal Server Connection
22. Appendix E: Other Types of Modems
22.1 Digital-to-Digital "Modems"
22.2 ISDN "Modems"
22.3 Digital Subscriber Line (DSL)
22.4 56k Digital-Modems
22.5 Leased Line Modems
23. Appendix F: Fax pixels (dots)
______________________________________________________________________
1. Introduction
1.1. DSL, Cable, and ISDN Modems in other HOWTOs
This HOWTO covers conventional analog modems for PCs on either the ISA
or PCI bus. For other types of modems:
� DSL modems: see the mini-howto: ADSL
� Cable-Modems-HOWTO (was once a LDP mini-Howto)
<
http://www.cs.unm.edu/~vuksan/linux/Cable-Modem.html>
� Cable-Modem-Providers-HOWTO
� ISDN Howto (not a LDP Howto)
<
http://sdb.suse.de/sdb/en/html/isdn.html>: drivers for ISDN
"Modems". Much related info on this is in German. For a tutorial
on ISDN see <
http://public.swbell.net/ISDN/overview.html>
See also ``Appendix D: Other Types of Modems''
1.2. Also not covered: PCMCIA Modems, PPP
For modems on the PCMCIA bus see the PCMCIA-HOWTO: PCMCIA serial and
modem devices. This HOWTO also doesn't cover PPP (used to connect to
the Internet via a modem) or communication programs. Except it does
show how to use communication programs to test that your modem works
OK and can make phone calls. If you want to use a modem to connect to
the Internet then you need to set up PPP. There's a lot of
documentation for PPP (including a PPP-HOWTO). More documentation
should be found in /usr/doc/ppp, /usr/share/doc/ppp or the like.
1.3. Copyright, Disclaimer, Trademarks, & Credits
1.3.1. Copyright
Copyright (c) 1998-2000 by David S. Lawyer <mailto:
[email protected]>
Please freely copy and distribute (sell or give away) this document in
any format. Send any corrections and comments to the document
maintainer. You may create a derivative work and distribute it
provided that you:
1. Send your derivative work to the LDP (Linux Documentation Project)
or the like for free distribution on the Internet in a format they
will accept. If not the LDP, then let the LDP know where it is
available. Except for a translation, send a copy to the previous
maintainer's url as shown in the latest version.
2. License the derivative work in the spirit of this license or use
GPL. Include a copyright notice and at least a pointer to the
license used.
3. Give due credit to previous authors and major contributors.
If you're considering making a derived work other than a translation,
it's requested that you discuss your plans with the current
maintainer.
1.3.2. Disclaimer
While I haven't intentionally tried to mislead you, there are likely a
number of errors in this document. Please let me know about them.
Since this is free documentation, it should be obvious that I cannot
be held legally responsible for any errors.
1.3.3. Trademarks.
Any brand names (starts with a capital letter) should be assumed to be
a trademark). Such trademarks belong to their respective owners.
"Hayes" is a trademark of Microcomputer Products Inc. I use
"winmodem" to mean any modem which requires MS-Windows and not in the
trademark sense. All other trademarks belong to their respective
owners.
1.3.4. Credits
The following is only a rough approximation of how this this document
(as of 2000) was created: About 1/4 of the material here was lifted
directly from Serial-HOWTO v. 1.11 (1997) by Greg Hankins.
<mailto:
[email protected]> (with his permission). About another 1/4
was taken from that Serial-HOWTO and revised. The remaining 1/2 is
newly created by the new author: David S. Lawyer
<mailto:
[email protected]>.
1.4. Contacting the Author
Since I don't follow the many different brands/models of modems please
don't email me with questions about them (or suggestions of which one
to buy). If you are interested in a certain model (to find out if it
works under Linux, etc.) see the huge list at ``Web Sites''. Also,
please don't ask me how to configure a modem unless you've looked over
this HOWTO and still can't do it. I've no personal experience with
software-based modems.
Please let me know of any errors in facts, opinions, logic, spelling,
grammar, clarity, links, etc. But first, if the date is over a month
old, check to see that you have the latest version. Please send me
any other info that you think belongs in this document.
1.5. New Versions of this HOWTO
New versions of this Modem-HOWTO come out every month or two since
modem situation is rapidly changing (and since I'm still learning).
Your problem might be solved in the latest version. It will be
available to browse and/or download at LDP mirror sites. For a list
of such sites see: <
http://metalab.unc.edu/LDP/mirrors.html> If you
only want to quickly compare the date of this the version v0.12,
December 2000 with the date of the latest version go to:
<
http://metalab.unc.edu/LDP/HOWTO/Modem-HOWTO.html>
1.6. New in this Version
v0.12 Zoltrix and Cirrus Logic linmodems; don't omit 0 in X0 (for some
modems) X3 if no dial-tone; init string locations; more on "Modem is
busy"; PCI non-winmodems are OK; terminology: software-based modem,
driver-based modem; Linmodem-HOWTO; types of software modems;
setserial irq5 to irq 5; Quick Install rewrite.
1.7. What is a Modem ?
A modem is a device that lets one send digital signals over ordinary
telephone lines not designed for digital signals. If telephone lines
were all digital then you wouldn't need a modem. It permits your
computer to connect to and communicate with the rest of the world.
When you use a modem, you normally use a communication program or web
browser (which includes such a program) to utilize the modem and dial-
out on a telephone line. Advanced modem users can set things up so
that others may phone in to them and use their computer. This is
called "dial-in".
There are two basic types of modems for a PC: external and internal.
The external sets on your desk outside the PC while the internal is
not visible since it's inside the PC. The external modem plugs into a
connector on the back of the PC known as a "serial port". The
internal modem is a card that is inserted inside the computer and has
an (invisible) serial port built into it. For a more detailed
comparison see ``External vs. Internal''. Thus when you get an
internal modem, you also get a dedicated serial port (which can only
be used with the modem and not with anything else such as another
modem or a printer). In Linux, the serial ports are named ttyS0,
ttyS1, etc. (usually corresponding respectively to COM1, COM2, etc. in
Dos/Windows).
The serial port is not to be confused with the "Universal Serial Bus"
(USB) which uses a special modular connector and may be used to
connect an external modem. See ``Modem & Serial Port Basics'' for
more details on modems and serial ports.
Modems usually include the ability to send Faxes (Fax Modems). See
``Fax'' for a list of fax software. "Voice" modems can work like an
automatic answering machine and handle voicemail. See ``Voicemail''.
1.8. Quick Install
1.8.1. External Modem Install
With a straight-thru or modem cable, connect the modem to an unused
serial port on the PC. Make sure you know the name of the serial
port: in most cases COM1 is ttyS0, COM2 is ttyS1, etc. You may need
to check the BIOS setup menu to determine this. Plug in the power
cord to provide power to the modem. See ``All Modems'' for further
instructions.
1.8.2. Internal Modems (Both ISA and PCI)
The first thing to do is to make sure that the modem will work under
Linux since (as of late 2000) most newer modems don't. See modem list
<
http://www.o2.net/~gromitkc/winmodem.html>. If the modem is both PnP
and directly supported by the serial driver (kernel 2.4 +) then you
have nothing to do since the driver will configure it. Otherwise read
on. It may or may not be easy.
If you have a modem that is not a winmodem (see ``Software-based
Modems (winmodems)'') and you must configure it yourself (the driver
doesn't do it) then you first need to decide which ttySx to assign it
to. Pick a ttySx that is not already in use by other serial ports.
Then you have the problem of setting an IRQ number and IO address.
For PnP modems: If the BIOS has already set these in the physical
device (which a PnP BIOS will do if it thinks you don't have a PnP OS)
then you need to determine the IRQ and IO address and then tell this
to "setserial". In other cases you may have some choice of IRQs and
IO addresses (including the case where you are able to change what the
BIOS has set). See ``Choosing Serial IRQs'' and ``Choosing
Addresses''. For ISA modems there are standard IO addresses to use
(corresponding to the ttySx). PCI modems seem to use different IO
addresses so as not to conflict with ISA modems. For example you may
find it feasible to use /dev/ttyS2 at IO address 0x3e8 and IRQ 11.
1.8.3. ISA Modems: What IOs and IRQs may be used?
For old modems with jumpers look at the manual (or jumpers if they
say). If the BIOS has already configured the ISA modem then "isapnp
--dumpregs" should show it. If you need to set or change them use
"isapnp". Use the "pnpdump" to see what you changes are possible.
1.8.4. PCI Modems: What IOs and IRQs have been set?
For PCI, the BIOS almost always sets the IRQ any may have set the IO
address as well. To see how it's set use "lspci -v" or look in
/proc/bus/pci. If more than one IO address is shown, the first one is
more likely to be it. You can't change the IRQ (at least not with
"setpci") If you must, change the IO address with "setpci" by
changing the BASE_ADDRESS_0 or the like. The _0 (or _1) after
BASE_ADDRESS must be the correct register.
1.8.5. Both PCI and ISA: Use setserial to tell the driver
You must find the file where "setserial" is run at boot-time and add a
line something like: "setserial /dev/ttyS2 irq 5 port 0x0b8". For
setserial v2.15 and later the results of running "setserial" on the
command line may (or may not) be saved to /etc/serial.conf so that it
runs each time you boot. See ``What is Setserial'' for more info.
See the next subsection ``All Modems'' for further instructions on
quick installation.
1.8.6. Use MS Windows to set the BIOS (A last resort method)
If you are using the BIOS to configure you may use MS Windows9x to
"force" the BIOS to set a certain IRQ and/or IO. It can set them into
the PnP BIOS's flash memory where they will be used to configure for
Linux as well as Windows. See "Plug-and-Play-HOWTO and search for
"forced" (occurs in several places). For Windows3.x you can do the
same thing using the ICU under Windows 3.x. A few modems have a way
to disable PnP in the modem hardware using software (under Windows)
that came with the modem.
1.8.7. All Modems
Plug the modem into a telephone line. Then configure a communication
program such as minicom or a ppp program (such as wvdial). Set the
serial port speed to a baud rate a few times higher than the bit rate
of your modem. See ``Speed Table'' for the "best" speeds to use.
Tell it the full name of your serial port such as /dev/ttyS1. Set
hardware flow control (RTS/CTS).
Minicom is the easiest to set up and to use to test your modem. But
if you are lucky you may get ppp to work the first time and not need
to bother with minicom. With minicom you may simply type the command:
AT and hit enter (before dialing) to check that your modem is there
and responds with OK.
2. Modems for a Linux PC
2.1. External vs. Internal
A modem for a PC may be either internal or external. The internal one
is installed inside of your PC (you must remove screws, etc. to
install it) and the external one just plugs into a serial port
connector on a PC. Internal modems are less expensive, are less
likely to to suffer data loss due to buffer overrun, usually use less
electricity, and use up no space on your desk.
External modems are usually easier to install and usually require less
configuration. They have lights which may give you a clue as to what
is happening and aid in troubleshooting. The fact that the serial
port and modem can be physically separated also aids in
troubleshooting. External modems are easy to move to another
computer.
Unfortunately most external modems have no switch to turn off the
power supply when not in use and thus are likely to consume a little
electricity even when turned off (unless you unplug the power supply
from the wall). Each watt they draw costs you about $1/yr. Another
possible disadvantage of an external is that you will be forced to use
an existing serial port which may not support a speed of over 115,200
bps (although as of late 1998 and late 2000 most new internal modems
don't either --but some do). If a new internal modem had say a 16650
UART it would put less load on the CPU.
Internal modems present a special problem for Linux, but will work
just as well as external modems provided you avoid the high percentage
of them that will work only for MS Windows, and also provided that you
spend time (sometimes a lot of time) to configure them correctly.
Some of the modems which will work only under MS Windows are,
unfortunately, not labeled as such. If you buy a new one, make sure
that you can return it for a refund if it will not work under Linux.
While most new modems are plug-and-play you have various ways to deal
with the PnP configuring:
� The serial driver does it all for you (more likely for a PCI modem)
� Use the "isapnp" program
� Let a PnP BIOS do the configuring
Each of the above has shortcomings. Isapnp documentation is
difficult to understand although reading the Plug-and-Play-HOWTO
(long) will aid in understanding it. If you want the PnP BIOS to
do the configuring, all you need to do is to make sure that it
knows you don't have a PnP operating system. But it may not do it
correctly and you may need to find out what it's done see ``What is
set in my serial port hardware?''.
There are many Linux users that say that it's a lot simpler just to
get an external modem and plug it in. But since new peripherals are
PnP today, you will sometime need to deal with it, so why delay the
inevitable? Still, the most expedient (and expensive) solution is an
external modem (if you have a free serial port).
2.2. External Modems
2.2.1. PnP External Modems
Many external modems are labeled "Plug and Play" (PnP) but they should
all work fine as non-PnP modems. While the serial port itself may
need to be configured (IRQ number and IO address) unless the default
configuration is OK an external modem uses no such IRQ/IO
configuration. You just plug the modem into the serial port. Since
you usually plug the modem into a serial port (and connect it to
power).
How can an external modem be called PnP since it can't be configured
by PnP? Well, it has a special PnP identification built into it that
can be read (thru the serial port) by a PnP operating system. Such an
operating system would then know that you have a modem on a certain
port and would also know the model number. Then you might not need to
configure application programs by telling them what port the modem is
on (such as /dev/ttyS2 or COM3). But since you don't have such a PnP
operating system you will need to configure your application program
manually by giving it the /dev id (such as /dev/ttyS2).
2.2.2. Cabling & Installation
Connecting an external modem is simple compared to connecting most
other devices to a serial port that require various types of "null
modem" cables (which will not work for modems). Modems use straight
through cable, with no pins crossed over. Most computer stores should
have this. Make sure you get the correct gender and number of pins.
Hook up your modem to one of your serial ports. If you are willing to
accept the default IRQ and IO address of the port you connect it to,
then you are ready to start your communication program and configure
the modem itself.
2.2.3. What the Lights (LED's) Mean (for some modems)
� TM Test Modem
� AA Auto Answer (If on, your modem will answer an incoming call)
� RD Receive Data line = RxD
� SD Send Data line = TxD
� TR data Terminal Ready = DTR (set by your PC)
� RI Ring Indicator (If on, someone is "ringing" your modem)
� OH Off Hook (If off, your modem has hung up the phone line)
� MR Modem Ready = DSR ??
� EC Error Correction
� DC Data Compression
� HS High Speed (for this modem)
2.3. Internal Modems
An internal modem is installed in a PC by taking off the cover of the
PC and inserting the modem card into a vacant slot on the motherboard.
There are modems for the ISA slots and others for the PCI slots. Some
new PC don't have any ISA slots. While external modems plug into the
serial port (via a short cable) the internal modems have the serial
port built into the modem. In other words, the modem card is both a
serial port and a modem.
Setting the IO address and IRQ for a serial port was formerly done by
jumpers on the card. These are little black rectangular "cubes" about
5x4x2 mm in size which push in over pins on the card. Plug-and-Play
modems (actually the serial port part of the modems) don't use jumpers
for setting these but instead are configured by sending configuration
commands to them over the bus inside the computer. Such configuration
commands can be sent by a PnP BIOS, by the isapnp program (for the ISA
bus only), or by newer serial device drivers for certain modems. Under
Linux you have a choice of how to configure the ones that don't get
io-irq configured by the serial driver.
1. Use "isapnp" which may be run automatically at every boot-time
2. Let a PnP BIOS do it, and then maybe tell setserial the IO and IRQ
2.4. Software-based Modems (winmodems)
2.4.1. Introduction software modems (winmodems)
Software modems turn over some (or even almost all) of the work of the
modem to the main processor (CPU) chip of your computer (such as a
Pentium chip). This requires special software (a modem driver) to do
the job. Until late 1999, such software was released only for MS
Windows and wouldn't work with Linux. Even worse was that the maker
of the modem kept the interface to the modem secret so that no one
could write a Linux driver for it (even though a few volunteers were
willing to write Linux drivers). With few exceptions, this is still
true today (late 2000). Also, there is no standard interface so that
different brands/models of software-modems need different drivers
(unless the different brands/models happen to use the same chipset
internally).
A third name for a software modem (used by MS) is "driver-based
modem". The conventional hardware-based modem (that works with Linux)
doesn't need a modem driver (but does use the Linux serial driver)
After about mid-1998 most new internal modems were winmodems and would
work only for MS Windows.
2.4.2. Linmodems
Finally in late 1999 two software-based modems appeared that could
work under Linux and were sometimes called "linmodems". Lucent
Technologies (LT) unofficially released a Linux binary-only code to
support its PCI modems. PC-TEL (includes "Zoltrix") introduced a new
software-based modem for Linux. There is a GPL'ed driver being
developed for the Modem Silicon Operation MD563X HaM chipset (nee
Ambient division of Cirrus Logic). Will other companies follow these
leads and thus create "linmodems"? For a list of modems which
work/don't_work under Linux see modem list
<
http://www.o2.net/~gromitkc/winmodem.html>. Links to "linmodem"
drivers may also be found there. A project to get winmodems to work
under Linux is at <
http://linmodems.org>. They also have a mailing
list.
There is some effort underway at reverse-engineering with at least one
report of a winmodem that has been made to work under Linux (but not
yet with full functionality). So by the time you read this there may
be more linmodems.
For details of how to get some winmodems to work under Linux see the
Linmodem-HOWTO (and/or Winmodems-and-Linux-HOWTO which is not as well
written). If code is made available to operate a "winmodem" under
Linux, then one may call it a "linmodem". Is it still a "winmodem"?
Well, it's still a software-based modem. The term "Winmodem" is also
a trademark for a certain model of "winmodem".
2.4.3. Software-based modem types
There are two basic types of software modems. In one type the
software does almost all of the work. The other is where the software
only does the "control" operations (which is everything except
processing the digital waveshapes --to be explained later). Since the
hardware doesn't do the control it's called a "controllerless" modem.
The first type is a all-software modem (sometimes just called a
software modem).
For both of these types there must be analog hardware in the modem to
generate an electrical waveshape to send out the phone line. It's
generated from a digital signal (which is sort of a "digital
waveshape"). It's something like the digital electronics creates a
lot of discrete points on graph paper and then the modem draws a
smooth curve thru them. There must also be hardware to convert the
incoming waveshape to digital. Then this digital waveshape must be
converted to a data byte stream. The modem can't just send this data
byte stream to the PC but must first do decompression, error
correction, and convert from serial to the parallel bus of the
computer.
The difference between the two types of software-based modems is where
these digital waveshapes are processed (generated and interpreted).
In the all-software modem this waveshape processing is done in the CPU
using a Host Signal Processor (HSP). In the controllerless modem it's
done in the modem but all other digital work is done by the CPU (data
compression, AT commands, etc.) For example the Rockwell HCF (Host
Controlled Family) does this. If the software that does these tasks
could be ported to Linux and then there wouldn't be a major problem.
2.4.4. Is this modem a software modem?
How do you determine if an internal modem will work under Linux?
First see if the name, description of it, or even the name of the MS
Windows driver for it indicates it's a software modem: HSP, HCF, HSF,
controllerless, host-controlled, host-based, and soft-... modem. If
it's one of these modem it will only work for the few cases (so far)
where a Linux driver is available.
If you don't know the model of the modem and you also have Windows on
your Linux PC, click on the "Modem" icon in the "Control Panel". Then
check out the modem list (see ``Web Sites''. If the above doesn't
work (or isn't feasible), you can look at the package it came in (or a
manual) find the section on the package that says something like
"Minimum System Requirements" or just "System Requirements". It may
be in fine print. Read it closely. If Windows or a Pentium CPU is
listed as one of the requirements then it is not too likely to work
under Linux.
Otherwise, it may work under Linux if it fails to state explicitly
that you must have Windows. By saying it's "designed for Windows" it
may only mean that it fully supports Microsoft's plug-and-play which
is OK since Linux uses the same plug-and-play specs (but it's harder
to configure under Linux). Being "designed for Windows" thus gives no
clue as to whether or not it will work under Linux. You might check
the Website of the manufacturer or inquire via email. I once saw a
web-page that specifically stated that one model worked under Linux
while implying that another model didn't.
2.4.5. Should I get a software modem?
Only if you know there is a Linux driver for it that works OK.
Besides the problems of getting a driver, what are the pros and cons
of software modems? Since the software modem uses the CPU to do much
of its work, the software modem requires less on-board electronics and
thus costs less. At the same time, the CPU is heavily loaded by the
modem which may result in slower operation. This is especially true
if other CPU-intensive tasks are running at the same time the modem is
being used. Of course when you're not using the software modem there
is no degradation in performance at all.
Is the cost savings worth it? In some cases yes, especially if you
seldom use the modem and/or are not running any other CPU intensive
tasks when the modem is in use. Thus there are cases where use of a
software modem is economically justified. The savings in modem cost
could be used for a better CPU which would speed things up a little.
But the on-board electronics of a modem can do the job more
efficiently than a general purpose CPU (except that it's not efficient
when it's not in use). So if you use the modem a lot it's probably
better to avoid software modems (and then you can use a less powerful
CPU :-).
2.5. PCI Modems
A PCI modem card is one which inserts into a PCI-bus slot on the
motherboard of a PC. While most PCI winmodems will not work under
Linux (no driver available) other PCI modems mostly work under Linux.
The Linux serial driver is being modified to support certain PCI modem
cards (but not winmodems). If the Linux serial driver supports it
then the driver will set up the PnP configuration for you. See ``PCI
Bus Support Underway'' If no special support is in the Linux serial
driver but it may still work OK but you have to do some work to
configure it.
2.6. Which Internal Modems might not work with Linux
� ``Software-based Modems (winmodems)'' only work in rare cases where
a Linux driver is available.
� ``MWave and DSP Modems'' might work, but only if you first start
Windows/Dos each time you power on your PC
� Modems with ``RPI (Rockwell)'' drivers work but with reduced
performance
2.6.1. MWave and DSP Modems
Such modems use DSP's (Digital Signal Processors) which are programmed
by driver which must be downloaded from the hard disk to the DSP's
memory just before using the modem. Unfortunately, such downloading
is normally done by Dos/Windows programs (which doesn't work for
Linux). But there has been substantial success in getting some of
these modems to work with Linux. For example, there is a Linux driver
available to run a Lucent (DSP) modem.
Ordinary modems that work fine with Linux (without needing a driver
for the modem) often have a DSP too (and may mention this on the
packaging), but the program that runs the DSP is stored inside the
modem. This is not a "DSP modem" in the sense of this section. An
example of a DSP modem is IBM's Aptiva MWAVE.
If a DSP modem modem simulates a serial port, then it may be usable
with Linux provided you're willing/able to boot from DOS. You must
have Dos/Windows on the same PC. You first install the driver under
DOS (using DOS and not Window drivers). Then start Dos/Windows and
start the driver for the modem so as to program the DSP. Then without
turning off the computer, go into Linux.
One may write a "batch" file (actually a script) to do this. Here is
an example but you must modify it to suit your situation.
rem mwave is a batch file supplied by the modem maker
call c:\mww\dll\mwave start
rem loadlin.exe is a DOS program that will boot Linux from DOS (See
rem Config-HOWTO).
c:\linux\loadlin f:\vmlinuz root=/dev/hda3 ro
One may create an icon for the Window's desktop which points to such a
batch file and set the icon properties to "Run in MSDOS Mode". Then
by clicking on this icon one sets up the modem and goes to Linux.
Another possible way to boot Linux from DOS is to press CTRL-ALT-DEL
and tell it to reboot (assuming that you have set things up so that
you can boot directly into Linux). The modem remains on the same com
port (same IO address) that it used under DOS.
The Newcom ifx modem needs a small kernel patch to work correctly
since its simulation of a serial port is non-standard. The patch and
other info for using this modem with Linux is at
<
http://maalox.pharmacy.ohio-state.edu/~ejolson/linux/newcom.html>.
2.6.2. Rockwell (RPI) Drivers
Some older Rockwell chips need Rockwell RPI (Rockwell Protocol
Interface) drivers. They can still be used with Linux even though the
driver software works only under MS Windows. This is because the MS
Windows software which you don't have does only compression and error
correction. If you are willing to operate the modem without
compression and error correction then it's feasible to use it with
Linux. To do this you will need to disable RPI by sending the modem
(via the initialization string) a "RPI disable" command each time you
power on your modem. On my modem this command was +H0. Not having
data compression available makes it slower to get webpages but is just
as fast when downloading files that are already compressed.
3. Modem Pools, Digital Modems
A modem pool is a number of modems on the same card (such as a
multiport modem card) or many modems in an external chassis (something
like an external modem). The modems may be analog modems similar to
modems used for home/office PCs (can't send at 56k even if they are
"56k modems"). They also could be "digital modems" which can send at
nearly 56k (if you have a good line). The "digital modems" require a
digital connection to the telephone line and don't use any serial
ports at all. All of these modem pools will require that you install
special drivers for them.
3.1. Analog Modem Pools, Multiport Modem Cards
These are just many analog modems (the common home/office modem)
provided either on a plug-in card or in an external chassis. Each
modem comes with a built-in serial port. There is usually a system of
sharing interrupts or of handling interrupts by their own electronics,
thus removing much of this burden from the CPU. Note that these
modems are not "digital modems" and will thus not be able to use 56k
for people who dial-in.
Here is a list of some companies that make multiport modem cards. 8
modems/card is common. The cards listed claim to work with Linux and
the websites should point you to a driver for them.
Multiport Modem Cards:
� MultiModemISI by Multi-Tech Systems. 56k or 33.6k, PCI or ISA, 4
or 8 ports. ISDN/56k hybrids.
<
http://www.multitech.com/products/>
� RAStel by Moreton Bay Products. 56k PCI or ISA, 4 or 8 ports. Also
2 modems + 2 vacant serial ports.
<
http://www.moretonbay.com.au/MBWEB/product/rastel/rastel.htm>
� RocketModem by Comtrol. ISA 33.6k, 4 or 8 port.
<
http://www.comtrol.com/SALES/SPECS/Rmodem.htm>
� AccelePort (RAS Family) by Digi.
<http:/www.dgii.com/digi.cfm?p=940564.pi.prd.00000046>
3.2. Digital Modems
"digital modems" are much different than the analog modems that most
people use in their PCs. They require a digital connection to the
telephone line and don't use serial ports for the interface to the
computer. Instead, they interface directly to the PC bus via a
special card (which may also contain the "digital modems"). They are
able to send at near 56k, something no analog modem can do. They are
often a component of "remote access servers" or "digital modem pools"
The cables from the phone company that carry digital signals have been
designed for high bandwidth so that the same cable carries multiple
telephone calls. It's done by "time-division multiplexing". So the
first task to be done is to separate the phone calls and send each
phone call to its own "digital modem". There is also the task in the
reverse direction of combining all of the calls onto a single line.
These tasks are done by what is sometimes called a "...
concentrator".
The digital modem gets the digital signal from the telephone company.
It converts the waveshape it represents back to the same data bytes
that were sent from the sending PC. It puts these bytes on its bus
(likely sending it to a buffer in memory). Likewise, it handles
sending digital signals in the opposite direction to a digital
telephone line. Thus it only makes digital-to-digital conversions and
doesn't deal in analog at all. It thus is not really a modem at all
since it doesn't modulate any analog carrier. So the name "digital
modem" is a misnomer but it does do the job formerly done by modems.
Thus Some "serial modems" call themselves "digital signal processors",
"remote access servers", etc. and may not even mention the word
"modem". This is technically correct terminology.
Such a system may be a stand-alone proprietary server, a chassis
containing digital modems that connects to a PC via a special
interface card, or just a card itself. Digi calls one such card a
"remote access server concentrator adapter". One incomplete
description of what is needed to become an ISP is: See What do I need
to be an ISP?. Cyclades promotes their own products here so please do
comparison shopping before buying anything.
4. Serial Port and Modem Basics
You don't have to understand the basics to use and install a modem.
But understanding it may help to determine what is wrong if you run
into problems. After reading this section, if you want to understand
it even better you may want to see ``How Modems Work'' in this
document (not yet complete). More details on the serial port
(including much of this section) will be found in Serial-HOWTO.
4.1. Modem Converts Digital to Analog (and conversely)
Most all telephone main lines are digital already but the lines
leading to your house (or business) are usually analog which means
that they were designed to transmit a voltage wave which is an exact
replica of the sound wave coming out of your mouth. Such a voltage
wave is called "analog". If viewed on an oscilloscope it looks like a
sine wave of varying frequency and amplitude. A digital signal is
like a square wave. For example 3 v (volts) might be a 1-bit and 0 v
could be a 0-bit. For most serial ports (used by external modems) +12
v is a 0-bit and -12 v is a 1-bit (some are + or - 5 v).
To send data from your computer over the phone line, the modem takes
the digital signal from your computer and converts it to "analog". It
does this by both creating an analog sine wave and then "MODulating"
it. Since the result still represents digital data, it could also be
called a digital signal instead of analog. But it looks something
like an analog signal and almost everyone calls it analog. At the
other end of the phone line another modem "DEModulates" this signal
and the pure digital signal is recovered. Put together the "mod" and
"dem" parts of the two words above and you get "modem" (if you drop
one of the two d's). A "modem" is thus a MODulator-DEModulator. Just
what modulation is may be found in the section ``Modulation Details''.
4.2. What is a Serial Port ?
4.2.1. Intro to Serial
The serial port is an I/O (Input/Output) device. Since modems have a
serial port between them and the computer, it's necessary to
understand the serial port as well as the modem.
Most PC's have one or two serial ports. Each has a 9-pin connector
(sometimes 25-pin) on the back of the computer. Computer programs can
send data (bytes) to the transmit pin (output) and receive bytes from
the receive pin (input). The other pins are for control purposes and
ground.
The serial port is much more than just a connector. It converts the
data from parallel to serial and changes the electrical representation
of the data. Inside the computer, data bits flow in parallel (using
many wires at the same time). Serial flow is a stream of bits over a
single wire (such as on the transmit or receive pin of the serial
connector). For the serial port to create such a flow, it must
convert data from parallel (inside the computer) to serial on the
transmit pin (and conversely).
Most of the electronics of the serial port is found in a computer chip
(or a part of a chip) known as a UART. For more details on UARTs see
the section "What are UARTS" in the Serial-HOWTO.
But you may want to finish this section first so that you will
hopefully understand how the UART fits into the overall scheme of
things.
4.2.2. Pins and Wires
Old PC's used 25 pin connectors but only about 9 pins were actually
used so today most connectors are only 9-pin. Each of the 9 pins
usually connects to a wire. Besides the two wires used for
transmitting and receiving data, another pin (wire) is signal ground.
The voltage on any wire is measured with respect to this ground. Thus
the minimum number of wires to use for 2-way transmission of data is
3. Except that it has been known to work with no signal ground wire
but with degraded performance and sometimes with errors.
There are still more wires which are for control purposes (signalling)
only and not for sending bytes. All of these signals could have been
shared on a single wire, but instead, there is a separate dedicated
wire for every type of signal. Some (or all) of these control wires
are called "modem control lines". Modem control wires are either in
the asserted state (on) of +12 volts or in the negated state (off) of
-12 volts. One of these wires is to signal the computer to stop
sending bytes out the serial port cable. Conversely, another wire
signals the device attached to the serial port to stop sending bytes
to the computer. If the attached device is a modem, other wires may
tell the modem to hang up the telephone line or tell the computer that
a connection has been made or that the telephone line is ringing
(someone is attempting to call in). See the Serial-HOWTO: Pinout and
Signals for more details.
4.2.3. Internal Modem Contains Serial Port
For an internal modem there is no 9-pin connector but the behavior is
almost exactly as if the above mentioned cable wires existed. Instead
of a a 12 volt signal in a wire giving the state of a modem control
line, the internal modem may just use a status bit in its own memory
(a register) to determine the state of this non-existent "wire". The
internal modem's serial port looks just like a real serial port to the
computer. It even includes the speed limits that one may set at
ordinary serial ports such as 115200 bits/sec. Unfortunately for
Linux, many internal modems today don't work exactly this way but
instead use software (running on the CPU) to do much of the modem's
work. Unfortunately, such software is often only available for the MS
Windows OS (it hasn't been ported to Linux). Thus you can't use most
of these modems with Linux See ``Software-based Modems (winmodems)''.
4.3. IO Address & IRQ
Since the computer needs to communicate with each serial port, the
operating system must know that each serial port exists and where it
is (its I/O address). It also needs to know which wire (IRQ number)
the serial port must use to request service from the computer's CPU.
It requests service by sending an interrupt on this wire. Thus every
serial port device must store in its non-volatile memory both its I/O
address and its Interrupt ReQuest number: IRQ. See ``Interrupts''.
For the PCI bus it doesn't work exactly this way since the PCI bus has
its own system of interrupts. But since the PCI-aware BIOS sets up
chips to map these PCI interrupts to IRQs, it seemingly behaves just
as described above except that sharing of interrupts is allowed (2 or
more devices may use the same IRQ number).
I/O addresses are not the same as memory addresses. When an I/O
addresses is put onto the computer's address bus, another wire is
energized. This both tells main memory to ignore the address and
tells all devices which have I/O addresses (such as the serial port)
to listen to the address to see if it matches the device's. If the
address matches, then the I/O device reads the data on the data bus.
4.4. Names: ttyS0, ttyS1, etc.
The serial ports are named ttyS0, ttyS1, etc. (and usually correspond
respectively to COM1, COM2, etc. in DOS/Windows). The /dev directory
has a special file for each port. Type "ls /dev/ttyS*" to see them.
Just because there may be (for example) a ttyS3 file, doesn't
necessarily mean that there exists a physical serial port there.
Which one of these names (ttyS0, ttyS1, etc.) refers to which physical
serial port is determined as follows. The serial driver (software)
maintains a table showing which I/O address corresponds to which ttyS.
This mapping of names (such as ttyS1) to I/O addresses (and IRQ's) may
be both set and viewed by the "setserial" command. See ``What is
Setserial''. This does not set the I/O address and IRQ in the
hardware itself (which is set by jumpers or by plug-and-play
software). Thus what physical port corresponds to say ttyS1 depends
both on what the serial driver thinks (per setserial) and what is set
in the hardware. If a mistake has been made, the physical port may
not correspond to any name (such as ttyS2) and thus it can't be used.
See ``Serial Port Devices /dev/ttyS2, etc.'' for more details>
4.5. Interrupts
Bytes come in over the phone line to the modem, are converted from
analog to digital by the modem and passed along to the serial port on
their way to their destination inside your computer. When the serial
port receives a number of bytes (may be set to 1, 4, 8, or 14) into
its FIFO buffer, it signals the CPU to fetch them by sending an
electrical signal known as an interrupt on a certain wire normally
used only by that port. Thus the FIFO waits for a number of bytes and
then issues an interrupt.
However, this interrupt will also be sent if there is an unexpected
delay while waiting for the next byte to arrive (known as a timeout).
Thus if the bytes are being received slowly (such as someone typing on
a terminal keyboard) there may be an interrupt issued for every byte
received. For some UART chips the rule is like this: If 4 bytes in a
row could have been received, but none of these 4 show up, then the
port gives up waiting for more bytes and issues an interrupt to fetch
the bytes currently in the FIFO. Of course, if the FIFO is empty, no
interrupt will be issued.
Each interrupt conductor (inside the computer) has a number (IRQ) and
the serial port must know which conductor to use to signal on. For
example, ttyS0 normally uses IRQ number 4 known as IRQ4 (or IRQ 4). A
list of them and more will be found in "man setserial" (search for
"Configuring Serial Ports"). Interrupts are issued whenever the
serial port needs to get the CPU's attention. It's important to do
this in a timely manner since the buffer inside the serial port can
hold only 16 (1 in old serial ports) incoming bytes. If the CPU fails
to remove such received bytes promptly, then there will not be any
space left for any more incoming bytes and the small buffer may
overflow (overrun) resulting in a loss of data bytes.
For an external modem, there is no way (such as flow control) to stop
the flow rapidly enough to prevent this. For an internal modem the
16-byte FIFO buffer is on the same card and a good modem will not
write to it if it's full. Thus a good internal modem will not overrun
the 16-byte buffers but it may need to use ``Modem-to-Modem Flow
Control'' to prevent the modem itself from being overrun. This is one
advantage of an internal modem over an external.
Interrupts are also issued when the serial port has just sent out all
16 of its bytes from its small transmit buffer out the external cable.
It then has space for 16 more outgoing bytes. The interrupt is to
notify the CPU of that fact so that it may put more bytes in the small
transmit buffer to be transmitted. Also, when a modem control line
changes state an interrupt is issued.
The buffers mentioned above are all hardware buffers. The serial port
also has large buffers in main memory. This will be explained later
Interrupts convey a lot of information but only indirectly. The
interrupt itself just tells a chip called the interrupt controller
that a certain serial port needs attention. The interrupt controller
then signals the CPU. The CPU runs a special program to service the
serial port. That program is called an interrupt service routine
(part of the serial driver software). It tries to find out what has
happened at the serial port and then deals with the problem such a
transferring bytes from (or to) the serial port's hardware buffer.
This program can easily find out what has happened since the serial
port has registers at IO addresses known to the the serial driver
software. These registers contain status information about the serial
port. The software reads these registers and by inspecting the
contents, finds out what has happened and takes appropriate action.
4.6. Data Compression (by the Modem)
Before continuing with the basics of the serial port, one needs to
understand about something done by the modem: data compression. In
some cases this task is actually done by software run on the
computer's CPU but unfortunately at present, such software only works
for MS Windows. The discussion here will be for the case where the
modem itself does the compression since this is what must happen in
order for the modem to work under Linux.
In order to send data faster over the phone line, one may compress
(encode it) using a custom encoding scheme which itself depends on the
data. The encoded data is smaller than the original (less bytes) and
can be sent over the Internet in less time. This process is called
"data compression".
If you download files from the Internet, they are likely already
compressed and it is not feasible for the modem to try to compress
them further. Your modem may sense that what is passing thru has
already been compressed and refrain from trying a compress it any
more. If you are receiving data which has been compressed by the
other modem, your modem will decompress it and create many more bytes
than were sent over the phone line. Thus the flow of data from your
modem into your computer will be higher than the flow over the phone
line to you. The ratio of this flow is called the compression ratio.
Compression ratios as high as 4 are possible, but not very likely.
4.7. Error Correction
Similar to data compression, modems may be set to do error correction.
While there is some overhead cost involved which slows down the
byte/sec flow rate, the fact that error correction strips off start
and stop bits actually increases the data byte/sec flow rate.
For the serial port's interface with the external world, each 8-bit
byte has 2 extra bits added to it: a start-bit and a stop-bit.
Without error correction, these extra start and stop bits usually go
right thru the modem and out over the phone lines. But when error
correction is enabled, these extra bits are stripped off and the 8-bit
bytes are put into packets. This is more efficient and results in
higher byte/sec flow in spite of the fact that there are a few more
bytes added for packet headers and error correction purposes.
4.8. Data Flow (Speeds)
Data (bytes representing letters, pictures, etc.) flows from your
computer to your modem and then out on the telephone line (and
conversely). Flow rates (such as 56k (56000) bits/sec) are
(incorrectly) called "speed". But almost everyone says "speed"
instead of "flow rate". If there were no data compression the flow
rate from the computer to the modem would be about the same as the
flow rate over the telephone line.
Actually there are two different speeds to consider at your end of the
phone line:
� The speed on the phone line itself (DCE speed) modem-to-modem
� The speed from your computer's serial port to your modem (DTE
speed)
When you dial out and connect to another modem on the other end of the
phone line, your modem often sends you a message like "CONNECT 28800"
or "CONNECT 115200". What do these mean? Well, its either the DCE
speed or the DTE speed. If it's higher than the advertised modem speed
it must be the DTE modem-to-computer speed. This is the case for the
115200 speed shown above. The 28800 must be a DCE (modem-to-modem)
speed since the serial port has no such speed. One may configure the
modem to report either speed. Some modems report both speeds and
report the modem-to-modem speed as (for example): CARRIER 28800.
If you have an internal modem you would not expect that there would be
any speed limit on the DTE speed from your modem to your computer
since you modem is inside your computer and is almost part of your
computer. But there is since the modem contains a dedicated serial
port within it.
It's important to understand that the average speed is often less than
the specified speed, especially on the short DTE computer-to-modem
line. Waits (or idle time) result in a lower average speed. These
waits may include long waits of perhaps a second due to ``Flow
Control''. At the other extreme there may be very short waits (idle
time) of several micro-seconds separating the end of one byte and the
start of the next byte. In addition, modems will fallback to lower
speeds if the telephone line conditions are less than pristine.
For a discussion of what DTE speed is best to use see section ``What
Speed Should I Use''.
4.9. Flow Control
Flow control means the ability to slow done the flow of bytes in a
wire. For serial ports this means the ability to stop and then
restart the flow without any loss of bytes. Flow control is needed
for modems to allow a jump in instantaneous flow rates.
4.9.1. Example of Flow Control
For example, consider the case where you connect a 36.6k external
modem via a short cable to your serial port. The modem sends and
receives bytes over the phone line at 36.6k bits per second (bps).
Assume it's not doing any data compression or error correction. You
have set the serial port speed to 115,200 bits/sec (bps), and you are
sending data from your computer to the phone line. Then the flow from
the your computer to your modem over the short cable is at 115.2k bps.
However the flow from your modem out the phone line is only 33.6k bps.
Since a faster flow (115.2k) is going into your modem than is coming
out of it, the modem is storing the excess flow (115.2k -33.6k = 81.6k
bps) in one of its buffers. This buffer would soon overrun (run out
of free storage space) unless the high 115.2k flow is stopped.
But now flow control comes to the rescue. When the modem's buffer is
almost full, the modem sends a stop signal to the serial port. The
serial port passes on the stop signal on to the device driver and the
115.2k bps flow is halted. Then the modem continues to send out data
at 33.6k bps drawing on the data it previous accumulated in its
buffer. Since nothing is coming into the buffer, the level of bytes
in it starts to drop. When almost no bytes are left in the buffer,
the modem sends a start signal to the serial port and the 115.2k flow
from the computer to the modem resumes. In effect, flow control
creates an average flow rate in the short cable (in this case 33.6k)
which is significantly less than the "on" flow rate of 115.2k bps.
This is "start-stop" flow control.
In the above simple example it was assumed that the modem did no data
compression. This would be true when the modem is sending a file
which is already compressed and can't be compressed further. Now
let's consider the opposite extreme where the modem is compressing the
data with a high compression ratio. In such a case the modem might
need an input flow rate of say 115.2k bps to provide an output (to the
phone line) of 33.6k bps (compressed data). The compression ratio is
3.43 (115.2/33.6) which is much higher than average. In this case the
modem is able to compress and the 115.2 bps PC-to-modem flow and send
the same data out on the phone line at 33.6bps. There's no need for
flow control here. But such a high compression ratio rarely happens
so that most of the time flow control is needed to slow down the flow
on the 115.2 bps PC-to-modem cable. The flow is stopped and started
so that the average flow is usually well under the "on" flow of 115.2
bps.
In the above example the modem was an external modem. But the same
situation exists (as of late 2000) for most internal modems. There is
still a speed limit on the PC-to-modem speed even though this flow
doesn't take place over an external cable. This makes the internal
modems compatible with the external modems.
In the above example of flow control the flow was from the computer to
a modem. But there is also flow control which is used for the
opposite direction of flow: from a modem (or other device) to a
computer. Each direction of flow involve 3 buffers: 1. in the modem
2. in the UART chip (called FIFOs) 3. in main memory managed by the
serial driver. Flow control protects certain buffers from
overflowing. The small UART FIFO buffers are not protected in this
way but rely instead on a fast response to the interrupts they issue.
FIFO stand for "First In, First Out" which is the way it handles
bytes. All the 3 buffers use the FIFO rule but only one of them also
uses it as a name. This is the essence of flow control but there are
still some more details.
You don't often need flow control in the direction from the modem to a
PC. For complex example of a case where it's needed see "Complex Flow
Control Example" in the Serial-HOWTO. But if you don't have a high
enough speed set between the modem and the computer (serial port
speed) then you do need to slow down the flow coming into the modem
from the telephone line. To do this your modem must tell the other
modem to stop sending. See ``Modem-to-Modem Flow Control''.
4.9.2. Hardware vs. Software Flow Control
If feasible it's best to use "hardware" flow control that uses two
dedicated "modem control" wires to send the "stop" and "start"
signals. Modern modems almost always use hardware flow control
between the modem and the serial port.
Software flow control uses the main receive and transmit wires to send
the start and stop signals. It uses the ASCII control characters DC1
(start) and DC3 (stop) for this purpose. They are just inserted into
the regular stream of data. Software flow control is not only slower
in reacting but also does not allow the sending of binary data unless
special precautions are taken. Since binary data will likely contain
DC1 and DC3, special means must be taken to distinguish between a DC3
that means a flow control stop and a DC3 that is part of the binary
code. Likewise for DC1. To get software flow control to work for
binary data requires both modem (hardware) and software support
4.9.3. Symptoms of No Flow Control
Understanding flow-control theory can be of practical use. For
example I used my modem to access the Internet and it seemed to work
fine. But after a few months I tried to send long files from my PC to
an ISP and a huge amount of retries and errors resulted (but
eventually Kermit could send a long file after many retries).
Receiving in the other direction (from my ISP to me) worked fine. The
problem turned out to be a modem with broken flow control. My modem's
buffer was overflowing (overrunning) on long outgoing files since no
"stop" signal was ever sent to the computer to halt sending to the
modem. There was no problem in the direction from the modem to my
computer since the capacity (say 115.2k) was always higher than the
flow over the telephone line. The fix was to enable flow control by
putting into the init string an enable-flow-control command for the
modem. (It should have been enabled by default but something was
wrong).
4.9.4. Modem-to-Modem Flow Control
This is the flow control of the data sent over the telephone lines
between two modems. Practically speaking, it only exists when you
have error correction enabled. Actually, even without error
correction it's possible to enable software flow control between
modems but it may interfere with sending binary data so it's not often
used.
4.10. Data Flow Path; Buffers
Much has been explained about this including flow control, a pair of
16-byte FIFO buffers (in the UART), and a pair of larger buffers
inside a device connected to the serial port (such as a modem. But
there is still another pair of buffers. These are large buffers
(perhaps 8k) in main memory also known as serial port buffers. When
an application program sends bytes to the serial port (and modem) they
first get stashed in the the transmit serial port buffer in main
memory. The pair consists of both this transmit buffer and a receive
buffer for the opposite direction of byte-flow. Here's an example
diagram for the case of browsing the Internet with a browser.
Transmit data flow is left to right while receive flow is right to
left.
application 8k-byte 16-byte 1k-byte tele-
BROWSER ------- MEMORY -------- UART --------- MODEM -------- phone
program buffer buffer buffer line
The serial device driver takes out say 16 bytes from this transmit
buffer, one byte at a time and puts them into the 16-byte transmit
buffer in the serial UART for transmission. Once in that transmit
buffer, there is no way to stop them from being transmitted. They are
then transmitted to the modem which also has a fair sized (say 1k)
buffer. When the device driver (on orders from flow control) stops
the flow of outgoing bytes from the computer, what it actually stops
is the flow of outgoing bytes from the large transmit buffer in main
memory. Even after this has happened and the flow to the modem has
stopped, an application program may keep sending bytes to the 8k
transmit buffer until it becomes fill.
When it gets fill, the application program can't send any more bytes
to it (a "write" statement in a C_program blocks) and the application
program temporarily stops running and waits until some buffer space
becomes available. Thus a flow control "stop" is ultimately able to
stop the program that is sending the bytes. Even though this program
stops, the computer does not necessarily stop computing. It may
switch to running other processes while it's waiting at a flow control
stop. The above was a little oversimplified since there is another
alternative of having the application program itself do something else
while it is waiting to "write".
4.11. Modem Commands
Commands to the modem are sent to it from the communication software
over the same conductor as used to send data. The commands are short
ASCII strings. Examples are "AT&K3" for enabling hardware flow
control (RTS/CTS) between your computer and modem; and "ATDT5393401
for Dialing the number 5393401. Note all commands are prefaced by
"AT". Some commands such as enabling flow control help configure the
modem. Other commands such as dialing a number actually do something.
There are about a hundred or so different possible commands. When
your communication software starts running, it first sends an "init"
string of commands to the modem to configure it. All commands are
sent on the ordinary data line before the modem dials (or receives a
call).
Once the modem is connected to another modem (on-line mode),
everything that is sent from your computer to your modem goes directly
to the other modem and is not interpreted by the modem as a command.
There is a way to "escape" from this mode of operation and go back to
command mode where everything sent to the modem will be interpreted as
a command. The computer just sends "+++" with a specified time
spacing before and after it. If this time spacing is correct, the
modem reverts to command mode. Another way to do this is by a signal
on a certain modem control line.
There are a number of lists of modem commands on the Internet. The
section ``Web Sites'' has links to a couple of such web-sites.
Different models and brands of modems do not use exactly the same set
of such commands. So what works for one modem might not work for
another. Some common command (not guaranteed to work on all modems)
are listed in this HOWTO in the section ``Modem Configuration''
4.12. Serial Driver Module
The device driver for the serial port is the software that operates
the serial port. It is now provided as a serial module. >From kernel
2.2 on, this module will normally get loaded automatically if it's
needed. In earlier kernels, you had to have kerneld running in order
to do auto-load modules on demand. Otherwise the serial module needed
to be explicitly listed in /etc/modules. Before modules became
popular with Linux, the serial driver was usually built into the
kernel (and sometimes still is). If it's built-in don't let the
serial module load or else you will have two serial drivers running at
the same time. With 2 drivers there are all sorts of errors including
a possible "I/O error" when attempting to open a serial port. Use
"lsmod" to see if the module is loaded.
When the serial module is loaded it displays a message on the screen
about the existing serial ports (often showing a wrong IRQ). But once
the module is used by setserial to tell the device driver the
(hopefully) correct IRQ then you should see a second display similar
to the first but with the correct IRQ, etc. See "Serial Module" in
the Serial-HOWTO. See ``What is Setserial'' for more info on
setserial.
5. Configuring Overview
If you want to use a modem only for MS Windows/Dos, then you can just
install almost any modem and it will work OK. With a Linux PC it's
not usually this easy unless you use an external modem. All external
modems should work OK (even if they are labeled "Plug and Play") But
most new internal modems are Plug-and-Play (PnP) and have PnP serial
ports. In some cases (depending both on the modem and your version of
Linux) The PnP configuring is built into the serial driver so you
don't need to do anything. If it's an ISA modem you may need to use
the Linux "isapnp" program to configure it (but this is planned to be
built into future drivers ??). See the Plug-and-Play-HOWTO and the
isapnp docs for more information.
Since each modem has an associated serial port there are two parts to
configuring a modem:
� Configuring the modem itself: Done by the communication program
� Configuring the modem's serial port: Done only *partly* by the
communication program
Most of the above configuring (but not necessarily most of the effort)
is done by the communication program that you use with the modem such
as minicom, seyon, wvdial (for PPP). If you use the modem for dial-
in, then the getty program which you use to present outsiders with a
login-prompt, will help configure. Thus to configure the modem (and
much of the serial port) you need to configure the communication
program (such as the PPP dialer or getty).
Unfortunately the above configuring doesn't do the low-level
configuring of the serial port: setting its IO address and IRQ in both
the hardware and the driver. If you are lucky, this will happen
automatically when you boot Linux. Setting these in the hardware was
formerly done by jumpers but today it's done by "Plug-and-Play"
software.
But there's a serious problem: Linux (as of late 1999) is not a Plug-
and-Play operating system but it does have Plug-and-Play tools which
you may use to set up the configuration although they are not always
very user friendly. This may create a difficult problem for you. The
next section will go into this in much more detail.
6. Configuring the Serial Port
6.1. PCI Bus Support Underway
Although most PCI modems are "winmodems" without a Linux driver (and
will not work under Linux), other PCI serial cards (usually modem
cards) will often work OK under Linux. Some need no special support
in the serial driver and work fine under Linux once setserial is used
to configure them. Other PCI cards need special support in the
kernel. Some of these cards are supported by kernel 2.4 (and in later
versions of the 2.3 series). Kernel 2.2 has no such support. If your
modem (or serial port) happens to be supported, then you shouldn't
need to do anything to PnP configure it. The new serial driver will
read the id number digitally stored on the card to determine how to
support the card. It should assign the I/O address to it, determine
it's IRQ, etc. So you don't need to use "setserial" for it.
If you have a PCI internal modem which you are convinced is not a
winmodem but it will not work because the latest serial driver doesn't
support it, you can help in attempting to create a driver for it. To
do this you'll need to contact the maintainer of the serial driver,
Theodore (Ted) Y. Ts'o. But first check out the modem list site
<
http://www.o2.net/~gromitkc/winmodem.html> for the latest info on PCI
modems and related topic. You will need to email Ted Ts'o a copy of
the output of "lspci -vv" with full information about the model and
manufacturer of the PCI modem (or serial port). Then he will try to
point you to a test driver which might work for it. You will then
need to get it, compile it and possibly recompile your kernel. Then
you will test the driver to see if it works OK for you and report the
results to Ted Ts'o. If you are willing to do all the above (and this
is the latest version of this HOWTO) then email the needed info to him
at: <mailto:
[email protected]>.
PCI modems are not well standardized. Some use main memory for
communication with the PC. It you see 8-digit hexadecimal addresses
it's not likely to work with Linux. Some require special enabling of
the IRQ. The output of "lspci" can help determine if one can be
supported. If you see a 4-digit IO port and no long memory address,
the modem might work by just telling "setserial" the IO port and the
IRQ. Some people have gotten a 3COM 3CP5610 PCI Modem to work that
way.
6.2. Configuring Overview
In many cases, configuring will happen automatically and you have
nothing to do. But sometimes you need to configure (or just want to
check out the configuration). If so, first you need to know about the
two parts to configuring the serial port under Linux:
The first part (low-level configuring) is assigning it an IO address,
IRQ, and name (such as ttyS2). This IO-IRQ pair must be set in both
the hardware and told to the serial driver. We might just call this
"io-irq" configuring for short. The setserial is used to tell the
driver. PnP methods, jumpers, etc, are used to set the hardware.
Details will be supplied later. If you need to configure but don't
understand certain details it's easy to get into trouble.
The second part (high-level configuring) is assigning it a speed (such
as 38.4k bits/sec), selecting flow control, etc. This is often done
by communication programs such as PPP, minicom, or by getty (which you
may run on the port so that others may log into your computer).
However you will need to tell these programs what speed you want, etc.
by using a menu or a configuration file. This high-level configuring
may also be done with the stty program. stty is also useful to view
the current status if you're having problems. See also the Serial-
HOWTO section: "Stty". When Linux starts, some effort is made to
detect and configure (low-level) a few serial ports. Exactly what
happens depends on your BIOS, hardware, Linux distribution, etc. If
the serial ports work OK, there may be no need for you to do any
configuring. Application programs often do the high-level configuring
but you may need to supply them with the required information. With
Plug-and-Play serial ports (often built into an internal modem), the
situation has become more complex. Here are cases when you need to do
low-level configuring (set IRQ and IO addresses):
� Plan to use more than 2 serial ports
� Installing a new serial port (such as an internal modem)
� Having problems with serial port(s)
For kernel 2.2+ you may be able to use more that 2 serial ports
without low-level configuring by sharing interrupts. This only works
if the serial hardware supports it and may be no easier than low-level
configuring. See ``Interrupt sharing and Kernels 2.2+''
The low-level configuring (setting the IRQ and IO address) seems to
cause people more trouble (than high-level), although for many it's
fully automatic and there is no configuring to be done. Thus most all
of this section is on that topic. Until the serial driver knows the
correct IRQ and IO address the port will not work at all. It may not
even be found by Linux. Even if it can be found, it may work
extremely slow if the IRQ is wrong. See ``Extremely Slow: Text
appears on the screen slowly after long delays''.
In the Wintel world, the IO address and IRQ are called "resources" and
we are thus configuring certain resources. But there are many other
types of "resources" so the term has many other meanings. In review,
the low-level configuring consists of putting two values (an IRQ
number and IO address) into two places:
1. the device driver (often by running "setserial" at boot-time)
2. memory registers of the serial port hardware itself
You may watch the start-up (= boot-time) messages. They are usually
correct. But if you're having problems, there's a good chance that
some of these messages don't show the true configuration of the
hardware (and they are not supposed to). See ``I/O Address & IRQ:
Boot-time messages''.
6.3. Common mistakes made re low-level configuring
Here are some common mistakes people make:
� setserial command: They run it (without the "autoconfig" and
auto_irq options) and think it has checked out the hardware (it
hasn't).
� setserial messages: They see them displayed on the screen at boot-
time (or by giving the setserial command) and erroneously think
that the result always shows how their hardware is actually
configured.
� /proc/interrupts: When their serial device isn't in use they don't
see its interrupt there, and erroneously conclude that their serial
port can't be found (or doesn't have an interrupt set).
� /proc/ioports and /proc/tty/driver/serial: People think this shows
the actual hardware configuration when it only shows about the same
info (possibly erroneous) as setserial.
6.4. I/O Address & IRQ: Boot-time messages
In many cases your ports will automatically get low-level configured
at boot-time (but not always correctly). To see what is happening,
look at the start-up messages on the screen. Don't neglect to check
the messages from the BIOS before Linux is loaded (no examples shown
here). These BIOS messages may be frozen by pressing the Pause key.
Use Shift-PageUp to scroll back to the messages after they have
flashed by. Shift-PageDown will scroll in the opposite direction.
The dmesg command may be used at any time to view some of the messages
but it often misses important ones. Here's an example of the start-up
messages (as of mid 1999). Note that ttyS00 is the same as
/dev/ttyS0.
At first you see what was detected (but the irq is only a wild guess):
Serial driver version 4.27 with no serial options enabled
ttyS00 at 0x03f8 (irq = 4) is a 16550A
ttyS01 at 0x02f8 (irq = 3) is a 16550A
ttyS02 at 0x03e8 (irq = 4) is a 16550A
Later you see what was saved, but it's not necessarily correct either:
Loading the saved-state of the serial devices...
/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
/dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
/dev/ttyS2 at 0x03e8 (irq = 5) is a 16550A
Note that there is a slight disagreement: The first message shows
ttyS2 at irq=4 while the second shows it at irq=5. Your may only have
the first message. In most cases the last message is the correct one.
But if your having trouble it may be misleading. Before reading the
explanation of all of this complexity in the rest of this section, you
might just try using your serial port and see if it works OK. If so
it may not be essential to read further.
The second message is from the setserial program being run at boot-
time. It shows what the device driver thinks is the correct
configuration. But this too could be wrong. For example, the irq
could actually be set to irq=8 in the hardware (both messages wrong).
The irq=5 could be there because someone incorrectly put this into a
configuration file (or the like). The fact that Linux sometimes gets
IRQs wrong is because it doesn't by default probe for IRQs. It just
assumes the "standard" ones (first message) or accepts what you told
it when you configured it (second message). Neither of these is
necessarily correct. If the serial driver has the wrong IRQ the
serial port is very slow or doesn't seem to work at all.
The first message is a result of Linux probing the serial ports but it
doesn't probe for IRQs. If a port shows up here it exists but the IRQ
may be wrong. Linux doesn't check IRQs because doing so is not
foolproof. It just assumes the IRQs are as shown because they are the
"standard" values. Your may check them manually with setserial using
the autoconfig and auto_irq options but this isn't guaranteed to be
correct either.
The data shown by the BIOS messages (which you see at first) is what
is set in the hardware. If your serial port is Plug-and-Play PnP then
it's possible that the isapnp will run and change these settings.
Look for messages about this after Linux starts. The last serial port
message shown in the example above should agree with the BIOS messages
(as possibly modified by isapnp). If they don't agree then you either
need to change the setting in the port hardware or use setserial to
tell the driver what is actually set in the hardware.
Also, if you have Plug-and-Play (PnP) serial ports, Linux will not
find them unless the IRQ and IO has been set inside the hardware by
Plug-and-Play software. Prior to kernel 2.4 this was a common reason
why the start-up messages did not show a serial port that physically
exists. The PC hardware (a PnP BIOS) may automatically low-level
configure this. PnP configuring will be explained later.
6.5. What is the current IO address and IRQ of my Serial Port ?
If your serial port seems to work OK, then you may type "setserial -g
/dev/ttyS*", look at /proc/tty/driver/serial, or inspect the start-up
messages. If you serial port doesn't work (or is very slow) then you
need to read further.
There are really two answers to the question "What is my IO and IRQ?"
1. What the device driver thinks has been set (This is what setserial
usually sets and shows). 2. What is actually set in the hardware.
They both should be the same. If they're not it spells trouble since
the driver has incorrect info on the physical serial port. If the
driver has the wrong IO address it will try to send data to a non-
existing serial port --or even worse, to some other device. If it has
the wrong IRQ the driver will not get interrupt service requests from
the serial port, resulting in a very slow or no response. See
``Extremely Slow: Text appears on the screen slowly after long
delays''. If it has the wrong model of UART there is also apt to be
trouble. To determine if both I0-IRQ pairs are identical you must
find out how they are set in both the driver and the hardware.
6.5.1. What does the device driver think?
This is easy to find out. Just look at the start-up messages or type
"setserial -g /dev/ttyS*". If everything works OK then what it tells
you is likely also set in the hardware. There are some other ways to
find this info by looking at "files" in the /proc directory. Be
warned that there is no guarantee that the same is set in the
hardware.
/proc/ioports will show the IO addresses that the drivers are using.
/proc/interrupts shows the IRQs that are used by drivers of currently
running processes (that have devices open). It shows how many
interrupts have actually be issued. /proc/tty/driver/serial shows
most of the above, plus the number of bytes that have been received
and sent (even if the device is not now open).
Note that for the IO addresses and IRQ assignments, you are only
seeing what the driver thinks and not necessarily what is actually set
in the hardware. The data on the actual number of interrupts issued
and bytes processed is real however. If you see a large number of
interrupts and/or bytes then it probably means that the device is (or
was in the case of bytes) working. If there are no bytes received
(rx:0) but bytes were transmitted (tx:3749 for example), then only one
direction of flow is working (or being utilized).
Sometimes a showing of just a few interrupts doesn't mean that the
interrupt is actually being physically generated by any serial port.
Thus if you see almost no interrupts for a port that you're trying to
use, that interrupt might not be set in the hardware and it implies
that the driver is using the wrong interrupt. To view
/proc/interrupts to check on a program that you're currently running
(such as "minicom") you need to keep the program running while you
view it.
6.5.2. What is set in my serial port hardware ?
How do you find out what IO address and IRQ are actually set in the
device hardware? Perhaps the BIOS messages will tell you some info
before Linux starts booting. Use the shift-PageUp key to step back
thru the boot-time messages and look at the very first ones which are
from the BIOS. This is how it was before Linux started. Setserial
can't change it but isapnp or pciutils can and starting with kernel
2.4, these will be built into the serial driver.
One crude method is try probing with setserial using the "autoconfig"
option. You'll need to guess the addresses to probe at. See ``What
is Setserial''. For a PCI serial port, use the "lspci" command (for
kernels <2.2 look at /proc/pci). If your serial port is is Plug-and-
Play see the next two subsections.
For a port set with jumpers, its how the jumpers were set. If the
port is not Plug-and-Play (PnP) but has been setup by using a DOS
program then it's set at whatever the person who ran that program set
it to.
6.5.3. What is set in my PnP serial port hardware ?
PnP ports don't store their configuration in the hardware when the
power is turned off. This is in contrast to Jumpers (non-PnP) which
remain the same with the power off. If you have an ISA PnP port, it
can reach a state where it doesn't have any IO address or IRQ and is
in effect disabled. It should still be possible to find the port
using the pnpdump program.
For Plug-and-Play (PnP) on the ISA bus one may try the pnpdump program
(part of isapnptools). If you use the --dumpregs option then it
should tell you the actual IO address and IRQ set in the port. The
address it "trys" is not the device's IO address, but a special
For PnP ports checking on how it's configured under DOS/Windows may
not be of much help. Windows stores its configuration info in its
Registry which is not used by Linux. It may supply the BIOS's non-
volatile memory with some info but it may not be kept in sync with the
current Window configuration in the Registry ?? If you let a PnP BIOS
automatically do the configuring when you start Linux (and have told
the BIOS that you don't have a PnP operating system when running
Linux) then Linux should use whatever configuration is in the BIOS's
non-volatile memory.
6.6. Choosing Serial IRQs
If you have Plug-and-Play ports then either a PnP BIOS or the serial
driver may configure all your devices for you and then you may not
need to choose any IRQs. PnP determines what it thinks is best and
assigns them. But if you use the tools in Linux for Plug-and-Play
(isapnp and pcitools) or jumpers then you have to choose. If you
already know what IRQ you want to use you could skip this section
except that you may want to know that IRQ 0 has a special use (see the
following paragraph).
6.6.1. IRQ 0 is not an IRQ
While IRQ 0 is actually the timer (in hardware) it has a special
meaning for setting a serial port with setserial. It tells the driver
that there is no interrupt for the port and the driver then will use
polling methods. This is quite inefficient but can be tried if there
is an interrupt conflict or mis-set interrupt. The advantage of
assigning this is that you don't need to know what interrupt is set in
the hardware. It should be used only as a temporary expedient until
you are able to find a real interrupt to use.
6.6.2. Interrupt sharing and Kernels 2.2+
The general rule is that every device should use a unique IRQ and not
share them. But there are situations where sharing is permitted such
as with most multi-port boards. Even when it is permitted, it may not
be as efficient since every time a shared interrupt is given a check
must be made to determine where it came from. Thus if it's feasible,
it's nice to allocate every device its own interrupt.
Prior to kernel 2.2, serial IRQs could be shared with each other only
for most multiport boards. Starting with kernel 2.2 serial IRQs may
be sometimes shared between all serial ports. In order for sharing to
work in 2.2 the kernel must have been compiled with
CONFIG_SERIAL_SHARE_IRQ, and the serial port hardware must support
sharing (so that if two serial cards put different voltages on the
same interrupt wire, only the voltage that means "this is an
interrupt" will prevail). Thus even if you have 2.2, it may be best
to avoid sharing.
6.6.3. What IRQs to choose?
The serial hardware often has only a limited number of IRQs it can be
set at. Also you don't want IRQ conflicts. So there may not be much
of a choice. Your PC may normally come with ttyS0 and ttyS2 at IRQ 4,
and ttyS1 and ttyS3 at IRQ 3. Looking at /proc/interrupts will show
which IRQs are being used by programs currently running. You likely
don't want to use one of these. Before IRQ 5 was used for sound
cards, it was often used for a serial port.
Here is how Greg (original author of Serial-HOWTO) set his up in
/etc/rc.d/rc.serial. rc.serial is a file (shell script) which runs at
start-up (it may have a different name of location). For versions of
"setserial" after 2.15 it's not always done this way anymore but this
example does show the choice of IRQs.
/sbin/setserial /dev/ttyS0 irq 3 # my serial mouse
/sbin/setserial /dev/ttyS1 irq 4 # my Wyse dumb terminal
/sbin/setserial /dev/ttyS2 irq 5 # my Zoom modem
/sbin/setserial /dev/ttyS3 irq 9 # my USR modem
Standard IRQ assignments:
IRQ 0 Timer channel 0 (May mean "no interrupt". See below.)
IRQ 1 Keyboard
IRQ 2 Cascade for controller 2
IRQ 3 Serial port 2
IRQ 4 Serial port 1
IRQ 5 Parallel port 2, Sound card
IRQ 6 Floppy diskette
IRQ 7 Parallel port 1
IRQ 8 Real-time clock
IRQ 9 Redirected to IRQ2
IRQ 10 not assigned
IRQ 11 not assigned
IRQ 12 not assigned
IRQ 13 Math coprocessor
IRQ 14 Hard disk controller 1
IRQ 15 Hard disk controller 2
There is really no Right Thing to do when choosing interrupts. Just
make sure it isn't being used by the motherboard, or any other boards.
2, 3, 4, 5, 7, 10, 11, 12 or 15 are possible choices. Note that IRQ 2
is the same as IRQ 9. You can call it either 2 or 9, the serial
driver is very understanding. If you have a very old serial board it
may not be able to use IRQs 8 and above.
Make sure you don't use IRQs 1, 6, 8, 13 or 14! These are used by
your motherboard. You will make her very unhappy by taking her IRQs.
When you are done, double-check /proc/interrupts when programs that
use interrupts are being run and make sure there are no conflicts.
6.7. Choosing Addresses --Video card conflict with ttyS3
The IO address of the IBM 8514 video board (and others like it) is
allegedly 0x?2e8 where ? is 2, 4, 8, or 9. This may conflict with the
IO address of ttyS3 at 0x02e8. Your may think that this shouldn't
happen since the addresses are different in the high order digit (the
leading 0 in 02e8). You're right, but a poorly designed serial port
may ignore the high order digit and respond to any address that ends
in 2e8. That is bad news if you try to use ttyS3 (ISA bus) at this IO
address.
For the ISA bus you should try to use the default addresses shown
below. Addresses shown represent the first address of an 8-byte
range. For example 3f8 is really the range 3f8-3ff. Each serial
device (as well as other types of devices that use IO addresses) needs
its own unique address range. There should be no overlaps
(conflicts). Here are the default addresses for commonly used serial
ports on the ISA bus:
ttyS0 address 0x3f8
ttyS1 address 0x2f8
ttyS2 address 0x3e8
ttyS3 address 0x2e8
Suppose there is an address conflict (as reported by setserial -g
/dev/ttyS*) between a real serial port and another port which does not
physically exist (and shows UART: unknown). Such a conflict shouldn't
cause problems but it sometimes does in older kernels. To avoid this
problem don't permit such address conflicts or delete /dev/ttySx if it
doesn't physically exist.
6.8. Set IO Address & IRQ in the hardware (mostly for PnP)
After it's set in the hardware don't forget to insure that it also
gets set in the driver by using setserial. For non-PnP serial ports
they are either set in hardware by jumpers or by running a DOS program
("jumperless") to set them (it may disable PnP). The rest of this
subsection is only for PnP serial ports. Here's a list of the
possible methods of configuring PnP serial ports:
� Using a PnP BIOS CMOS setup menu (usually only for external modems
on ttyS0 (Com1) and ttyS1 (Com2))
� Letting a PnP BIOS automatically configure a PnP serial port See
``Using a PnP BIOS to I0-IRQ Configure''
� Doing nothing if you have both a PnP serial port and a PnP Linux
operating system (see Plug-and-Play-HOWTO).
� Using isapnp for a PnP serial port non-PCI)
� Using pciutils (pcitools) for the PCI bus
The IO address and IRQ must be set (by PnP) in their registers each
time the system is powered on since PnP hardware doesn't remember how
it was set when the power is shut off. A simple way to do this is to
let a PnP BIOS know that you don't have a PnP OS and the BIOS will
automatically do this each time you start. This might cause problems
in Windows (which is a PnP OS) if you start Windows with the BIOS
thinking that Windows is not a PnP OS. See Plug-and-Play-HOWTO.
Plug-and-Play was designed to automate this io-irq configuring, but
for Linux at present, it has made life more complicated. The standard
kernels for Linux don't support plug-and-play very well. If you use a
patch to the Linux kernel to covert it to a plug-and-play operating
system, then all of the above should be handled automatically by the
OS. But when you want to use this to automate configuring devices
other that the serial port, you may find that you'll still have to
configure the drivers manually since many Linux drivers are not
written to support a Linux PnP OS. If you use isapnptools or the BIOS
for configuring plug-and-play this will only put the two values into
the registers of the serial port section of the modem card and you
will likely still need to set up setserial. None of this is easy or
very well documented as of early 1999. See Plug-and-Play-HOWTO and
the isapnptools FAQ.
6.8.1. Using a PnP BIOS to I0-IRQ Configure
While the explanation of how to use a PnP OS or isapnp for io-irq
configuring should come with such software, this is not the case if
you want to let a PnP BIOS do such configuring. Not all PnP BIOS can
do this. The BIOS usually has a CMOS menu for setting up the first
two serial ports. This menu may be hard to find and for an "Award"
BIOS it was found under "chipset features setup" There is often
little to choose from. Unless otherwise indicated in a menu, these
first two ports normally get set at the standard IO addresses and
IRQs. See ``Serial Port Device Names & Numbers''
Whether you like it or not, when you start up a PC a PnP BIOS starts
to do PnP (io-irq) configuring of hardware devices. It may do the job
partially and turn the rest over to a PnP OS (which you probably don't
have) or if thinks you don't have a PnP OS it may fully configure all
the PnP devices but not configure the device drivers. This is what
you want but it's not always easy to figure out exactly what the PnP
BIOS has done.
If you tell the BIOS that you don't have a PnP OS, then the PnP BIOS
should do the configuring of all PnP serial ports --not just the first
two. An indirect way to control what the BIOS does (if you have
Windows 9x on the same PC) is to "force" a configuration under
Windows. See Plug-and-Play-HOWTO and search for "forced". It's
easier to use the CMOS BIOS menu which may override what you "forced"
under Windows. There could be a BIOS option that can set or disable
this "override" capability.
If you add a new PnP device, the BIOS should change its PnP
configuration to accommodate it. It could even change the io-irq of
existing devices if required to avoid any conflicts. For this
purpose, it keeps a list of non-PnP devices provided that you have
told the BIOS how these non-PnP devices are io-irq configured. One
way to tell the BIOS this is by running a program called ICU under
DOS/Windows.
But how do you find out what the BIOS has done so that you set up the
device drivers with this info? The BIOS itself may provide some info,
either in its setup menus of via messages on the screen when you turn
on your computer. See ``What is set in my serial port hardware?''
6.9. Giving the IRQ and IO Address to Setserial
Once you've set the IRQ and IO address in the hardware (or arranged
for it to be done by PnP) you also need to insure that the "setserial"
command is run each time you start Linux. See the subsection ``Boot-
time Configuration''
6.10. Other Configuring
6.10.1. Configuring Hardware Flow Control (RTS/CTS)
See ``Flow Control'' for an explanation of it. You should always use
hardware flow control if possible. Your communication program or
"getty" should have an option for setting it (and if you're in luck it
might be enabled by default). It needs to be set both inside your
modem (by an init string or default) and in the device driver. Your
communication program should set both of these (if you configure it
right).
If none of the above will fully enable hardware flow control. Then
you must do it yourself. For the modem, make sure that it's either
done by the init string or is on by default. If you need to tell the
device driver to do it is best done on startup by putting it in a file
that runs at boot-time. See the subsection ``Boot-time
Configuration'' You need to add the following to such a file for each
serial port (example is ttyS2) you want to enable hardware flow
control on:
stty crtscts < /dev/ttyS2
If you want to see if flow control is enabled do the following: In
minicom (or the like) type AT&V to see how the modem is configured and
look for &K3 which means hardware flow control. Then see if the
device driver knows about it by typing: stty -a < /dev/ttyS2 Look for
"crtscts" (without a disabling minus sign).
7. Modem Configuration (excluding serial port)
7.1. Finding Your Modem
Before spending a lot of time configuring your modem, you need to make
sure it can be found and that AT commands and the like can be sent to
it. So I suggest you first give it a very simple configuration using
the communication program you will be using on the port and see it it
works. If so, then it's been found. If not then see ``My Modem is
Physically There but Can't be Found''. A winmodem may be hard to find
and will not work under Linux.
7.2. AT Commands
While the serial port on which a modem resides requires configuring,
so does the modem itself. The modem is configured by sending AT
commands (or the like) to it on the same serial line that is used to
send data.
Most modems use an AT command set. These are cryptic and short ASCII
commands where all command strings are prefaced by the letters AT.
For example: ATZ&K3<return> There are two commands here: Z and &K3.
Z is short for Z0 and some modems require that you use Z0 instead of
just Z. Similarly for all commands ending in 0. The command string
is terminated by a return character (use the <enter> key if you are
manually typing it). Unfortunately there are many different
variations of the AT command set so that what works for one modem may
or may not work for another modem. Thus there is no guarantee that
the AT commands given in this section will work on your modem.
Such command strings are either automatically sent to the modem by
communication programs or are manually typed in by you. Most
communication programs provide a screen where you may type such
commands. You may type in some commands to create the configuration
you want and then save this this configuration (profile) for later
use. It gets saved inside the modem itself.
If you have a manual for your modem you can likely look up the AT
command set. Otherwise, you may try to find it on the Internet. One
may use a search engine and include some actual commands in the search
terms to avoid finding sites that just talk about such commands but
fail to list them. You might also try a few of the sites listed in
the subsection ``Web Sites''
7.3. Init Strings: Saving and Recalling
The examples given in this subsection are from the Hayes AT modem
command set. All command strings must be prefaced by the two letters
AT. For example: AT&C1&D3^M (^M is the return character). When a
modem is powered on, it automatically configures itself with one of
the configurations it has stored in its non-volatile memory. If this
configuration is satisfactory there is nothing further to do.
If it's not satisfactory, then one may either alter the stored
configuration or configure the modem each time you use it by sending
it a string of commands known as an "init string" (= initialization
string). Normally, a communication program does this. What it sends
will depend on how you configured the communications program. Your
communication program should allow you to edit the init string and
change it to whatever you want. Sometimes the communications program
will let you select the model of your modem and then it will use an
init string that it thinks is best for that modem.
The configuration of the modem when it's first powered on may be
expressed by an init string. You might think of this as the default
"string" (called a profile). If your communications program sends the
modem another string (the init string), then this string will modify
the default configuration. For example, if the init string only
contains two commands, then only those two items will be changed.
However, some commands will recall a stored profile from inside the
modem so a single such command in the init string can thereby change
everything in the configuration.
Modern modems have a few different stored profiles to choose from that
are stored in the modem's non-volatile memory (it's still there when
you turn it off). In my modem there are two factory profiles (0 and
1, neither of which you can change) and two user defined profiles (0
and 1) that the user may set and store. Your modem may have more. To
view some of these profiles send the command &V. At power-up one of
the user-defined profiles is loaded. For example, if you type the
command &Y0 then in the future profile 0 will be used at power-on.
There are also commands to load (activate) any of the stored profiles.
Such a load command may be put in an init string. Of course if it
loads the same profile that was automatically loaded at power-up,
nothing is changed (unless the active profile has been modified since
power-up). Since it could have been modified it's a good idea to use
some kind of an init string even if it does nothing more than load a
stored profile.
Examples of loading saved profiles:
Z0 loads user-defined profile 0 and resets (hangs up, etc.)
&F1 loads factory profile 1
Once you have sent commands to the modem to configure it the way you
want (such as loading a factory profile and modifying it a little) you
may save this as a user-defined profile:
&W0 saves the current configuration to user-profile 0
Many people don't bother saving a good configuration in their modem,
but instead, send the modem a longer init string each time the modem
is used. Another method is to restore the factory default by &F1 at
the start of the init string and then modify it a little by adding a
few other commands to the end of the init string. By doing it this
way no one can cause problems by modifying (and saving) the user-
defined profile which is loaded at power-on.
You may choose an init string supplied by someone else that they think
is right for your modem. Some communication programs have a library
of init strings to select from. The most difficult method (and one
which will teach you the most about modems) is to study the modem
manual and write one yourself. You could save this configuration
inside the modem so that you don't need an init string. A third
alternative is to start with an init string that someone else wrote,
but modify it to suit your purposes.
If you look at init strings used by communication programs you may see
symbols which are not valid modem commands. These symbols are
commands to the communication program itself and will not be sent to
the modem. For example, means to pause briefly.
7.3.1. Where is my "init string" so I can modify it ?
This depends on your communication program (often a PPP program). If
this is the latest version of Modem-HOWTO send me info for other
cases.
� Gnome: run pppsetup
� wvdial: edit /etc/wvdial.conf
� minicom: hit ^Ao, then select "Modem and Dialing"
7.4. Other Modem Commands
Future editions of Modem-HOWTO may contain more AT commands but the
rest of this section is mostly what was in the old Serial-HOWTO. All
strings must start with AT. Here's a few Hayes AT codes that should
be in the string (if they are not set by using a factory default or by
a saved configuration).
E1 command echo ON
Q0 result codes are reported
V1 result codes are verbose
S0=0 never answer (uugetty does this with the WAITFOR option)
Here's some more codes:
� &C1 DCD is on only after connect
� &S0 DSR is always on
� &X3 Dial even if there is no dialtone (Use where dial-tones
don't exist).
What DTR does may also be set up but it's more complicated since it
depends on both the &D and &Q commands.
Greg Hankins had a collection of setups for different types of modems.
It's not currently maintained and covers modems prior to 1998.
ftp://ftp.cc.gatech.edu/pub/people/gregh/modem-configs.
Note: to get his USR Courier V.34 modem to reset correctly when DTR
drops, Greg Hankins had to set &D2 and S13=1 (this sets bit 0 of
register S13). This has been confirmed to work on USR Sportster V.34
modems as well.
Note: some Supra modems treat DCD differently than other modems. If
you are using a Supra, try setting &C0 and not &C1. You must also set
&D2 to handle DTR correctly.
8. Serial Port Devices /dev/ttyS2, etc.
For creating devices in the device directory see the Serial-HOWTO:
"Creating Devices In the /dev directory".
8.1. Serial Port Device Names & Numbers
Devices in Linux have major and minor numbers. Each serial port may
have 2 possible names in the /dev directory: ttyS and cua. Their
drivers behave slightly differently. The cua device is deprecated and
will not be used in the future. See ``The cua Device''.
Dos/Windows use the COM name while the setserial program uses tty00,
tty01, etc. Don't confuse these with dev/tty0, dev/tty1, etc. which
are used for the console (your PC monitor) but are not serial ports.
The table below is for the "standard" case (but yours could be
different).
IO
dos major minor major minor address
COM1 /dev/ttyS0 4, 64; /dev/cua0 5, 64 3F8
COM2 /dev/ttyS1 4, 65; /dev/cua1 5, 65 2F8
COM3 /dev/ttyS2 4, 66; /dev/cua2 5, 66 3E8
COM4 /dev/ttyS3 4, 67; /dev/cua3 5, 67 2E8
Note that all distributions should come with ttyS devices (and many
distributions have the obsolete cua device). You can verify this by
typing (don't feel bad if you don't find any obsolete cua devices):
linux% ls -l /dev/cua*
linux% ls -l /dev/ttyS*
8.2. Link ttySN to /dev/modem ?
On some installations, two extra devices will be created, /dev/modem
for your modem and /dev/mouse for your mouse. Both of these are
symbolic links to the appropriate device in /dev which you specified
during the installation (unless you have a bus mouse, then /dev/mouse
will point to the bus mouse device).
There has been some discussion on the merits of /dev/mouse and
/dev/modem. The use of these links is discouraged. In particular, if
you are planning on using your modem for dialin you may run into
problems because the lock files may not work correctly if you use
/dev/modem. However, if you change or remove this link, some
applications might need reconfiguration.
8.3. The cua Device
Each ttyS device has a corresponding cua device. But the cua device
is deprecated so it's best to use ttyS (unless cua is required).
There is a difference between cua and ttyS but a savvy programmer can
make a ttyS port behave just like a cua port so there is no real need
for the cua anymore. Except that some older programs may need to use
the cua.
What's the difference? The main difference between cua and ttyS has
to do with what happens in a C-program when an ordinary "open" command
tries to open the port. If a cua port has been set to check modem
control signals, the port can be opened even if the DCD modem control
signal says not to. Astute programming (by adding additional lines to
the program) can force a ttyS port to behave this way also. But a cua
port can be more easily programmed to open for dialing out on a modem
even when the modem fails to assert DCD (since no one has called into
it and there's no carrier). That's why cua was once used for dial-out
and ttyS used for dial-in.
Starting with Linux kernel 2.2, a warning message is put in the kernel
log when one uses cua. This is an omen that cua is defunct and should
be avoided if possible.
9. Interesting Programs You Should Know About
9.1. What is setserial ?
This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal. There are
some minor differences, depending on which HOWTO it appears in.
9.1.1. Introduction
Don't ever use setserial with Laptops (PCMCIA). setserial is a
program which allows you to tell the device driver software the I/O
address of the serial port, which interrupt (IRQ) is set in the port's
hardware, what type of UART you have, etc. Since theres a good chance
that the serial ports will be automatically detected and set, many
people never need to use setserial. In any case setserial will not
work without either serial support built into the kernel or loaded as
a module. The module may get loaded automatically if you (or a
script) tries to use setserial.
Setserial can also show how the driver is currently set. In addition,
it can be made to probe the hardware and try to determine the UART
type and IRQ, but this has severe limitations. See ``Probing''. Note
that it can't set the IRQ or the port address in the hardware of PnP
serial ports (but the plug-and-play features of the serial driver can
do this).
If you only have one or two built-in serial ports, they will usually
get set up correctly without using setserial. Otherwise (or if there
are problems with the serial port) you will likely need to deal with
setserial. Besides the man page for setserial, check out info in
/usr/doc/setserial.../ or /usr/share/doc/setserial. It should tell
you how setserial is handled in your distribution of Linux.
Setserial is often run automatically at boot-time by a start-up shell-
script for the purpose of assigning IRQs, etc. to the driver.
Setserial will only work if the serial module is loaded (or if the
equivalent was compiled into your kernel). If the serial module gets
unloaded later on, the changes previously made by setserial will be
forgotten by the kernel. But recent (2000) distributions may contain
scripts that save and restore this. If not, then setserial must be
run again to reestablish them. In addition to running via a start-up
script, something akin to setserial also runs earlier when the serial
module is loaded (or the like). Thus when you watch the start-up
messages on the screen it may look like it ran twice, and in fact it
has.
Setserial can set the time that the port will keep operating after
it's closed (in order to output any characters still in its buffer in
main RAM). This is needed at slow baud rates of 1200 or lower. It's
also needed at higher speeds if there are a lot of "flow control"
waits. See "closing_wait" in the man pg.
Setserial does not set either IRQ's nor I/O addresses in the serial
port hardware itself. That is done either by jumpers or by plug-and-
play. You must tell setserial the identical values that have been set
in the hardware. Do not just invent some values that you think would
be nice to use and then tell them to setserial. However, if you know
the I/O address but don't know the IRQ you may command setserial to
attempt to determine the IRQ.
You can see a list of possible commands by just typing setserial with
no arguments. This fails to show you the one-letter options such as
-v for verbose which you should normally use when troubleshooting.
Note that setserial calls an IO address a "port". If you type:
setserial -g /dev/ttyS*
you'll see some info about how that device driver is configured for
your ports. Note that where it says "UART: unknown" it probably means
that no uart exists. In other words you probably have no such serial
port and the other info shown about the port is meaningless and should
be ignored. If you really do have such a serial port, setserial
doesn't recognize it and that needs to be fixed.
If you add -a to the option -g you will see more info although few
people need to deal with (or understand) this additional info since
the default settings you see usually work fine. In normal cases the
hardware is set up the same way as "setserial" reports, but if you are
having problems there is a good chance that "setserial" has it wrong.
In fact, you can run "setserial" and assign a purely fictitious I/O
port address, any IRQ, and whatever uart type you would like to have.
Then the next time you type "setserial ..." it will display these
bogus values without complaint. They will also be officially
registered with the kernel as seen by the "scanport" command. Of
course the serial port driver will not work correctly (if at all) if
you attempt to use such a port. Thus when giving parameters to
"setserial" anything goes. Well almost. If you assign one port a
base address that is already assigned (such as 3e8) it will not accept
it. But if you use 3e9 it will accept it. Unfortunately 3e9 is
already assigned since it is within the range starting at base address
3e8. Thus the moral of the story is to make sure of your data before
assigning resources with setserial.
While assignments made by setserial are lost when the PC is powered
off, a configuration file may restore them (or a previous
configuration) when the PC is started up again. In newer versions,
what you change by setserial gets automatically saved to a
configuration file. In older versions, the configuration file only
changes if you edit it manually so the configuration remains the same
from boot to boot. See ``Configuration Scripts/Files''
9.1.2. Probing
With appropriate options, setserial can probe (at a given I/O address)
for a serial port but you must guess the I/O address. If you ask it
to probe for /dev/ttyS2 for example, it will only probe at the address
it thinks ttyS2 is at (2F8). If you tell setserial that ttyS2 is at a
different address, then it will probe at that address, etc. See
``Probing''
The purpose of this is to see if there is a uart there, and if so,
what its IRQ is. Use "setserial" mainly as a last resort as there are
faster ways to attempt it such as wvdialconf to detect modems, looking
at very early boot-time messages, or using pnpdump --dumpregs. To try
to detect the physical hardware use the -v (verbose) and autoconfig
command to setserial. If the resulting message shows a uart type such
as 16550A, then you're OK. If instead it shows "unknown" for the uart
type, then there is supposedly no serial port at all at that I/O
address. Some cheap serial ports don't identify themselves correctly
so if you see "unknown" you still might have a serial port there.
Besides auto-probing for a uart type, setserial can auto-probe for
IRQ's but this doesn't always work right either. In one case it first
gave the wrong irq but when the command was repeated it found the
correct irq. In versions of setserial >= 2.15, the results of your
last probe test may be saved and put into the configuration file
/etc/serial.conf which will be used next time you start Linux. At
boot-time when the serial module loads (or the like), a probe for
UARTs is made automatically and reported on the screen. But the IRQs
shown may be wrong. The second report of the same is the result of a
script which usually does no probing and thus provides no reliable
information as to how the hardware is actually set. It only shows
configuration data someone wrote into the script or data that got
saved in /etc/serial.conf.
It may be that two serial ports both have the same IO address set in
the hardware. Of course this is not permitted but it sometimes
happens anyway. Probing detects one serial port when actually there
are two. However if they have different IRQs, then the probe for IRQs
may show IRQ = 0. For me it only did this if I first used setserial
to give the IRQ a ficticious value.
9.1.3. Boot-time Configuration
When the kernel loads the serial module (or if the "module equivalent"
is built into the kernel) then only ttyS{0-3} are auto-detected and
the driver is set to use only IRQs 4 and 3 (regardless of what IRQs
are actually set in the hardware). You see this as a boot-time
message just like as if setserial had been run.
To correct possible errors in IRQs (or for other reasons) there may be
a file somewhere that runs setserial again. Unfortunately, if this
file has some IRQs wrong, the kernel will still have incorrect info
about the IRQs. This file should run early at boot-time before any
process uses the serial port. In fact, your distribution may have set
things up so that the setserial program runs automatically from a
start-up script at boot-time. More info about how to handle this
situation for your particular distribution might be found in file
named "setserial..." or the like located in directory /usr/doc/ or
/usr/share/doc/.
Before modifying a configuration file, you can test out a "proposed"
setserial command by just typing it on the command line. In some
cases the results of this use of setserial will automatically get
saved in /etc/serial.conf when you shutdown. So if it worked OK (and
solved your problem) then there's no need to modify any configuration
file. See ``New configuration method using /etc/serial.conf''.
9.1.4. Configuration Scripts/Files
Your objective is to modify (or create) a script file in the /etc tree
that runs setserial at boot-time. Most distributions provide such a
file (but it may not initially reside in the /etc tree). In addition,
setserial 2.15 and higher often have an /etc/serial.conf file that is
used by the above script so that you don't need to directly edit the
script that runs setserial. In addition just using setserial on the
command line (2.15+) may ultimately alter this configuration file.
So prior to version 2.15 all you do is edit a script. After 2.15 you
may need to either do one of three things: 1. edit a script. 2. edit
/etc/serial.conf or 3. run "setserial" on the command line which may
result in /etc/serial.conf automatically being edited. Which one of
these you need to do depends on both your particular distribution, and
how you have set it up.
9.1.5. Edit a script (required prior to version 2.15)
Prior to setserial 2.15 (1999) there was no /etc/serial.conf file to
configure setserial. Thus you need to find the file that runs
"setserial" at boot time and edit it. If it doesn't exist, you need
to create one (or place the commands in a file that runs early at
boot-time). If such a file is currently being used it's likely
somewhere in the /etc directory-tree. But Redhat <6.0 has supplied it
in /usr/doc/setserial/ but you need to move it to the /etc tree before
using it. You might use "locate" to try to find such a file. For
example, you could type: locate "*serial*".
The script /etc/rc.d/rc.serial was commonly used in the past. The
Debian distribution used /etc/rc.boot/0setserial. Another file once
used was /etc/rc.d/rc.local but it's not a good idea to use this since
it may not be run early enough. It's been reported that other
processes may try to open the serial port before rc.local runs
resulting in serial communication failure. Today it's most likely in
/etc/init.d/ but it isn't normally intended to be edited.
If such a file is supplied, it should contain a number of commented-
out examples. By uncommenting some of these and/or modifying them,
you should be able to set things up correctly. Make sure that you are
using a valid path for setserial, and a valid device name. You could
do a test by executing this file manually (just type its name as the
super-user) to see if it works right. Testing like this is a lot
faster than doing repeated reboots to get it right.
For versions >= 2.15 (provided your distribution implemented the
change, Redhat didn't) it may be more tricky to do since the file that
runs setserial on startup, /etc/init.d/setserial or the like was not
intended to be edited by the user. See ``New configuration method
using /etc/serial.conf''.
If you want setserial to automatically determine the uart and the IRQ
for ttyS3 you would add something like:
/sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig
Do this for every serial port you want to auto configure. Be sure to
give a device name that really does exist on your machine. In some
cases this will not work right due to the hardware. If you know what
the uart and irq actually are, you may want to assign them explicitly
with "setserial". For example:
/sbin/setserial /dev/ttyS3 irq 5 uart 16550A skip_test
9.1.6. New configuration method using /etc/serial.conf
Prior to setserial version 2.15, the way to configure setserial was to
manually edit the shell-script that ran setserial at boot-time. See
``Edit a script (after version 2.15: perhaps not)''. Starting with
version 2.15 (1999) of setserial this shell-script is not edited but
instead gets its data from a configuration file: /etc/serial.conf.
Furthermore you may not even need to edit serial.conf because using
the "setserial" command on the command line may automatically cause
serial.conf to be edited appropriately.
This was intended to make it so that you don't need to edit any file
in order to set up (or change) setserial so it will do the right thing
each time that Linux is booted. But there are serious pitfalls
because it's not really "setserial" that edits serial.conf. Confusion
is compounded because different distributions handle this differently.
In addition, you may modify it so it works differently.
What often happens is this: When you shut down your PC the script
that runs "setserial" at boot-time is run again, but this time it only
does what the part for the "stop" case says to do: It uses
"setserial" to find out what the current state of "setserial" is and
puts that info into the serial.conf file. Thus when you run
"setserial" to change the serial.conf file, it doesn't get changed
immediately but only when and if you shut down normally.
Now you can perhaps guess what problems might occur. Suppose you
don't shut down normally (someone turns the power off, etc.) and the
changes don't get saved. Suppose you experiment with "setserial" and
forget to run it a final time to restore the original state (or make a
mistake in restoring the original state). Then your "experimental"
settings are saved.
If you manually edit serial.conf, then your editing is destroyed when
you shut down because it gets changed back to the state of setserial
at shutdown. There is a way to disable the changing of serial.conf at
shutdown and that is to remove "###AUTOSAVE###" or the like from first
line of serial.conf. In at least one distribution, the removal of
"###AUTOSAVE###" from the first line is automatically done after the
first time you shutdown just after installation. The serial.conf file
will hopefully contain some comments to help you out.
The file most commonly used to run setserial at boot-time (in
conformance with the configuration file) is now /etc/init.d/setserial
(Debian) or /etc/init.d/serial (Redhat), or etc., but it should not
normally be edited. For 2.15 Redhat 6.0 just had a file
/usr/doc/setserial-2.15/rc.serial which you have to move to
/etc/init.d/ if you want setserial to run at boot-time.
To disable a port, use setserial to set it to "uart none". The format
of /etc/serial.conf appears to be just like that of the parameters
placed after "setserial" on the command line with one line for each
port. If you don't use autosave, you may edit /etc/serial.conf
manually.
BUG: As of July 1999 there is a bug/problem since with ###AUTOSAVE###
only the setserial parameters displayed by "setserial -Gg /dev/ttyS*"
get saved but the other parameters don't get saved. Use the -a flag
to "setserial" to see all parameters. This will only affect a small
minority of users since the defaults for the parameters not saved are
usually OK for most situations. It's been reported as a bug and may
be fixed by now.
In order to force the current settings set by setserial to be saved to
the configuration file (serial.conf) without shutting down, do what
normally happens when you shutdown: Run the shell-script
/etc/init.d/{set}serial stop. The "stop" command will save the
current configuration but the serial ports still keep working OK.
In some cases you may wind up with both the old and new configuration
methods installed but hopefully only one of them runs at boot-time.
Debian labeled obsolete files with "...pre-2.15".
9.1.7. IRQs
By default, both ttyS0 and ttyS2 will share IRQ 4, while ttyS1 and
ttyS3 share IRQ 3. But actually sharing serial interrupts (using them
in running programs) is not permitted unless you: 1. have kernel 2.2
or better, and 2. you've complied in support for this, and 3. your
serial hardware supports it. See
``Interrupt sharing and Kernels 2.2+'' If you only have two serial
ports, ttyS0 and ttyS1, you're still OK since IRQ sharing conflicts
don't exist for non-existent devices.
If you add an internal modem and retain ttyS0 and ttyS1, then you
should attempt to find an unused IRQ and set it both on your serial
port (or modem card) and then use setserial to assign it to your
device driver. If IRQ 5 is not being used for a sound card, this may
be one you can use for a modem. To set the IRQ in hardware you may
need to use isapnp, a PnP BIOS, or patch Linux to make it PnP. To
help you determine which spare IRQ's you might have, type "man
setserial" and search for say: "IRQ 11".
9.2. What is isapnp ?
isapnp is a program to configure Plug-and-Play (PnP) devices on the
ISA bus including internal modems. It comes in a package called
"isapnptools" and includes another program, "pnpdump" which finds all
your ISA PnP devices and shows you options for configuring them in a
format which may be added to the PnP configuration file:
/etc/isapnp.conf. It may also be used with the --dumpregs option to
show the current IO address and IRQ of the modem's serial port. The
isapnp command may be put into a startup file so that it runs each
time you start the computer and thus will configure ISA PnP devices.
It is able to do this even if your BIOS doesn't support PnP. See
Plug-and-Play-HOWTO.
9.3. What is wvdialconf ?
wvdialconf will try to find which serial port (ttyS?) has a modem on
it. It also creates a configuration program for the wvdial program.
wvdial is used for simplified dialing out using the PPP protocol to an
ISP. But you don't need to install PPP in order to use wvdialconf.
It will only find modems which are not in use. It will also
automatically devise a "suitable" init strings but sometimes gets it
wrong. Since this command has no options, it's simple to use but you
must give it the name of a file to put the init string (and other
data) into. For example type: wvdialconf my_config_file_name.
9.4. What is stty ?
stty is like setserial but it sets the baud rate and other parameters
of a serial port. Typing "stty -a < /dev/ttyS2" should show you how
ttyS2 is configured. Most of the settings are for things that you
never need to use with modems (such as some used only for old
terminals of the 1970s). Your communication package should
automatically set up all the setting correctly for modems. But stty
is sometimes useful for trouble-shooting.
Two items set by stty are: 1. Hardware flow control by "crtscts" and
2. Ignore the DCD signal from the modem: "clocal". If the modem is
not sending a DCD signal and clocal is disabled (stty shows -clocal)
then a program may not be able to open the serial port. If the port
can't open, the program may just hang, waiting (often in vain) for a
DCD signal from the modem.
Minicom sets clocal automatically when it starts up so there is no
problem. But version 6.0.192 of Kermit hung when I set -clocal and
tried to "set line ..." If -clocal is set and there is no DCD signal
then even the "stty" command will hang and there is seemingly no way
to set clocal (except by running minicom). But minicom will restore
-clocal when it exits. One way to get out of this is to use minicom
to send the "AT&C" to the modem (to get the DCD signal) and then exit
minicom with no reset so that the DCD signal remains on. Then you may
use stty again.
10. Trying Out Your Modem (Dialing Out)
10.1. Are You Ready to Dial Out ?
Once you've plugged in your modem and know which serial port it's on
you're ready to try using it. Before you try to get the Internet on
it or have people call in to you, you could first try something
simpler like dialing out to some number to see if your modem is
working OK. Find a phone number that is connected to a modem. If you
don't know what number to call, ask at computer stores for such phone
numbers of bulletin boards, etc. or see if a local library has a phone
number for their on-line catalog.
Then make sure you are ready to phone. Do you know what serial port
(such as ttyS2) your modem is on? You should have found this out when
you io-irq configured your serial ports. Have you decided what speed
you are going to use for this port? See ``Speed Table'' for a quick
selection or ``What Speed Should I Use'' for more details. If you
have no idea what speed to set, just set it a few times faster than
the advertised speed of your modem. Also remember that if you see a
menu where an option is "hardware flow control" and/or "RTS/CTS" or
the like, select it. Is a live telephone cable plugged in to your
modem? You may want to connect the cable to a real telephone to make
sure that it can produce a dial tone.
Now you need to select a communication (dialing) program to use to
dial out. Dialing programs include: minicom, seyon (X-windows), and
kermit. See section ``Communications Programs'' about some
communications programs. Two examples are presented next: ``Dialing
Out with Minicom'' and ``Dialing Out with Kermit''
10.2. Dialing Out with Minicom
Minicom comes with most Linux distributions. To configure it you
should be the root user. Type "minicom -s" to configure. This will
take you directly to the configuration (set-up) menus. Alternatively
you could just run "minicom" and then type ^A to see the bottom status
line. This shows to type ^A Z for help (you've already typed the ^A
so just type z). From the help menu go to the Configuration menu.
Most of the options don't need to be set for just simply dialing out.
To configure you have to supply a few basic items: the name of the
serial port your modem is on such as /dev/ttyS2 and the speed such as
115200. These are set at the serial port menu. Go to it and set
them. Also (if possible) set hardware flow control (RTS/CTS). Then
save them. When typing in the speed, you should also see something
like "8N1" which you should leave alone. It means: 8-bit bytes, No
parity, 1 stop-bit appended to each byte. If you can't find the speed
you want, a lower speed will always work for a test. Exit (hit
return) when done and save the configuration as default (dfl) using
the menu. You may want to exit minicom and start it again so it can
now find the serial port and initialize the modem, or you could go to
help and tell minicom to initialize the modem.
Now you are ready to dial. But first at the main screen you get after
you first type "minicom" make sure there's a modem there by typing AT
and then hit the <enter> key. It should display OK. If it doesn't
something is wrong and there is no point of trying to dial.
If you got the "OK" go back to help and select the dialing directory.
You may edit it and type in a phone number, etc. into the directory
and then select "dial" to dial it. Alternatively, you may just dial
manually (by selecting "manual" and then type the number at the
keyboard). If it doesn't work, carefully note any error messages and
try to figure out what went wrong.
10.3. Dialing Out with Kermit
You can find the latest version of kermit at
http://www.columbia.edu/kermit/. For example, say your modem was on
ttyS3, and its speed was 115200 bps. You would do the following:
linux# kermit
C-Kermit 6.0.192, 6 Sep 96, for Linux
Copyright (C) 1985, 1996,
Trustees of Columbia University in the City of New York.
Default file-transfer mode is BINARY
Type ? or HELP for help.
C-Kermit>set line /dev/ttyS3
C-Kermit>set carrier-watch off
C-Kermit>set speed 115200
/dev/ttyS3, 115200 bps
C-Kermit>c
Connecting to /dev/ttyS3, speed 115200.
The escape character is Ctrl-\ (ASCII 28, FS)
Type the escape character followed by C to get back,
or followed by ? to see other options.
ATE1Q0V1 ; you type this and then the Enter key
OK ; modem should respond with this
If your modem responds to AT commands, you can assume your modem is
working correctly on the Linux side. Now try calling another modem by
typing:
ATDT7654321
where 7654321 is a phone number. Use ATDP instead of ATDT if you have
a pulse line. If the call goes through, your modem is working.
To get back to the kermit prompt, hold down the Ctrl key, press the
backslash key, then let go of the Ctrl key, then press the C key:
Ctrl-\-C
(Back at linux)
C-Kermit>quit
linux#
This was just a test using the primitive "by-hand" dialing method.
The normal method is to let kermit do the dialing for you with its
built-in modem database and automatic dialing features, for example
using a US Robotics (USR) modem:
linux# kermit
C-Kermit 6.0.192, 6 Sep 1997, for Linux
Copyright (C) 1985, 1996,
Trustees of Columbia University in the City of New York.
Default file-transfer mode is BINARY
Type ? or HELP for help
C-Kermit>set modem type usr ; Select modem type
C-Kermit>set line /dev/ttyS3 ; Select communication device
C-Kermit>set speed 115200 ; Set the dialing speed
C-Kermit>dial 7654321 ; Dial
Number: 7654321
Device=/dev/ttyS3, modem=usr, speed=115200
Call completed.<BEEP>
Connecting to /dev/ttyS3, speed 115200
The escape character is Ctrl-\ (ASCII 28, FS).
Type the escape character followed by C to get back,
or followed by ? to see other options.
Welcome to ...
login:
11. Dial-In
11.1. Overview
Dial-in is where you set up your PC so that others may dial in to your
phone number and use your PC. The "point of view" is your PC. When
you dial out from your PC you are also dialing in to another computer
(but not dialing in to your own computer)
Dial-in works like this. Someone with a modem dials your telephone
number. Your modem answers the call and connects. Once the caller is
connected, your PC (via the getty program) starts the login process
for the caller. The original method was to send a login prompt to the
caller's screen (manual login). But a more modern method (if you use
mgetty) is to start PPP (pppd) and let PPP automatically login the
caller (no need to manually type in a name or password). See the PPP-
HOWTO (new revision expected soon) and docs for mgetty for more
details.
After the caller has logged in, the caller uses your PC. Using your
PC may mean that the caller has a shell account and can use your PC
just as if they logged in at the console (or from a text-terminal).
It could also mean that they get connected to the Internet thru your
PC (via PPP). The program that you use at your PC to handle dialin is
called getty or mgetty. See ``About mgetty''.
If you expect that people will be able to dial-in to you at 56k, it
can't be done unless:
1. You have a digital connection to the telephone company such as a
trunkside-T1 or ISDN line
2. You use special digital modems (see ``Digital Modems'')
3. You have a "... concentrator", or the like to interface your
digital-modems to the digital lines of the telephone company.
A "... concentrator" may be called a "modem concentrator" or a
"remote access concentrator" or it could be included in a "remote
access server" which includes the digital modems, etc. This type
of setup is used by ISPs (Internet Service Providers).
11.2. Getty
getty is the program you run for dialin. You don't need it for
dialout. In addition to presenting a login prompt, it also answers
the telephone. Originally getty was used for logging in to a computer
from a dumb terminal. A major use of it today is for logging in to a
Linux console. There are a few different getty programs with slightly
different names. Only certain ones work with modems for dialin. The
getty program is usually started at boot-time. It must be called from
the /etc/inittab file. You may find an example in this file of a call
to getty which you will likely need to edit a bit. If you use a
different getty program than the one shown in such an example, then
you will need to edit it quite a bit since the options will have a
different format.
There are four different getty programs to choose from that may be
used with modems for dialin: mgetty, uugetty, getty_em, and agetty. A
brief overview is given in the following subsections. agetty is the
simplest (and weakest) of the four and some consider it mainly for use
with directly connected text-terminals. mgetty has support for fax and
voice mail but Uugetty doesn't. mgetty allegedly lacks a few of the
features of uugetty. getty_em is a simplified version of uugetty.
Thus mgetty is likely your best choice unless you are already familiar
with uugetty (or find it difficult to get mgetty). The syntax for
these getty programs differs, so be sure to check that you are using
the correct syntax in /etc/inittab for whichever getty you use.
In order to see what documentation exists about the various gettys on
your computer, use the "locate" command. Type: locate "*getty*"
(including the quotes may help). Note that many distributions just
call the program getty even though it may actually be agetty, uugetty,
etc. But if you read the man page (type: man getty), it might
disclose which getty it is. This should be the getty program with
path /sbin/getty.
11.2.1. About mgetty
mgetty was written as a replacement for uugetty which was in existence
long before mgetty. Both are for use with modems. Although mgetty
may be also used for directly connected terminals the documentation
for this is hard to pinpoint and mgetty will not (as of mid 1999)
support software flow control (used on many terminals) without
recompiling. This defect is listed as a bug. In addition to allowing
dialup logins, mgetty also provides FAX support and auto PPP
detection. There is a supplemental program called vgetty which
handles voicemail for some modems. mgetty documentation is good
(except for voice mail), and does not need supplementing. Please
refer to it for installation instructions. You can find the latest
information on mgetty at
http://www.leo.org/~doering/mgetty/ and
<
http://alpha.greenie.net/mgetty>
11.2.2. About uugetty
getty_ps contains two programs: getty is used for console and
terminal devices, and uugetty for modems. Greg Hankins SUAL --(former
author of Serial-HOWTO) used uugetty so his writings about it are
included here. See ``Uugetty''. The other gettys are well covered by
the documentation that comes with them.
11.2.3. About getty_em
This is a simplified version of ``uugetty''. It was written by Vern
Hoxie after he became fully confused with complex support files needed
for getty_ps and uugetty.
It is part of the collection of serial port utilities and information
by Vern Hoxie available via ftp from <scicom.alphacdc.com/pub/linux>.
The name of the collection is ``serial_suite.tgz''. When logging into
``scicom'' as "anonymous", you must use your full e-mail address as
the password. For example:
[email protected]
11.2.4. About agetty and mingetty
agetty is a simple, completely functional implementation of getty
which is best suited for virtual consoles or terminals rather than
modems. But it works fine with modems under favorable 48% Waiting for
a call). agetty in the Debian distribution is just named getty.
mingetty is a small getty that will work only for consoles (monitors)
so you can't use it with modems for dialin.
11.3. What Happens when Someone Dials In ?
The caller runs some sort of communication program that dials your
telephone number and your telephone rings. There are two different
ways that your PC can answer the phone. One way is for the modem to
automatically answer the call. The other way is for getty to sense
the ringing and send a command to the modem to answer the call. Once
the call is answered, your modem sends tones to the other modem (and
conversely). The two modems negotiate how they will communicate and
when this is done your modem sends a "CONNECTed" message (or the like)
to getty. When getty gets this message, it sends a login prompt out
the serial port. Sometimes getty just calls on a program named login
to handle the logging in. getty usually starts running at boot-time
but it must wait until a connection is made before sending out a
"login" prompt.
Now for more details on the two methods of answering the call. By
setting the S0 register of the modem to 3, the modem will
automatically answer on the 3rd ring. If it's set to 0 then the modem
will only answer the call if getty sends it an "A" (= Answer) command
while the phone is ringing. Actually an "ATA" is sent since all modem
commands are prefixed by "AT". You might think it best to utilize the
ability of the modem to automatically answer the call, but it's
actually better if getty answers it. If the modem doesn't
automatically answer, it's called manual answer (even though getty
automatically handles it).
For the "manual" answer case, getty opens the port at boot-time and
listens. When the phone rings, a "RING" message is sent to the
listening getty. Then if getty wants to answer this ring, it sends
the modem an "ATA" command. The modem then makes a connection and
sends a "CONNECT ..." message to getty which then sends a login prompt
to the caller.
The automatic answer case uses the CD (Carrier Detect) wire from the
modem to the serial port to detect when a connection is made. It
works like this. At boot-time getty tries to open the serial port but
the attempt fails since there is normally no CD signal from the modem.
Then the getty program waits at the open statement in the program
until a CD signal appears. When a CD signal arrives (perhaps hours
later) then the port is opened and getty sends the login prompt.
While getty is waiting (sleeping) at the open statement, other
processes can run since Linux is a multiprocessing operating system.
What actually wakes getty up is an interrupt which is issued when the
CD line from the modem changes state to on.
You may wonder how getty is able to open the serial port in the
manual-answer case since there is no CD signal. Well, there's a way
to write a program to force the port to open even if there is no CD
signal present.
11.4. Why Manual Answer is Best
The difference between the two ways of answering will show itself when
the computer happens to be down but the modem is still working. For
the manual case, the "RING" message is sent to getty but since the
computer is down, getty isn't there and the phone never gets answered.
There are no telephone charges when there is no answer. For the
automatic answer case, the phone is answered but no login message is
ever sent since the computer is down. The phone bill runs up as the
waiting continues. If the phone call is toll-free, it doesn't make
much difference, although it may be frustrating waiting for a login
prompt that never arrives. mgetty uses manual answer. Uugetty can do
this too using a configuration script.
11.5. Callback
Callback is where someone first dials in to your modem. Then, you get
a little info from the caller and then call it right back. Why would
you want to do this? One reason is to save on telephone bills if you
can call the caller cheaper than the caller can call you. Another is
to make sure that the caller really is who it claims to be. If a
caller calls you and claims to be calling from its usual phone number,
then one way to verify this is to actually place a new call to that
number.
There's a program for Linux called "callback" that works with mgetty.
It's at <
ftp://ftp.icce.rug.nl/pub/unix/>. Step-by-step instructions
on how someone installed it (and PPP) is at
<
http://www.stokely.com/unix.serial.port.resources/callback.html>
11.6. Voice Mail
Voice mail is like an answering machine run by a computer. To do this
you must have a modem that supports "voice" and supporting software.
Instead of storing the messages on tape, they are stored in digital
format on a disk. When a person phones you, they hear a "greeting"
message and can then leave a message for you. More advanced systems
would have caller-selectable mail boxes and caller-selectable messages
to listen to. Free software is available in Linux for simple
answering, but doesn't seem to be available yet for the more advanced
stuff.
I know of two different voicemail packages for Linux. One is a very
minimal package (see ``Voicemail Software''). The other, more
advanced, but currently poorly documented, is vgetty. It's an
optional addition to the well documented and widely distributed mgetty
program. It supports ZyXEL-like voice modem commands. In the Debian
distribution, you must get the mgetty-voice package in addition to the
mgetty package and mgetty-doc package. Obsolete documentation has
been removed from mgetty but replacement documentation is lacking
(except if you use the -h (help) option when running certain programs,
etc.). But one sees postings about using it on the mgetty newsgroup.
See ``About mgetty''. It seems that vgetty is currently not very
stable but it's successfully being used and development of it
continues. If this is the latest version of this HOWTO can someone
who is familiar with vgetty please let me know its current status.
12. Uugetty for Dial-In (from the old Serial-HOWTO)
Be aware that you could use mgetty as a (better?) alternative to
uugetty. mgetty is newer and more popular than uugetty. See ``What
is getty?'' for a brief comparison of these 2 gettys.
12.1. Installing getty_ps
Since uugetty is part of getty_ps you'll first have to install
getty_ps. If you don't have it, get the latest version from
metalab.unc.edu:/pub/Linux/system/serial. In particular, if you want
to use high speeds (57600 and 115200 bps), you must get version 2.0.7j
or later. You must also have libc 5.x or greater.
By default, getty_ps will be configured to be Linux FSSTND (File
System Standard) compliant, which means that the binaries will be in
/sbin, and the config files will be named /etc/conf.{uu}getty.ttySN.
This is not apparent from the documentation! It will also expect lock
files to go in /var/lock. Make sure you have the /var/lock directory.
If you don't want FSSTND compliance, binaries will go in /etc, config
files will go in /etc/default/{uu}getty.ttySN, and lock files will go
in /usr/spool/uucp. I recommend doing things this way if you are
using UUCP, because UUCP will have problems if you move the lock files
to where it isn't looking for them.
getty_ps can also use syslogd to log messages. See the man pages for
syslogd(1) and syslog.conf(5) for setting up syslogd, if you don't
have it running already. Messages are logged with priority LOG_AUTH,
errors use LOG_ERR, and debugging uses LOG_DEBUG. If you don't want
to use syslogd you can edit tune.h in the getty_ps source files to use
a log file for messages instead, namely /var/adm/getty.log by default.
Decide on if you want FSSTND compliance and syslog capability. You
can also choose a combination of the two. Edit the Makefile, tune.h
and config.h to reflect your decisions. Then compile and install
according to the instructions included with the package.
12.2. Setting up uugetty
With uugetty you may dial out with your modem while uugetty is
watching the port for logins. uugetty does important lock file
checking. Update /etc/gettydefs to include an entry for your modem.
For help with the meaning of the entries that you put into
/etc/gettydefs, see the "serial_suite" collected by Vern Hoxie. How
to get it is in section See``About getty_em''. When you are done
editing /etc/gettydefs, you can verify that the syntax is correct by
doing:
linux# getty -c /etc/gettydefs
12.2.1. Modern Modems
If you have a 9600 bps or faster modem with data compression, you can
lock your serial port to one speed. For example:
# 115200 fixed speed
F115200# B115200 CS8 # B115200 SANE -ISTRIP HUPCL #@S @L @B login: #F115200
If you have your modem set up to do RTS/CTS hardware flow control, you
can add CRTSCTS to the entries:
# 115200 fixed speed with hardware flow control
F115200# B115200 CS8 CRTSCTS # B115200 SANE -ISTRIP HUPCL CRTSCTS #@S @L @B login: #F115200
12.2.2. Old slow modems
If you have a slow modem (under 9600 bps) Then, instead of one line
for a single speed, your need several lines to try a number of speeds.
Note the these lines are linked to each other by the last "word" in
the line such as #38400. Blank lines are needed between each entry.
# Modem entries
115200# B115200 CS8 # B115200 SANE -ISTRIP HUPCL #@S @L @B login: #57600
57600# B57600 CS8 # B57600 SANE -ISTRIP HUPCL #@S @L @B login: #38400
38400# B38400 CS8 # B38400 SANE -ISTRIP HUPCL #@S @L @B login: #19200
19200# B19200 CS8 # B19200 SANE -ISTRIP HUPCL #@S @L @B login: #9600
9600# B9600 CS8 # B9600 SANE -ISTRIP HUPCL #@S @L @B login: #2400
2400# B2400 CS8 # B2400 SANE -ISTRIP HUPCL #@S @L @B login: #115200
12.2.3. Login Banner
If you want, you can make uugetty print interesting things in the
login banner. In Greg's examples, he has the system name, the serial
line, and the current bps rate. You can add other things:
@B The current (evaluated at the time the @B is seen) bps rate.
@D The current date, in MM/DD/YY.
@L The serial line to which uugetty is attached.
@S The system name.
@T The current time, in HH:MM:SS (24-hour).
@U The number of currently signed-on users. This is a
count of the number of entries in the /etc/utmp file
that have a non-null ut_name field.
@V The value of VERSION, as given in the defaults file.
To display a single '@' character, use either '\@' or '@@'.
12.3. Customizing uugetty
There are lots of parameters you can tweak for each port you have.
These are implemented in separate config files for each port. The
file /etc/conf.uugetty will be used by all instances of uugetty, and
/etc/conf.uugetty.ttySN will only be used by that one port. Sample
default config files can be found with the getty_ps source files,
which come with most Linux distributions. Due to space concerns, they
are not listed here. Note that if you are using older versions of
uugetty (older than 2.0.7e), or aren't using FSSTND, then the default
file will be /etc/default/uugetty.ttySN. Greg's
/etc/conf.uugetty.ttyS3 looked like this:
# sample uugetty configuration file for a Hayes compatible modem to allow
# incoming modem connections
#
# line to initialize
INITLINE=ttyS3
# timeout to disconnect if idle...
TIMEOUT=60
# modem initialization string...
# format: <expect> <send> ... (chat sequence)
INIT="" AT\r OK\r\n
WAITFOR=RING
CONNECT="" ATA\r CONNECT\s\A
# this line sets the time to delay before sending the login banner
DELAY=1
#DEBUG=010
Add the following line to your /etc/inittab, so that uugetty is run on
your serial port, substituting in the correct information for your
environment - run-levels (2345 or 345, etc.) config file location,
port, speed, and default terminal type:
S3:2345:respawn:/sbin/uugetty -d /etc/default/uugetty.ttyS3 ttyS3 F115200 vt100
Restart init:
linux# init q
For the speed parameter in your /etc/inittab, you want to use the
highest bps rate that your modem supports.
Now Linux will be watching your serial port for connections. Dial in
from another machine and login to you Linux system.
uugetty has a lot more options, see the man page for uugetty) (often
just called getty) for a full description. Among other things there
is a scheduling feature, and a ringback feature.
13. What Speed Should I Use with My Modem?
By "speed" we really mean the "data flow rate" but almost everybody
incorrectly calls it speed. For all modern modems you have no choice
of the speed that the modem uses on the telephone line since it will
automatically choose the highest possible speed that is feasible under
the circumstances. But you do have a choice as to what speed will be
used between your modem and your computer (PC-to-modem speed). This
is sometimes called "DTE speed" where "DTE" stands for Data Terminal
Equipment (Your computer is a DTE.) You need to set this speed high
enough so this part of the signal path will not be a bottleneck. The
setting for the DTE speed is the maximum speed of this link. Most of
the time it should operate at lower speeds.
For an external modem, DTE speed is the speed (in bits/sec) of the
flow over the cable between you modem and PC. For an internal modem,
it's the same idea since the modem also emulates a serial port. It
may seem ridiculous having a speed limit on communication between a
computer and a modem card that is directly connected inside the
computer to a much higher speed bus. But it's that way since the
modem card probably includes a dedicated serial port which does have
speed limits (and settable speeds).
13.1. Speed and Data Compression
What speed do you choose? If it were not for "data compression" one
might try to choose a DTE speed exactly the same as the modem speed.
Data compression takes the bytes sent to the modem from your computer
and encodes them into a fewer number of bytes. For example, if the
flow (speed) from the PC to the modem was 20,000 bytes/sec (bps) and
the compression ratio was 2 to 1, then only 10,000 bytes/sec would
flow over the telephone line. Thus for a 2:1 compression ratio you
need to set the DTE speed to double the maximum modem speed on the
phone line. If the compression ratio were 3 to 1 you need to set it 3
times faster, etc.
13.2. Where do I Set Speed ?
This DTE (PC-to-modem) speed is normally set by a menu in your
communications program or by an option given to the getty command if
someone is dialing in. You can't set the DCE modem-to-modem speed.
13.3. Can't Set a High Enough Speed
You need to find out the highest speed supported by your hardware. As
of late 1998 (and late 2000) most serial ports only supported speeds
up to 115.2k bps. Some 56k internal modems support 230.4k bps (but it
may be hard to find out which ones do). Recent Linux kernels support
high speeds (over 115.2k) but you might have some problems using it
because of one or both of the following reasons:
1. Older versions of an application program (or stty) may not accept
the high speed.
2. Setserial may show the wrong speed unless you use the baud_base
option. In any case the high speed will work OK.
13.3.1. How speed is set in hardware: the divisor and baud_base
Here's a list of commonly used divisors and their corresponding speeds
(assuming a maximum speed of 115,200): 1 (115.2k), 2 (57.6k), 3
(38.4k), 6 (19.2k), 12 (9.6k), 24 (4.8k), 48 (2.4k), 96 (1.2k), etc.
The serial driver sets the speed in the hardware by sending the
hardware only a "divisor" (a positive integer). This "divisor"
divides the maximum speed of the hardware resulting in a slower speed
(except a divisor of 1 obviously tells the hardware to run at maximum
speed).
Normally, if you specify a speed of 115.2k (in your communication
program or by stty) then the serial driver sets the port hardware to
divisor 1 which obviously sets the highest speed. If you happen to
have hardware with a maximum speed of say 230.4k, then specifying
115.2k will result in divisor 1 and will actually give you 230.4k.
This is double the speed that you set. In fact, for any speed you
set, the actual speed will be double. If you had hardware that could
run at 460.8k then the actual speed would be quadruple what you set.
13.3.2. Work-arounds for setting speed
To correct this accounting (but not always fix the problem) you may
use "setserial" to change the baud_base to the actual maximal speed of
your port such as 230.4k. Then if you set the speed (by your
application or by stty) to 230.4k, a divisor of 1 will be used and
you'll get the same speed as you set. PROBLEM: stty and many
communication programs (as of mid 1999) still have 115.2k as their
maximum speed setting and will not let you set 230.4k, etc. So in
these cases one solution is not to change anything with setserial but
mentally keep in mind that the actual speed is always double what you
set.
There's another work-around which is not much better. To use it you
set the baud_base (with setserial) to the maximal speed of your
hardware. This corrects the accounting so that if you set say 115.2k
you actually get 115.2k. Now you still have to figure out how to set
the highest speed if your communication program (or the like) will not
let you do it. Fortunately, setserial has a way to do this: use the
"spd_cust" parameter with "divisor 1". Then when you set the speed to
38400 in a communication program, the divisor will be set to 1 in the
port and it will operate at maximum speed. For example:
setserial /dev/ttyS2 spd_cust baud_base 230400 divisor 1
Don't try using "divisor" for any other purpose other than the special
use illustrated above (with spd_cust).
If there are two or more high speeds that you want to use that your
communication program can't set, then it's not quite as easy as above.
But the same principles apply. You could just keep the default
baud_base and understand that when you set a speed you are really only
setting a divisor. So your actual speed will always be your maximum
speed divided by whatever divisor is set by the serial driver. See
``How speed is set in hardware: the divisor and baud_base''
13.3.3. Crystal frequency is not baud_base
Note that the baud_base setting is usually much lower than the
frequency of the crystal oscillator in the hardware since the crystal
frequency is often divided by 16 in the hardware to get the actual top
speed. The reason the crystal frequency needs to be higher is so that
this high crystal speed can be used to take a number of samples of
each bit to determine if it's a 1 or a 0.
13.4. Speed Table
It's best to have at least a 16650 UART for a 56k modem but few modems
or serial ports provide it. Second best is a 16550 that has been
tweaked to give 230,400 bps (230.4 kbps). Most people still use a
16550 that is only 115.2 kbps but it's claimed to only slow down
thruput by a few percent (on average). This is because a typical
compression ratio is 2 to 1 and for downloading compressed files
(packages) it's 1 to 1. There's no degradation for these cases. Here
are some suggested speeds to set your serial line if your modem speed
is:
� 56k (V.90): use 115.2 kbps or 230.4 kbps (best)
� 28.8k (V.34): use 115.2 kbps
� 33.6k (V.34): use 115.2 kbps
� 14400 bps (V.32bis), with V.42bis data compression: use 57600 bps
� 9600 bps (V.32), with V.42bis data compression: use 38400 bps
� slower than a 9600 bps (V.32) modem: set your speed to the highest
speed your modem supports.
14. Communications Programs And Utilities
While PPP is used for Internet access, there are various dialer
programs that dial a phone number, possibly login, and then start PPP.
Dialer programs include wvdial, chap scripts, kppp, and gnome-ppp.
There are various dialers which can be For dialing out to public
libraries, bulletin boards, etc. minicom is the most popular followed
by Seyon (X-Windows only) and Kermit.
14.1. Minicom vs. Kermit
Minicom is only a communications program while Kermit is both a
communications program and a file transfer protocol. But one may use
the Kermit protocol from within Minicom (provided one has Kermit
installed on one's PC). Minicom is menu based while Kermit is command
line based (interactive at the special Kermit prompt). While the
Kermit program is free software, the documentation is not all free.
There is no detailed manual supplied and it is suggested that you
purchase a book as the manual. However Kermit has interactive online
help which tells all but lacks tutorial explanations for the beginner.
Commands may be put in a script file so you don't have to type them
over again each time. Kermit (as a communications program) is more
powerful than Minicom.
Although all Minicom documentation is free, it's not as extensive as
Kermit's. Since permission is required to include Kermit in a
commercial distribution, and since the documentation is not entirely
free, some distributions don't include Kermit. In my opinion it's
easier to set up Minicom and there is less to learn.
14.2. List of Communication Software
Here is a list of some communication software you can choose from, If
they didn't come with your distribution they should be available via
FTP. I would like comparative comments on the dialout programs. Are
the least popular ones obsolete?
14.2.1. Least Popular Dialout
� ecu - a communications program
� pcomm - procomm-like communications program with zmodem
� xc - xcomm communication package
14.2.2. Most Popular Dialout
� ppp dialers for getting on the internet: chat, wvdial
� minicom - telix-like communications program. Supports scripts,
zmodem, kermit
� C-Kermit <
http://www.columbia.edu/kermit/> - portable, scriptable,
serial and TCP/IP communications including file transfer,
character-set translation, and zmodem support
� seyon - X based communication program
14.2.3. Fax
By using a fax program, you may use most modems to send faxes. In
this case you dial out directly and not via ppp and an ISP. You also
pay any long-distance telephone charges. email is more efficient.
� efax a small fax program
� hylafax a large fax program based on the client-server model.
� mgetty+fax handles fax stuff and login for dial-ins
14.2.4. Voicemail Software
� mvm <
http://www-internal.alphabet.ch/~schaefer/mvm/> is a Minimal
VoiceMail for Linux
� vgetty is an extension to mgetty that handles voicemail for some
modems. It should come with recent releases of mgetty.
14.2.5. Dial-in (uses getty)
� mgetty+fax is for modems and is well documented (except for
voicemail as of early 1999). It also handles fax stuff and
provides an alternative to uugetty. It's incorporating voicemail
(using vgetty) features. See ``About mgetty''
� uugetty is also for modems. It comes as a part of the ps_getty
package. See ``About getty_ps''
14.2.6. Other
� callback is where you dial out to a remote modem and then that
modem hangs up and calls you back (to save on phone bills).
� SLiRP and term provide a PPP-like service that you can run in user
space on a remote computer with a shell account. See ``term and
SLiRP'' for more details
� ZyXEL is a control program for ZyXEL U-1496 modems. It handles
dialin, dialout, dial back security, FAXing, and voice mailbox
functions.
� SLIP and PPP software can be found at
ftp://metalab.unc.edu/pub/Linux/system/network/serial.
� Other things can be found on
ftp://metalab.unc.edu/pub/Linux/system/serial and
ftp://metalab.unc.edu/pub/Linux/apps/serialcomm or one of the many
mirrors. These are the directories where serial programs are kept.
14.3. SLiRP and term
SLiRP and term are programs which are of use if you only have a dial-
up shell account on a Unix-like machine and want to get the equivalent
of a PPP account (or the like) without being authorized to have it
(possibly because you don't want to pay extra for it, etc.). SLiRP is
more popular than term which is almost obsolete.
To use SLiRP you install it in your shell account on the remote
computer. Then you dial up the account and run SLiRP on the remote
and PPP on your local PC. You now have a PPP connection over which
you may run a web browser on your local PC such as Netscape, etc.
There may be some problems as SLiRP is not as good as a real PPP
account. Some accounts may provide SLiRP since it saves on IP
addresses (You have no IP address while using SLiRP).
term is something like SLiRP only you need to run term on both the
local and remote computer. There is no PPP on the phone line since
term uses its own protocol. To use term from your PC you need to use
a term-aware version of ftp to do ftp, etc. Thus it's easier to use
SLiRP since the ordinary version of ftp works fine with SLiRP. There
is an unmaintained Term HOWTO.
15. Troubleshooting
15.1. My Modem is Physically There but Can't be Found
The error messages could be something like "No modem detected", "Modem
not responding", or (strange) "You are already online" (from Minicom).
If you have installed an internal modem (serial port is builtin) or
are using an external one and don't know what serial port it's
connected to then the problem is to find the serial port. See ``My
Serial Port is Physically There but Can't be Found''. This section is
about finding out which serial port has the modem on it.
There's a program that looks for modems on commonly used serial ports
called "wvdialconf". Just type "wvdialconf <a-new-file-name>". It
will create the new file as a configuration file but you don't need
this file unless you are going to use "wvdial" for dialing. See
``What is wvdialconf ?'' Unfortunately, if your modem is in "online
data" mode, wvdialconf will report "No modem detected" See ``No
response to AT''
Your problem could be due to a winmodem (or the like) which usually
can't be used with Linux. See ``Software-based Modems (winmodems)''.
The "setserial program may be used to detect serial ports but will not
detect modems on them. Thus "wvdialconf" is best to try first.
Another way try to find out if there's a modem on a port is to start
"minicom" on the port (after first setting up minicom for the correct
serial port --you will need to save the setup and then exit minicom
and start it again). Then type "AT" and you should see OK (or 0 if
it's set for "digit result codes"). The results may be:
� No response. See ``No response to AT''
� It takes many seconds to get an expected truncated response
(including only the cursor moving down one line). See ``Extremely
Slow: Text appears on the screen slowly after long delays''
� Some strange characters appear but they are not in response to AT.
This likely means that your modem is still connected to something
at the other end of the phone line which is sending some cryptic
packets or the like.
15.1.1. No response to AT
The modem should send you "OK" in response to your "AT" which you type
to the modem (using minicom or the like). If you don't see "OK" (and
in most cases don't even see the "AT" you typed either) then the modem
is not responding (often because what you type doesn't even get to the
modem).
A common cause is that there is no modem on the serial port you are
typing to. For the case of an internal modem, that serial port likely
doesn't exist either. That's because the PnP modem card (which has a
built-in serial port) has either not been configured (by isapnp or the
like) or has been configured incorrectly. See ``My Serial Port is
Physically There but Can't be Found''.
If what you type is really getting thru to a modem, then the lack of
response could be due to the modem being in "online data" mode where
it can't accept any AT commands. You may have been using the modem
and then abruptly disconnected (such as killing the process with
signal 9). In that case your modem did not get reset to "command
mode" where it can interact to AT commands. Thus the message from
minicom "You are already online. Hangup first." Well, you are sort
of online but you are may not be connected to anything over the phone
line. Wvdial will report "modem not responding" for the same
situation.
To fix this as a last resort you could reboot the computer. Another
way to try to fix this is to send +++ to the modem to tell it to
escape back to "command mode" from "online data mode". On both sides
of the +++ sequence there must be about 1 second of delay (nothing
sent during "guard time"). This may not work if another process is
using the modem since the +++ sequence could wind up with other
characters inserted in between them or after the +++ (during the guard
time). Ironically, even if the modem line is idle, typing an
unexpected +++ is likely to set off an exchange of control packets
(that you never see) that will violate the required guard time so that
the +++ doesn't do what you wanted. +++ is usually in the string that
is named "hangup string" so if you command minicom (or the like) to
hangup it might work. Another way to do this is to just exit minicom
and then run minicom again.
15.2. "Modem is busy"
What this means depends on what program sent it. The modem could
actually be in use (busy). Another cause reported for the SuSE
distribution is that there may be two serial drivers present instead
of one. One driver was built into the kernel and the second was a
module.
In kppp, this message is sent when an attempt to get/set the serial
port parameters fails. These parameters are the one you see if you
give the "stty -a" command. It's similar to the "Input/output error"
one may get when trying to use "stty -F /dev/ttySx". Although the
port is already opened when you see this message, I think it's
possible to open a non-existent device (or one with the wrong IRQ or
IO address). Now getting certain port parameters (such as the speed)
means communicating with the serial port hardware. So this error
message may mean that there is no serial port there (or that the
modem's serial port has an incorrect (or no) IRQ or I0 address. It
could be a wrong ttySx number. If /dev/modem is used it should be
linked to the correct ttySx. In these cases the error message should
have said: "Modem can't be found" which really means that it's serial
port (often built into the modem) can't be found.
15.3. I can't get near 56k on my 56k modem
There must be very low noise on the line for it to work at even close
to 56k. Some phone lines are so bad that the speeds obtainable are
much slower than 56k (like 28.8k or even slower). Sometimes extension
phones connected to the same line can cause problems. To test this
you might connect your modem directly at the point where the telephone
line enters the building with the feeds for everything else on that
line disconnected (if others can tolerate such a test).
15.4. Uploading (downloading) files is broken/slow
Flow control (both at your PC and/or modem-to-modem) may not be
enabled. For the uploading case: If you have set a high DTE speed
(like 115.2k) then flow from your modem to your PC may work OK but
uploading flow in the other direction will not all get thru due to the
telephone line bottleneck. This will result in many errors and the
resending of packets. It may thus take far too long to send a file.
In some cases, files don't make it thru at all.
For the downloading case: If you're downloading long uncompressed
files or web pages (and your modem uses data compression) or if you've
set a low DTE speed, then downloading may also be broken due to no
flow control.
15.5. For Dial-in I Keep Getting "line NNN of inittab invalid"
Make sure you are using the correct syntax for your version of init.
The different init's that are out there use different syntax in the
/etc/inittab file. Make sure you are using the correct syntax for
your version of getty.
15.6. I Keep Getting: ``Id "S3" respawning too fast: disabled for 5
minutes''
Id "S3" is just an example. In this case look on the line which
starts with "S3" in /etc/inittab. This is causing the problem. Make
sure the syntax for this line is correct and that the device (ttyS3)
exists and can be found.
Make sure your modem is configured correctly. Look at registers E and
Q. This can occur when your modem is chatting with getty.
If you use uugetty, verify that your /etc/gettydefs syntax is correct
by doing the following:
linux# getty -c /etc/gettydefs
This can also happen when the uugetty initialization is failing. See
section ``uugetty Still Doesn't Work''.
15.7. My Modem is Hosed after Someone Hangs Up, or uugetty doesn't
respawn
This can happen when your modem doesn't reset when DTR is dropped.
Greg Hankins saw his RD and SD LEDs go crazy when this happened. You
need to have your modem reset. Most Hayes compatible modems do this
with &D3, but on his USR Courier, he had to set &D2 and S13=1. Check
your modem manual (if you have one).
15.8. uugetty Still Doesn't Work
There is a DEBUG option that comes with getty_ps. Edit your config
file /etc/conf.{uu}getty.ttySN and add DEBUG=NNN. Where NNN is one of
the following combination of numbers according to what you are trying
to debug:
D_OPT 001 option settings
D_DEF 002 defaults file processing
D_UTMP 004 utmp/wtmp processing
D_INIT 010 line initialization (INIT)
D_GTAB 020 gettytab file processing
D_RUN 040 other runtime diagnostics
D_RB 100 ringback debugging
D_LOCK 200 uugetty lockfile processing
D_SCH 400 schedule processing
D_ALL 777 everything
Setting DEBUG=010 is a good place to start.
If you are running syslogd, debugging info will appear in your log
files. If you aren't running syslogd info will appear in
/tmp/getty:ttySN for debugging getty and /tmp/uugetty:ttySN for
uugetty, and in /var/adm/getty.log. Look at the debugging info and
see what is going on. Most likely, you will need to tune some of the
parameters in your config file, and reconfigure your modem.
You could also try mgetty. Some people have better luck with it.
15.9. (The following subsections are in both the Serial and Modem
HOWTOs)
15.10. My Serial Port is Physically There but Can't be Found
If a physical device (such as a modem) doesn't work at all it may mean
that the device is not at the I/O address that setserial thinks it's
at. It could also mean (for a PnP card) that is doesn't yet have an
address. Thus it can't be found.
Check the BIOS menus and BIOS messages. For the PCI bus use lspci or
scanpci. If it's an ISA bus PnP serial port, try "pnpdump --dumpregs"
and/or see Plug-and-Play-HOWTO. Using "scanport" will scan all ISA
bus ports and may discover an unknown port that could be a serial port
(but it doesn't probe the port). It could hang your PC. You may try
probing with setserial. See ``Probing''. If nothing seems to get
thru the port it may be accessible but have a bad interrupt. See
``Extremely Slow: Text appears on the screen slowly after long
delays''. Use setserial -g to see what the serial driver thinks and
check for IRQ and I0 address conflicts. Even if you see no conflicts
the driver may have incorrect information (view it by "setserial" and
conflicts may still exist.
If two ports have the same IO address then probing it will erroneously
indicate only one port. Plug-and-play detection will find both ports
so this should only be a problem if at least one port is not plug-and-
play. All sorts of errors may be reported/observed for devices
illegally "sharing" a port but the fact that there are two devices on
the same a port doesn't seem to get detected (except hopefully by
you). In the above case, if the IRQs are different then probing for
IRQs with setserial might "detect" this situation by failing to detect
any IRQ. See ``Probing''.
15.11. Extremely Slow: Text appears on the screen slowly after long
delays
It's likely mis-set/conflicting interrupts. Here are some of the
symptoms which will happen the first time you try to use a modem,
terminal, or serial printer. In some cases you type something but
nothing appears on the screen until many seconds later. Only the last
character typed may show up. It may be just an invisible <return>
character so all you notice is that the cursor jumps down one line.
In other cases where a lot of data should appear on the screen, only a
batch of about 16 characters appear. Then there is a long wait of
many seconds for the next batch of characters. You might also get
"input overrun" error messages (or find them in logs).
For more details on the symptoms and why this happens see the Serial-
HOWTO section: "Interrupt Problem Details".
If it involves Plug-and-Play devices, see also Plug-and-Play-HOWTO.
As a quick check to see if it really is an interrupt problem, set the
IRQ to 0 with "setserial". This will tell the driver to use polling
instead of interrupts. If this seems to fix the "slow" problem then
you had an interrupt problem. You should still try to solve the
problem since polling uses excessive computer resources.
Checking to find the interrupt conflict may not be easy since Linux
supposedly doesn't permit any interrupt conflicts and will send you a
``/dev/ttyS?: Device or resource busy'' error message if it thinks you
are attempting to create a conflict. But a real conflict can be
created if "setserial" has told the kernel incorrect info. The kernel
has been lied to and thus doesn't think there is any conflict. Thus
using "setserial" will not reveal the conflict (nor will looking at
/proc/interrupts which bases its info on "setserial"). You still need
to know what "setserial" thinks so that you can pinpoint where it's
wrong and change it when you determine what's really set in the
hardware.
What you need to do is to check how the hardware is set by checking
jumpers or using PnP software to check how the hardware is actually
set. For PnP run either "pnpdump --dumpregs" (if ISA bus) or run
"lspci" (if PCI bus). Compare this to how Linux (e.g. "setserial")
thinks the hardware is set.
15.12. Somewhat Slow: I expected it to be a few times faster
One reason may be that whatever is on the serial port (such as a
modem, terminal, printer) doesn't work as fast as you thought it did.
A 56k Modem seldom works at 56k and the Internet often has congestion
and bottlenecks that slow things down. If the modem on the other end
does not have a digital connection to the phone line (and uses a
special "digital modem" not sold in most computer stores), then speeds
above 33.6k are not possible.
Another possible reason is that you have an obsolete serial port: UART
8250, 16450 or early 16550 (or the serial driver thinks you do). See
"What are UARTS" in the Serial-HOWTO.
Use "setserial -g /dev/ttyS*". If it shows anything less than a
16550A, this may be your problem. If you think that "setserial" has
it wrong check it out. See ``What is Setserial'' for more info. If
you really do have an obsolete serial port, lying about it to
setserial will only make things worse.
15.13. The Startup Screen Show Wrong IRQs for the Serial Ports.
Linux does not do any IRQ detection on startup. When the serial
module loads it only does serial device detection. Thus, disregard
what it says about the IRQ, because it's just assuming the standard
IRQs. This is done, because IRQ detection is unreliable, and can be
fooled. But if and when setserial runs from a start-up script, it
changes the IRQ's and displays the new (and hopefully correct) state
on on the startup screen. If the wrong IRQ is not corrected by a
later display on the screen, then you've got a problem.
So, even though I have my ttyS2 set at IRQ 5, I still see
ttyS02 at 0x03e8 (irq = 4) is a 16550A
at first when Linux boots. (Older kernels may show "ttyS02" as
"tty02" which is the same as ttyS2). You may need to use setserial to
tell Linux the IRQ you are using.
15.14. "Cannot open /dev/ttyS?: Permission denied"
Check the file permissions on this port with "ls -l /dev/ttyS?"_ If
you own the ttyS? then you need read and write permissions: crw with
the c (Character device) in col. 1. It you don't own it then it
should show rw- in cols. 8 & 9 which means that everyone has read and
write permission on it. Use "chmod" to change permissions. There are
more complicated ways to get access like belonging to a "group" that
has group permission.
15.15. "Operation not supported by device" for ttyS?
This means that an operation requested by setserial, stty, etc.
couldn't be done because the kernel doesn't support doing it.
Formerly this was often due to the "serial" module not being loaded.
But with the advent of PnP, it may likely mean that there is no modem
(or other serial device) at the address where the driver (and
setserial) thinks it is. If there is no modem there, commands (for
operations) sent to that address obviously don't get done. See ``What
is set in my serial port hardware?''
If the "serial" module wasn't loaded but "lsmod" shows you it's now
loaded it might be the case that it's loaded now but wasn't loaded
when you got the error message. In many cases the module will
automatically loaded when needed (if it can be found). To force
loading of the "serial" module it may be listed in the file:
/etc/modules.conf or /etc/modules. The actual module should reside
in: /lib/modules/.../misc/serial.o.
15.16. "Cannot create lockfile. Sorry"
When a port is "opened" by a program a lockfile is created in
/var/lock/. Wrong permissions for the lock directory will not allow a
lockfile to be created there. Use "ls -ld /var/lock" to see if the
permissions are OK: usually rwx for everyone (repeated 3 times). If
it's wrong, use "chmod" to fix it. Of course, if there is no "lock"
directory no lockfile can be created there. For more info on
lockfiles see the Serial-HOWTO subsection: "What Are Lock Files".
15.17. "Device /dev/ttyS? is locked."
This means that someone else (or some other process) is supposedly
using the serial port. There are various ways to try to find out what
process is "using" it. One way is to look at the contents of the
lockfile (/var/lock/LCK...). It should be the process id. If the
process id is say 100 type "ps 100" to find out what it is. Then if
the process is no longer needed, it may be gracefully killed by "kill
100". If it refuses to be killed use "kill -9 100" to force it to be
killed, but then the lockfile will not be removed and you'll need to
delete it manually. Of course if there is no such process as 100 then
you may just remove the lockfile but in most cases the lockfile should
have been automatically removed if it contained a stale process id
(such as 100).
15.18. "/dev/tty? Device or resource busy"
This means that the device you are trying to access (or use) is
supposedly busy (in use) or that a resource it needs (such as an IRQ)
is supposedly being used by another device (the resource is "busy").
This message is easy to understand if it only means that the device is
busy (in use). But it often means that a resource is in use. What
makes it even more confusing is that in some cases neither the device
not the resources that it needs are actually "busy".
The ``resource busy'' part often means (example for ttyS2) ``You can't
use ttyS2 since another device is using ttyS2's interrupt.'' The
potential interrupt conflict is inferred from what "setserial" thinks.
A more accurate error message would be ``Can't use ttyS2 since the
setserial data (and kernel data) indicates that another device is
using ttyS2's interrupt''. If two devices use the same IRQ and you
start up only one of the devices, everything is OK because there is no
conflict yet. But when you next try to start the second device
(without quitting the first device) you get a "... busy" error
message. This is because the kernel only keeps track of what IRQs are
actually in use and actual conflicts don't happen unless the devices
are in use (open). The situation for I/O address (such as 0x3f8)
conflict is similar.
This error is sometimes due to having two serial drivers: one a module
and the other compiled into the kernel. Both drivers try to grab the
same resources and one driver finds them "busy".
There are two possible cases when you see this message:
1. There may be a real resource conflict that is being avoided.
2. Setserial has it wrong and the only reason ttyS2 can't be used is
that setserial erroneously predicts a conflict.
What you need to do is to find the interrupt setserial thinks ttyS2 is
using. Look at /proc/tty/driver/serial (if you have it). You should
also be able to find it with the "setserial" command for ttyS2. But
due to a bug (reported by me in Nov. 2000) you get the same "... busy"
error message when you try this with "setserial".
To try to resolve this problem reboot or: exit or gracefully kill all
likely conflicting processes. If you reboot: 1. Watch the boot-time
messages for the serial ports. 2. Hope that the file that runs
"setserial" at boot-time doesn't (by itself) create the same conflict
again.
If you think you know what IRQ say ttyS2 is using then you may look at
/proc/interrupts to find what else (besides another serial port) is
currently using this IRQ. You might also want to double check that
any suspicious IRQs shown here (and by "setserial") are correct (the
same as set in the hardware). A way to test whether or not it's a
potential interrupt conflict is to set the IRQ to 0 (polling) using
"setserial". Then if the busy message goes away, it was likely a
potential interrupt conflcit. It's not a good idea to leave it
permanently set at 0 since it will make the CPU work too hard.
15.19. "Input/output error" from setserial or stty
You may have typed "ttys" instead of "ttyS". You will see this error
message if you try to use the setserial command for any device that is
not a serial port. It also may mean that the serial port is in use
(busy or opened) and thus the attempt to get/set parameters by
setserial or stty failed. It could also mean that there isn't any
serial port at the IO address that setserial thinks your port is at.
15.20. Overrun errors on serial port
This is an overrun of the hardware FIFO buffer and you can't increase
its size. See "Higher Serial Thruput" in the Serial-HOWTO.
15.21. Modem doesn't pick up incoming calls
This paragraph is for the case where a modem is used for both dial-in
and dial-out. If the modem generates a DCD (=CD) signal, some
programs (but not mgetty) will think that the modem is busy. This
will cause a problem when you are trying to dial out with a modem and
the modem's DCD or DTR are not implemented correctly. The modem
should assert DCD only when there is an actual connection (ie someone
has dialed in), not when getty is watching the port. Check to make
sure that your modem is configured to only assert DCD when there is a
connection (&C1). DTR should be on (asserted) by the communications
program whenever something is using, or watching the line, like getty,
kermit, or some other comm program.
15.22. Troubleshooting Tools
These are some of the programs you might want to use in
troubleshooting:
� "lsof /dev/ttyS*" will list serial ports which are open.
� "setserial" shows and sets the low-level hardware configuration of
a port (what the driver thinks it is). See ``What is Setserial''
� "stty" shows and sets the configuration of a port (except for that
handled by "setserial"). See the Serial-HOWTO section: "Stty".
� "modemstat" or "statserial" will show the current state of various
modem signal lines (such as DTR, CTS, etc.)
� "irqtune" will give serial port interrupts higher priority to
improve performance.
� "hdparm" for hard-disk tuning may help some more.
� "lspci" shows the actual IRQs, etc. of hardware on the PCI bus.
� "pnpdump --dumpregs" shows the actual IRQs, etc. of hardware for
PnP devices on the ISA bus.
� Some "files" in the /proc tree (such as ioports, interrupts, and
tty/driver/serial).
16. Flash Upgrades
Many modems can be upgraded by reprogramming their flash memories with
an upgrade program which you get from the Internet. By sending this
"program" from the PC via the serial port to the modem, the modem will
store this program in its non-volatile memory (it's still there when
the power is turned off). The instructions on installing it are
usually on how to do in under Windows so you'll need to figure out how
to do the equivalent under Linux (unless you want to install the
upgrade under Windows). Sending the program to the modem is often
called a download.
If the latest version of this HOWTO still contains this request (see
``New Versions of this HOWTO'') please send me your experiences with
installing such upgrades that will be helpful to others.
Here's the general idea of doing an upgrade. First, there may be a
command that you need to send your modem to tell it that what follows
is a flash ROM upgrade. In one case this was AT** You can do this by
starting a communications program (such as minicom) and type. First
type AT <enter> to see if your modem is there and answers "OK".
Next, you need to send an file (sometimes two files) directly to the
modem. Communication programs (such as minicom) often use zmodem or
kermit to send files to the modem (and beyond) but these put the file
into packets which append headers and you want the exact file sent to
the modem, not a modified one. But the kermit communications program
has a "transmit" command that will send the file directly (without
using the kermit packets) so this is one way to send a file directly.
Minicom didn't have this feature in 1998.
Another way to send the file(s) would be to escape from the
communications program to the shell (in minicom this is ^AJ) and then:
cat upgrade_file_name > /dev/ttyS2 (if your serial port is ttyS2).
Then go back to the communication program (type fg at the command line
prompt in minicom) to see what happened.
Here's an example session for a certain Rockwell modem (C-a is ^A):
- Run minicom
- Type AT** : see "Download initiated ..."
- C-a J
- cat FLASH.S37 > /dev/modem
- fg : see "Download flash code ..."
- C-a J
- cat 283P1722.S37 > /dev/modem
- fg : see "Device successfully programmed"
17. Other Sources of Information
17.1. Misc
� man pages for: agetty(8), getty(1m), gettydefs(5), init(1),
isapnp(8), login(1), mgetty(8), setserial(8)
� Your modem manual (if it exists). Some modems come without
manuals.
� Serial Suite <
ftp://scicom.alphacdc.com/pub/linux> by Vern Hoxie
is a collection of blurbs about the care and feeding of the Linux
serial port plus some simple programs.
� The Linux serial mailing list. To subscribe, send email to
[email protected], with ``subscribe linux-serial'' in the
message body. If you send ``help'' in the message body, you get a
help message. The server also serves many other Linux lists. Send
the ``lists'' command for a list of mailing lists.
17.2. Books
I've been unable to find a good up-to-date book on modems.
� The Complete Modem Reference by Gilbert Held, 1997. Contains too
much info about obsolete topics. More up-to-date info may be found
on the Internet.
� Modems For Dummies by Tina Rathbone, 1996. (Have never seen it.)
� The Modem Technical Guide by Douglas Anderson, 1996.
� Ultimate Modem Handbook by Cass R. Lewart, 1998.
17.3. HOWTOs
� Cable-Modem mini-howto
� ISDN Howto (not a LDP Howto)
<
http://www.suse.de/Support/sdb_e/isdn.html>: drivers for ISDN
"Modems". Much related info on this is in German.
� Linux-Modem-Sharing mini-howto. Computers on a network share a
single modem for dial-out (like a shared printer).
� Modems-HOWTO: In French (Not used in creating this Modem-HOWTO)
� NET-3-4-HOWTO: all about networking, including SLIP, CSLIP, and PPP
� PPP-HOWTO: help with PPP including modem set-up
� Serial-HOWTO has info on Multiport Serial Cards used for both
terminals and banks of modems. Covers the serial port in more
detail than in the HOWTO.
� Serial-Programming-HOWTO: for some aspects of serial-port
programming
� Text-Terminal-HOWTO: (including connecting up with modems)
� UUCP-HOWTO: for information on setting up UUCP
17.4. Usenet newsgroups
� comp.os.linux.answers FAQs, How-To's, READMEs, etc. about Linux.
� comp.os.linux.hardware Hardware compatibility with the Linux
operating system.
� comp.os.linux.setup Linux installation and system administration.
� comp.dcom.modems Modems for all OS's
17.5. Web Sites
� Modem List of modems which work/don't_work under Linux
<
http://www.o2.net/~gromitkc/winmodem.html>
� Linux Serial Driver home page <
http://serial.sourceforge.net/>
Includes info about support for PCI modems.
� Hayes AT modem commands Technical Reference for Hayes (tm) Modem
Users <
http://www.hayes.com/TechSupport/techref/>
� Rockwell-based modem commands
<
http://www.rss.rockwell.com/techinfo/>
� Modem FAQs:
Navas 28800 Modem FAQ
<
http://web.aimnet.com/~jnavas/modem/faq.html>
� Curt's High Speed Modem Page
<
http://www.teleport.com/~curt/modems.html>
� Much info on 56k modems 56k Modem = v.Unreliable
<
http://808hi.com/56k/>
� Links to modem manufacturers
<
http://www.56k.com/links/Modem_Manufacturers/>
� Identifying modems by FCC ID
<
http://www.sbsdirect.com/fccenter.html>
18. Appendix A: How Analog Modems Work (technical) (unfinished)
18.1. Modulation Details
18.1.1. Intro to Modulation
This part describes the modulation methods used for conventional
modems. It doesn't cover the high speed methods (modulus conversion)
sometimes used by ``56k Modems (v.90)''. But 56k modems also use the
modulation methods described here.
Modulation is the conversion of a digital signal represented by binary
binary (0 or 1) into an analog signal something like a sine wave. The
modulated signal consists pure sine wave "carrier" signal which is
modified to convey information. A pure carrier sine wave, unchanging
in frequency and voltage, provides no flow of information at all
(except that a carrier is present). To make it convey information we
modify (or modulate) this carrier. There are 3 basic types of
modulation: frequency, amplitude, and phase. They will be explained
next.
18.1.2. Frequency Modulation
The simplest modulation method is frequency modulation. Frequency is
measured in cycles per second (of a sine wave). It's the count of the
number of times the sine wave shape repeats itself in a second. This
is the same as the number of times it reaches it peak value during a
second. The word "Hertz" (abbreviated Hz) is used to mean "cycles per
second".
A simple example of frequency modulation is where one frequency means
a binary 0 and another means a 1. For example, for some obsolete 300
baud modems 1070 Hz meant a binary 0 while 1270 Hz meant a binary 1.
This was called "frequency shift keying". Instead of just two
possible frequencies, more could be used to allow more information to
be transmitted. If we had 4 different frequencies (call them A, B, C,
and D) then each frequency could stand for a pair of bits. For
example, to send 00 one would use frequency A. To send 01, use
frequency B; for 10 use C; for 11 use D. In like manner, by using 8
different frequencies we could send 3 bits with each shift in
frequency. Each time we double the number of possible frequencies we
increase the number of bits it can represent by 1.
18.1.3. Amplitude Modulation
Once one understands frequency modulation example above including the
possibilities of representing a few bits by a single shift in
frequency, it's easier to understand both amplitude modulation and
phase modulation. For amplitude modulation, one just changes the
height (voltage) of the sine wave analogous to changing the frequency
of the sine wave. For a simple case there could only be 2 allowed
amplitude levels, one representing a 0-bit and another representing a
1-bit. As explained for the case of frequency modulation, having more
possible amplitudes will result in more information being transmitted
per change in amplitude.
18.1.4. Phase Modulation
To change the phase of a sine wave at a certain instant of time, we
stop sending this old sine wave and immediately begin sending a new
sine wave of the same frequency and amplitude. If we started sending
the new sine wave at the same voltage level (and slope) as existed
when we stopped sending the old sine wave, there would be no change in
phase (and no detectable change at all). But suppose that we started
up the new sine wave at a different point on the sine wave curve.
Then there would likely be a sudden voltage jump at the point in time
where the old sine wave stopped and the new sine wave began. This is
a phase shift and it's measured in degrees (deg.) A 0 deg. (or a 360
deg.) phase shift means no change at all while a 180 deg. phase shift
just reverses the voltage (and slope) of the sine wave. Put another
way, a 180 deg. phase shift just skips over a half-period (180 deg.)
at the point of transition. Of course we could just skip over say 90
deg. or 135 deg. etc. As in the example for frequency modulation, the
more possible phase shifts, the more bits a single shift in phase can
represent.
18.1.5. Combination Modulation
Instead of just selecting either frequency, amplitude, or phase
modulation, we may chose to combine modulation methods. Suppose that
we have 256 possible frequencies and thus can send a byte (8 bits) for
each shift in frequency (since 2 to the 8 power is 256). Suppose also
that we have another 256 different amplitudes so that each shift in
amplitude represents a byte. Also suppose there are 256 possible
phase shifts. Then a certain points in time we may make a shift in
all 3 things: frequency, amplitude and phase. This would send out 3
bytes for each such transition.
No modulation method in use today actually does this. It's not
practical due to the relatively long time it would take to detect all
3 types of changes. The main problem is that frequent shifts in phase
can make it appear that a shift in frequency has happened when it
actually didn't.
To avoid this difficulty one may simultaneous change only the phase
and amplitude (with no change in frequency). This is called phase-
amplitude modulation (sometimes also called quadrature amplitude
modulation = QAM). This method is used today for the common modem
speeds of 14.4k, 28.8k, and 33.6k. The only significant case where
this modulation method is not used today is for 56k modems. But even
56k modems exclusively use QAM (phase-amplitude modulation) in the
direction from your PC out the telephone line. Sometimes even the
other direction will also fall back to QAM when line conditions are
not good enough. Thus QAM (phase-amplitude modulation) still remains
the most widely used method on ordinary telephone lines.
18.2. 56k Modems (v.90)
The "modulation" method used above 33.6k is entirely different than
the common phase-amplitude modulation. Since ordinary telephone calls
are converted to digital signals at the local offices of the telephone
company, the fastest speed that you can send digital data by an
ordinary telephone call is the same speed that the telephone company
uses over its digital portion of the phone call transmission. What is
this speed? Well, it's close to 64kbps. It would be 64k but
sometimes bits are "stolen" for signalling purposes. But if the phone
Co. knows that the link is not for voice, bits may not get stolen.
The case of 64k will be presented and then it will be explained why
the actual speed is lower (56k or less --usually significantly less).
Thus 64k is the absolute top speed possible for an ordinary telephone
call using the digital portion of the circuit that was designed to
send digital encodings of the human voice. In order to use 64k, the
modem must know exactly how the telephone company is doing its digital
encoding of the analog signals. This task is far too complicated if
both sides of a telephone call have only an analog interface to the
telephone company. But if one side has a digital interface, then it's
possible (at least in one direction). Thus if your ISP has a digital
interface to the phone company, the ISP may send out a certain digital
signal over the phone lines toward your PC. The digital signal from
the ISP gets converted to analog at the local telephone office near
your PC's location (perhaps near your home). Then it's your modem's
task to try to figure out exactly what that digital signal was. If it
could do this then transmission at 64k (the speed of the telephone
company's digital signal) is possible in this direction.
What method does the telephone company use to digitally encode analog
signals? It uses a method of sampling the amplitude of the analog
signal at a rate of 8000 samples per second. Each sample amplitude is
encoded as a 8-bit (ASCII-like) byte. (Note: 8 x 8000 = 64k) This is
called "Pulse Code Modulation" = PCM. These bytes are then sent
digitally on the telephone company's digital circuits where many calls
share a single circuit using a time-sharing scheme known as "time
division multiplexing". Then finally at a local telephone office near
your home, the digital signal is de-multiplexed resulting in the same
digital signal as was originally created by PCM. Then this signal is
converted back to analog and sent to your home. Each 8-bit byte
creates a certain amplitude of the analog signal. Your modem's task
is to determine just what that PCM 8-bit byte was based on the analog
amplitude it detects.
This is (sort of) "amplitude demodulation" but not really. It's not
amplitude demodulation because there is no carrier. Actually, it's
called "modulus conversion" which is the inverse of PCM. In order to
determine the digital codes the telephone Co. used to create the
analog signal, the modem must sample this analog signal amplitude at
exactly the same points in time the phone Co. used when it created the
analog signal. To do this a timing signal is generated from a
residual 4kHz signal on the analog phone line. The creation of
amplitudes to go out to your home/office at 8k amplitudes/sec sort of
creates a 4kHz signal. Suppose every other amplitude was of opposite
polarity. Then there would be a 4kHz sine-like wave created. Each
amplitude is in a sense a 8-bit symbol and when to sample amplitudes
is known as "symbol timing".
Now the encoding of amplitudes in PCM is not linear. At low
amplitudes an increment of 1 in the PCM byte represents a much smaller
increment (delta) in analog signal amplitude than would be the case if
the amplitude being sampled were much higher. Thus for low amplitudes
it's difficult to distinguish between adjacent byte values. To make
it easier to do this (for 56k modems) certain PCM codes representing
very low amplitudes are not used. This gives a larger delta between
possible amplitudes and makes correct detection of them by your modem
easier. Thus half the amplitude levels are not used by v.90. This is
tantamount to each symbol (allowed amplitude level) representing 7
bits instead of 8. This is where 56k comes from: 7 bits/symbol x 8k
symbols/sec = 56k bps. Of course each symbol is actually generated by
8-bits but only 128 bytes of the possible 256 bytes are actually used.
There is a code table mapping these 128 8-bit bytes to 128 7-bit
bytes.
But it's a little more complicated that this. If the line conditions
are not nearly perfect, then even fewer possible levels (symbols) are
used resulting in speeds under 56k. Also due to US government rules
prohibiting high power levels on phone lines, certain high amplitudes
levels can't be used resulting in only about 53.3k at best for "56k"
modems.
Note that the digital part of the telephone network is bi-directional.
Two such circuits are used for a phone call, one in each direction.
The 56k signal is only used in one of these directions: from your ISP
to your PC. The other direction, from your home/office to the ISP,
uses the conventional phase-amplitude modulation scheme with a maximum
of 36.6kbps (and not 53.3kbps). Yet due to sophisticated cancellation
methods it's able to send simultaneously in both directions over the
analog portion of the telephone line as explained in the next
subsection.
18.3. Full Duplex on One Circuit
Modern modems are able to both send and receive signals
simultaneously. One could call this "bidirectional" or "full duplex".
This was once done by using one frequency for sending and another for
receiving. Today, the same frequency is used for both sending and
receiving. How this works is not easy to comprehend.
Most of the telephone system "main lines" are digital with two
channels in use when you make a telephone call. What you say goes
over one digital channel and what the other person says goes over the
other (reverse) digital channel. Unfortunately, the part of the
telephone system which goes to homes (and many offices) is not digital
but only a single analog channel. If both modems were directly
connected to the digital part of the phone system then bidirectional
communication (sending and receiving at the same time) would be no
problem because two channels would be available.
But the end portions of the signal path go over just one circuit. How
can there be two-way communication on it simultaneously? It works
something like this. Suppose your modem is receiving a signal from
the other modem and is not transmitting. Then there's no problem.
But if it were to start transmitting (with the other received signal
still flowing into the modem) it would drown out the received signal.
If the transmitted signal was a "solid" voltage wave applied to the
end of the line then there is no way any received signal could be
present at that point.
But the transmitter has "internal impedance" and the transmitted
signal applied to the end of the line is not solid (or strong enough)
to completely eliminate the received signal coming from the other end.
Thus while the voltage at the end of the line is mostly the stronger
transmitted signal a small part of it is the desired received signal.
All that is needed is to filter out this stronger transmitted signal
and then what remains will be the signal from the other end which we
want. To do this, one only needs to get the pure transmitted signal
directly from the transmitter (before it's applied to the line)
amplify it a determined amount, and then subtract it from the total
signal present at the end of the line. Doing this in the receiver
circuits leaves a signal which mostly came from the other end of the
line.
18.4. Echo Cancellation
A signal traveling down a line in one direction may encounter changes
in the line that will cause part of the signal to echo back in the
opposite direction. Since the same circuit is used for bi-directional
flow of data such echos will result in garbled reception. One way to
ameliorate this problem is to send training signals once in a while to
determine the echo characteristic of the line. This will enable one
to predict the echos that will be generated by any given signal. Then
this prediction method is used to predict what echos the transmitted
signal will cause. Then this predicted echo signal is subtracted
from the received signal. This cancels out the echoes.
19. Appendix B: Digital Modem Signal Processing (not done)
20. Appendix C: "baud" vs. "bps"
20.1. A simple example
``baud'' and ``bps'' are perhaps one of the most misused terms in the
computing and telecommunications field. Many people use these terms
interchangeably, when in fact they are not! bps is simply the number
of bits transmitted per second. The baud rate is a measure of how
many times per second a signal changes (or could change). For a
typical serial port a 1-bit is -12 volts and a 0-bit is +12 v (volts).
If the bps is 38,400 a sequence of 010101... would also be 38,400 baud
since the voltage shifts back and forth from positive to negative to
positive, etc. and there are 38,400 shifts per second. For another
sequence say 111000111... there will be fewer shifts of voltage since
for three 1's in sequence the voltage just stays at -12 volts yet we
say that its still 38,400 baud since there is a possibility that the
number of changes per second will be that high.
Looked at another way, put an imaginary tic mark separating each bit
(even though the voltage may not change). 38,400 baud then means
38,400 tic marks per second. The tic marks at at the instants of
permitted change and are actually marked by a synchronized clock
signal generated in the hardware but not sent over the external cable.
Suppose that a "change" may have more than the two possible outcomes
of the previous example (of +- 12 v). Suppose it has 4 possible
outcomes, each represented by a unique voltage level. Each level may
represent a pair of bits (such as 01). For example, -12v could be 00,
-6v 01, +6v 10 and +12v 11. Here the bit rate is double the baud
rate. For example, 3000 changes per second will generate 2 bits for
each change resulting in 6000 bits per second (bps). In other words
3000 baud results in 6000 bps.
20.2. Real examples
The above example is overly simple. Real examples are more
complicated but based on the same idea. This explains how a modem
running at 2400 baud, can send 14400 bps (or higher). The modem
achieves a bps rate greater than baud rate by encoding many bits in
each signal change (or transition). Thus, when 2 or more bits are
encoded per baud, the bps rate exceeds the baud rate. If your modem-
to-modem connection is at 14400 bps, it's going to be sending 6 bits
per signal transition (or symbol) at 2400 baud. A speed of 28800 bps
is obtained by 3200 baud at 9 bits/baud. When people misuse the word
baud, they may mean the modem speed (such as 33.6k).
Common modem bps rates were formerly 50, 75, 110, 300, 1200, 2400,
9600. These were also the bps rates over the serial_port-to-modem
cables. Today the bps modem-to-modem (maximum) rates are 14.4k,
28.8k, 33.6k, and 56k, but the common rates over the serialPort-to-
modem cables are not the same but are: 19.2k, 38.4k, 57.6k, 115.2k,
230.4k. The high speed of 230.4k is (as of late 2000) unfortunately
not provided by most new (and old) hardware. Using modems with
V.42bis compression (max 4:1 compression), rates up to 115.2k bps are
possible for 33.6k modems. 203.2k (4 x 53.3k) is possible for 56k
modems.
Except for 56k modems, most modems run at 2400, 3000, or 3200 baud.
Even the 56k modems use these bauds for transmission and sometimes
fall back to them for reception. Because of the bandwidth limitations
on voice-grade phone lines, baud rates greater than 2400 are harder to
achieve, and only work under conditions of good phone line quality.
How did this confusion between bps and baud start? Well, back when
antique low speed modems were high speed modems, the bps rate actually
did equal the baud rate. One bit would be encoded per phase change.
People would use bps and baud interchangeably, because they were the
same number. For example, a 300 bps modem also had a baud rate of
300. This all changed when faster modems came around, and the bit rate
exceeded the baud rate. ``baud'' is named after Emile Baudot, the
inventor of the asynchronous telegraph printer. One way this problem
gets resolved is to use the term "symbol rate" instead of "baud" and
thus avoid using the term "baud". However when talking about the
"speeds" between the modem and the serial port (DTE speed) baud and
the symbol rate are the same. And even "speed" is a misnomer since we
really mean flow rate.
21. Appendix D: Terminal Server Connection
This section was adapted from Text-Terminal-HOWTO.
A terminal server is something like an intelligent switch that can
connect many modems (or terminals) to one or more computers. It's not
a mechanical switch so it may change the speeds and protocols of the
streams of data that go thru it. A number of companies make terminal
servers: Xyplex, Cisco, 3Com, Computone, Livingston, etc. There are
many different types and capabilities. Another HOWTO is needed to
compare and describe them (including the possibility of creating your
own terminal server with a Linux PC). Most are used for modem
connections rather than directly connected terminals.
One use for them is to connect many modems (or terminals) to a high
speed network which connects to host computers. Of course the
terminal server must have the computing power and software to run
network protocols so it is in some ways like a computer. The terminal
server may interact with the user and ask what computer to connect to,
etc. or it may connect without asking. One may sometimes send jobs to
a printer thru a terminal server.
A PC today has enough computing power to act like a terminal server
except that each serial port should have its own hardware interrupt.
PC's only have a few spare interrupts for this purpose and since they
are hard-wired you can't create more by software. A solution is to
use an advanced multiport serial card which has its own system of
interrupts (or on lower cost models, shares one of the PC's interrupts
between a number of ports). See Serial-HOWTO for more info. If such
a PC runs Linux with getty running on many serial ports it might be
thought of as a terminal server. It is in effect a terminal server if
it's linked to other PC's over a network and if its job is mainly to
pass thru data and handle the serial port interrupts every 14 (or so)
bytes. Software called "radius" is sometimes used.
Today real terminal servers serve more than just terminals. They also
serve PC's which emulate terminals, and are sometimes connected to a
bank of modems connected to phone lines. Some even include built-in
modems. If a terminal (or PC emulating one) is connected directly to
a modem, the modem at the other end of the line could be connected to
a terminal server. In some cases the terminal server by default
expects the callers to use PPP packets, something that real text
terminals don't generate.
22. Appendix E: Other Types of Modems
This HOWTO currently only deals with the common type of modem used to
connect PC's to ordinary analog telephone lines. There are various
other types of modems, including devices called modems that are not
really modems.
22.1. Digital-to-Digital "Modems"
The standard definition of a modem is sometimes broadened to include
"digital" modems. Today direct digital service is now being provided
to many homes and offices so a computer there sends out digital
signals directly (well almost) into the telephone lines. But a device
is still needed to convert the computer digital signal into the type
allowed on telephone circuits and this device is sometimes called a
modem. This HOWTO doesn't cover such modems but some links to
documents that do may be found at the start of this HOWTO. The next 3
sections: ISDN, DSL and 56k, concern digital-to-digital "modems".
22.2. ISDN "Modems"
The "modem" is really a Terminal Adapter (TA). A Debian package
"isdnutils" is available. There is a ISDN Howto in German with an
English translation: <
http://www.suse.de/Support/sdb_e/isdn.html>.
It's put out by the SuSE distribution of Linux and likely is about
drivers available in that distribution. There is an isdn4linux
package and a newsgroup: de.alt.comm.isdn4linux. Many of the postings
are in German. You might try using a search engine (such as DejaNews)
to find "isdn4linux".
22.3. Digital Subscriber Line (DSL)
DSL uses the existing twisted pair line from your home (etc.) to the
local telephone office. This can be used if your telephone line can
accept higher speeds than an ordinary modem (say 56k) sends over it.
It replaces the analog-to-digital converter at the local telephone
office with a converter which can accept a much faster flow of data
(in a different format of course). The device which converts the
digital signals from your computer to the signal used to represent
digital data on the local telephone line is also called a modem.
22.4. 56k Digital-Modems
For any 56k modem to work as a 56k modem in your home or office the
other end must be connected directly to the digital system of the
telephone company. Thus ISPs at the other end of the line must obtain
special digital modems to provide customers with 56k service. There's
more to it than this since banks of many modems are multiplexed onto a
high capacity telephone cable that transports a large number of phone
calls simultaneously (such as a T1, E1, ISDN PRI, or better line).
This requires a concentrator or "remote access server". This has
usually been done by stand-alone units (like PC's but they cost much
more and have proprietary OSs). Now there are some cards one may
insert into a PC's PCI bus to do this.
22.5. Leased Line Modems
These are analog and not digital modems. These special modems are
used on lines leased from the telephone company or sometimes on just a
long direct wire hookup. Ordinary modems for a telephone line will
not normally work on such a line. An ordinary telephone line has
about 40-50 volts (know as the "battery) on it when not in use and the
conventional modem uses this voltage for transmission. Furthermore,
the telephone company has special signals indicating a ring, line
busy, etc. Conventional modems expect and respond to these signals.
Connecting two such modems by a long cable will not provide the
telephone signals on the cable and thus the modems will not work.
A common type of leased line used two pairs of wires (one for each
direction) using V.29 modulation at 9600 baud. Some brands of leased
line modems are incompatible with other brands.
23. Appendix F: Fax pixels (dots)
Here's some info on the bloated bandwidth required for standard fax
including the dot density. You can of course send a fax via your
modem if you dial the real telephone number of the recipinet.
A4 paper: 216mm (horizontal) * 297mm (vertical)
normal mode 8dots/mm * 3.85dots/mm
fine mode * 7.7dots/mm
extra fine mode *15.4dots/mm
Each dot is either white or black and thus 1 bit. One sheet of A4
paper using fine mode is (216*8) * (297*7.7) = about 4 million dots.
With a compression ratio of 8:1 it takes about 50 seconds at 9600bps
for transmission.
END OF Modem-HOWTO
