The Linux Serial HOWTO

The Linux Serial HOWTO
revised by David S.Lawyer [email protected] original by Greg
Hankins
v1.12, July 1998

This document describes serial port features not covered by Text-Ter�
minal-HOWTO or Serial-Programming-HOWTO. It covers mostly getty, mul�
tiport serial cards and modems.
______________________________________________________________________

Table of Contents

1. Introduction

1.1 Copyright
1.2 Related HOWTO's re: the Serial Port
1.3 Release Notes
1.4 New Versions Of This Document
1.5 Feedback
1.6 Disclaimer

2. Supported Serial Hardware

2.1 Standard PC Serial Boards
2.2 Plug And Play (PnP) Modems
2.3 Dumb Multiport Serial Boards (with 8250/16450/16550A UART's)
2.4 Intelligent Multiport Serial Boards
2.5 Unsupported Serial Hardware
2.5.1 Winmodems Don't Work
2.5.2 Rockwell (RPI) Drivers May Work

3. What Are The Names Of The Serial Ports?

3.1 Devices: modem, mouse
3.2 The cua Device
3.3 Serial Port Devices and Numbers In
3.3.1 Creating Devices In
3.4 Notes For Dumb Multiport Boards
3.5 Notes For Intelligent Multiport Boards

4. Interesting Programs You Should Know About

4.1 What is
4.1.1 About
4.1.2 About
4.1.3 About
4.2 What is

5. How Do I Dial Out With My Modem?

5.1 Hardware Requirements
5.1.1 External Modems
5.1.2 Internal Modems
5.2 Talking To Your Modem
5.3 Dial Out Modem Configuration
5.4 Hardware Flow Control

6. How Do I Dial In And Out With My Modem Using

6.1 Dial In And Out Modem Configuration
6.2 Installing

7. uugetty

7.1 Setting up
7.1.1 Modern Modems
7.1.2 Old slow modems
7.1.3 Login Banner
7.2 Customizing

8. How Do I Set Up A Terminal Connected To My PC?

8.1 Hardware Requirements
8.2 Setting up
8.3 Notes On Setting Up A PC As A Terminal

9. Can I Use More Than Two Serial Devices?

9.1 Choosing Serial Device Interrupts
9.2 Setting Serial Device Addresses

10. How Do I Set Up My Serial Ports For Higher Speeds? What Speed Should I Use With My Modem?

11. Communications Programs And Utilities

12. Serial Tips And Miscellany

12.1 kermit and zmodem
12.2 Setting Terminal Types Automagically
12.3 Color
12.4 Printing To A Printer Connected To A Terminal
12.5 Can Linux Configure The Serial Devices Automagically?
12.5.1 Notes For Multiport Boards
12.6 Using A Serial Console
12.7 Higher Serial Throughput

13. One Step Further...

13.1 What Are Lock Files?
13.2 ``baud'' Vs. ``bps''
13.3 What Are UARTs? How Do They Affect Performance?

14. Troubleshooting

14.1 I Keep Getting ``line
14.2 When I Try To Dial Out, It Says ``/dev/cua
14.3 I Keep Getting ``Getty respawning too fast: disabled for 5 minutes''
14.4 Serial Devices Are Slow, Or Serial Devices Can Only Send In One Direction
14.5 My Modem Is Hosed After Someone Hangs Up, Or
14.6 I Have My Terminal Connected To My PC, But After I Type In A Login Name, It Just Locks Up
14.7 At High Speeds, My Modem Looses Data
14.8 On Startup, Linux Doesn't Report The Serial Devices The Way I Have Them Configured
14.9 (TT
14.10 My Screen Is Displaying Funny Looking Characters
14.11 (TT

15. Other Sources Of Information

16. Contributions

______________________________________________________________________

1. Introduction

This is the Linux Serial HOWTO. This covers info on the Serial Port
that is not available in other HOWTO's. It includes how to set up
modems under Linux, some serial tips, and troubleshooting advice.
Another HOWTO exists for terminals (see ``Related HOWTO's''). This
HOWTO addresses Linux running on Intel x86 hardware, although it might
work for other architectures.

1.1. Copyright

Copyright (c) 1993 - 1997 by Greg Hankins, 1998 by David Lawyer. This
document may be distributed under the terms set forth in the LDP
license at http://sunsite.unc.edu/LDP/COPYRIGHT.html. This document
may not be distributed in modified form without consent of the author.

1.2. Related HOWTO's re: the Serial Port

� Serial-Programming-HOWTO helps you write C programs (or parts of
them) that handle the serial port. You may do the equivalent of
"stty ...", open ports in various modes, and more.

� Text-Terminal-HOWTO is all about character-cell terminals (dumb
terminals ?).

1.3. Release Notes

I, David Lawyer, am the new maintainer (Greg Hankins was the original
author and maintainer). This is a minor revision and I haven't
checked out much of the info to see if it's up-to-date. There is much
work to do on this. Let me know what's wrong or missing. In the
future, I may do a 3-way split on this Howto: 1. Info on terminals
into Text-Terminal-HOWTO. 2. Info on modems into a new Howto: Modem-
HOWTO. 3. More info on the Serial Port added to this Howto. Thus
look for major new versions in Aug. or Sept. 1998.

1.4. New Versions Of This Document

New versions of the Serial HOWTO will be available at
ftp://sunsite.unc.edu:/pub/Linux/docs/HOWTO/Serial-HOWTO and mirror
sites. There are other formats, such as Postscript and DVI versions
in the other-formats directory. The Serial HOWTO is also available at
http://sunsite.unc.edu/LDP/HOWTO/Serial-HOWTO.html and will be posted
to comp.os.linux.answers monthly.

1.5. Feedback

Please send me any questions, comments, suggestions, or additional
material. I'm always eager to hear about what you think about this
HOWTO. I'm also always on the lookout for improvements! Tell me
exactly what you don't understand, or what could be clearer. You can
reach me at [email protected] (David Lawyer) via email.

1.6. Disclaimer

Your mileage may vary. The answers given may not work for all systems
and all setup combinations.

2. Supported Serial Hardware

Linux is known to work with the following serial hardware.

2.1. Standard PC Serial Boards

� standard PC serial boards (COM1 - COM4), to which external serial
devices (modems, serial mice, etc...) can be connected. Since PC's
no longer come with them (but have the chips for this purpose
mounted on the motherboard), they are hard to find in retail
stores.

� standard PC internal modems (COM1 - COM4)

� Quickpath Systems Port-Folio 550e (allows IRQs of 3, 4, 5, 9, 10,
11, 12, and 15)
Note: due to address conflicts, you cannot use COM4 and IBM8514 video
board simultaneously. This is due to a bug in the IBM8514 board.

2.2. Plug And Play (PnP) Modems

Most modems today are PnP and most will (with some effort) work with
Linux. It's best to get one that has jumpers (shunts) to disable PnP.
In many cases, if you simply set the jumpers for a certain COM number
and IRQ number, this will disable PnP. You may also want (need ??) to
disable PnP in your computer's BIOS. If you can't disable PnP then
you may try:

� Boot Win95/98, configure the modem, and see if the settings are
preserved when you boot Linux. If not, then:

� Boot Win95/98, configure the modem, and then warm boot Linux so
that the settings are preserved. You can do this with the loadlin
boot loader.

� Get the PnP tools from http://www.redhat.com/linux-info/pnp.

2.3. Dumb Multiport Serial Boards (with 8250/16450/16550A UART's)

They are also called "serial adapters".
* => "setserial" shows details of configuring

� AST FourPort and clones (4 ports) *

� Accent Async-4 (4 ports) *

� Arnet Multiport-8 (8 ports)

� Bell Technologies HUB6 (6 ports)

� Boca BB-1004 (4 ports), BB-1008 (8 ports), BB-2016 (16 ports) *

� Boca IOAT66 (6 ports)

� Boca 2by4 (4 serial ports, 2 parallel ports)

� Byterunner (claims low prices)

� Computone ValuePort V4-ISA (AST FourPort compatible) *

� Digi PC/8 (8 ports)

� GTEK BBS-550 (8 ports) See the mini-howto for this.

� HUB-6 See Bell Technologies.

� Longshine LCS-8880, Longshine LCS-8880+ (AST FourPort compatible)

� Moxa C104, Moxa C104+ (AST FourPort compatible)

� PC-COMM (4 ports)

� Sealevel Systems <http://www.sealevel.com> COMM-2 (2 ports), COMM-4
(4 ports) and COMM-8 (8 ports)

� SIIG I/O Expander 2S IO1812 (4 ports)

� STB-4COM (4 ports)

� Twincom ACI/550

� Usenet Serial Board II (4 ports) *

In general, Linux will support any serial board which uses a 8250,
16450, 16550, 16550A, 16650 (or compatible) UART, or an internal modem
which emulates one of the above UARTs.

Note: the BB-1004 and BB-1008 do not support DCD and RI lines, and
thus are not usable for dialin modems. They will work fine for all
other purposes.

2.4. Intelligent Multiport Serial Boards

Make sure that a Linux computable driver is available. This list is a
little out of date.

� Comtrol RocketPort (36MHz ASIC; 4, 8, 16 or 32 ports)
contact: [email protected] or http://www.comtrol.com
driver status: supported by Comtrol
driver location: ftp://tsx-11.mit.edu/pub/linux/packages/comtrol

� Computone IntelliPort II (16MHz 80186; 4, 8, or 16 ports),
IntelliPort II EXpandable (20MHz 80186; 16 - 64 ports)
contact: Michael H. Warfield, [email protected]
driver status: pre-ALPHA

� Cyclades Cyclom-Y (Cirrus Logic CD1400 UARTs; 8 - 32 ports),
Cyclom-Z (25MHz MIPS R3000; 8 - 128 ports)
contact: [email protected] or http://www.cyclades.com
driver status: supported by Cyclades
driver location: ftp://ftp.cyclades.com/pub/cyclades and included
in Linux kernel since version 1.1.75

� Decision PCCOM8 (8 ports)
contact: [email protected]
driver location: ftp://ftp.signum.se/pub/pccom8

� Digi PC/Xi (12.5MHz 80186; 4, 8, or 16 ports),
PC/Xe (12.5/16MHz 80186; 2, 4, or 8 ports),
PC/Xr (16MHz IDT3041; 4 or 8 ports),
PC/Xem (20MHz IDT3051; 8 - 64 ports)
contact: [email protected] or http://www.dgii.com
driver status: supported by Digi
driver location: ftp://ftp.dgii.com/drivers/linux and included in
Linux kernel since version 2.0

� Digi COM/Xi (10MHz 80188; 4 or 8 ports)
contact: Simon Park, [email protected]
driver status: ALPHA
note: Simon is often away from email for months at a time due to
his job. Mark Hatle, [email protected] has graciously
volunteered to make the driver available if you need it. Mark is
not maintaining or supporting the driver.

� Equinox SuperSerial Technology (30MHz ASIC; 2 - 128 ports)
contact: [email protected] or http://www.equinox.com
driver status: supported by Equinox
driver location: ftp://ftp.equinox.com/library/sst

� GTEK Cyclone (16C654 UARTs; 6, 16 and 32 ports),
SmartCard (24MHz Dallas DS80C320; 8 ports),
BlackBoard-8A (16C654 UARTs; 8 ports),
PCSS (15/24MHz 8032; 8 ports)
contact: [email protected] or http://www.gtek.com
driver status: supported by GTEK
driver location: ftp://ftp.gtek.com/pub

� Hayes ESP (COM-bic; 1 - 8 ports)
contact: Andrew J. Robinson, [email protected] or
http://www.nyx.net/~arobinso
driver status: supported by author
driver location: http://www.nyx.net/~arobinso and included in Linux
kernel since version 2.1.15

� Maxpeed SS (Toshiba; 4, 8 and 16 ports)
contact: [email protected] or http://www.maxpeed.com
driver status: supported by Maxpeed
driver location: ftp://maxpeed.com/pub/ss

� Moxa C218 (12MHz 80286; 8 ports),
Moxa C320 (40MHz TMS320; 8 - 32 ports)
contact: [email protected] or http://www.moxa.com.tw
driver status: supported by Moxa
driver location: ftp://ftp.moxa.com.tw/drivers/c218-320/linux

� SDL RISCom/8 (Cirrus Logic CD180; 8 ports)
contact: [email protected] or http://www.sdlcomm.com
driver status: supported by SDL
driver location: ftp://ftp.sdlcomm.com/pub/drivers

� Specialix SIO (20MHz Zilog Z280; 4 - 32 ports),
XIO (25MHz Zilog Z280; 4 - 32 ports)
contact: Simon Allen, [email protected]
driver status: BETA
driver location:
ftp://sunsite.unc.edu/pub/Linux/kernel/patches/serial

� Stallion EasyIO-4 (4 ports), EasyIO-8 (8 ports), and
EasyConnection (8 - 32 ports) - each with Cirrus Logic CD1400
UARTs,
Stallion (8MHz 80186 CPU; 8 or 16 ports),
Brumby (10/12 MHz 80186 CPU; 4, 8 or 16 ports),
ONboard (16MHz 80186 CPU; 4, 8, 12, 16 or 32 ports),
EasyConnection 8/64 (25MHz 80186 CPU; 8 - 64 ports)
contact: [email protected] or http://www.stallion.com
driver status: supported by Stallion
driver location: ftp://ftp.stallion.com/drivers/ata5/Linux and
included in linux kernel since 1.3.27

A review of Comtrol, Cyclades, Digi, and Stallion products was printed
in the June 1995 issue of the Linux Journal. The article is available
at http://www.ssc.com/lj/issue14.

2.5. Unsupported Serial Hardware

2.5.1. Winmodems Don't Work

Modems that claim to be ``Winmodems'', for example the USR Sportster
Winmodem and IBM Aptiva MWAVE, are not supported under Linux. These
modems use proprietary designs that require special Windows drivers.
Avoid these types of modems.

2.5.2. Rockwell (RPI) Drivers May Work

Modems that require Rockwell RPI drivers are not fully usable since
the driver software doesn't work on Linux. The RPI does compression
and error correction using MS software on your computer's CPU. If you
are willing to operate the modem without using the RPI (and thus have
no compression nor error correction) then you may easily disable RPI
by sending the modem (via the initialization string) a RPI disable
command: +H0

3. What Are The Names Of The Serial Ports?

An I/O port is a way to get data into and out of a computer. There
are many types of I/O ports such as serial ports, parallel ports, disk
drive controllers, ethernet boards, etc. We will be dealing with
serial ports since modems and terminals are serial devices. Each
serial port must have an I/O address, and an interrupt (IRQ). There
are the four serial ports corresponding to COM1 - COM4:

ttyS0 (COM1) address 0x3f8 IRQ 4
ttyS1 (COM2) address 0x2f8 IRQ 3
ttyS2 (COM3) address 0x3e8 IRQ 4
ttyS3 (COM4) address 0x2e8 IRQ 3

If Linux does not detect any serial ports when it boots, then make
sure that serial support is enabled and compiled into the kernel. In
this document, I refer to COM1 as ttyS0, COM2 as ttyS1, COM3 as ttyS2,
and COM4 as ttyS3. Notice that by default these devices have overlap�
ping IRQs. You cannot use all of the ports in this default configura�
tion, and you must reassign different IRQs. See section ``Can I Use
More Than Two Serial Devices?'' on setting IRQs.

3.1. Devices: modem, mouse

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. I strongly discourage the use of these links. 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. Use them if you like, but be sure they point to the
right device. However, if you change or remove this link, some
applications (minicom for example) might need reconfiguration.

3.2. The cua Device

Each ttyS device has a corresponding cua device. There has been some
talk about abolishing cua so perhaps it's best to use ttyS. The main
difference between cua and ttyS is that for an ordinary "open" command
(without certain flags) in an application program, cua will open the
port even if the modem control signals (such as DCD) say not to.
(stty must have been set to check modem control signals.) A ttyS port
would refuse to open in such a case, but it can be forced to open by
giving a certain flag to the "open" command.

Thus a ttyS port can do everything a cua port can. Except that a
"forced" open results in read statements in the program behaving in a
certain way but the programmer may prevent this by adding some other
statements to the program. Eliminating cua would brings Linux more
into compliance with the Posix standard and avoids certain problems
with lock files.

3.3. Serial Port Devices and Numbers In /dev

/dev/ttyS0 major 4, minor 64 /dev/cua0 major 5, minor 64
/dev/ttyS1 major 4, minor 65 /dev/cua1 major 5, minor 65
/dev/ttyS2 major 4, minor 66 /dev/cua2 major 5, minor 66
/dev/ttyS3 major 4, minor 67 /dev/cua3 major 5, minor 67

Note that all distributions should come with these devices already
made correctly (unless cua is abolished). You can verify this by typ�
ing:

linux% ls -l /dev/cua*
linux% ls -l /dev/ttyS*

3.3.1. Creating Devices In /dev

If you don't have a device, you will have to create it with the mknod
command. Example, suppose you needed to create devices for ttyS0:

linux# mknod -m 666 /dev/cua0 c 5 64
linux# mknod -m 666 /dev/ttyS0 c 4 64

You can use the MAKEDEV script, which lives in /dev. This simplifies
the making of devices. For example, if you needed to make the devices
for ttyS0 you would type:

linux# cd /dev
linux# ./MAKEDEV ttyS0

This handles the devices creation for the incoming and outgoing
devices, and should set the correct permissions.

3.4. Notes For Dumb Multiport Boards

The devices your multiport board uses depends on what kind of board
you have. Some of these are listed in detail in rc.serial or in
0setserial which comes with the setserial package. I highly recommend
getting the latest version of setserial if you are trying to use
multiport boards. You will probably need to create these devices.
Either use the mknod command, or the MAKEDEV script. Devices for
multiport boards are made by adding ``64 + port number''. So, if you
wanted to create devices for ttyS17, you would type:

linux# mknod -m 666 /dev/cua17 c 5 81
linux# mknod -m 666 /dev/ttyS17 c 4 81

Note that ``64 + 17 = 81''. Using the MAKEDEV script, you would type:

linux# cd /dev
linux# ./MAKEDEV ttyS17

Note: the SIIG manual for the IO1812 listing for COM5-COM8 is wrong.
They should be COM5=0x250, COM6=0x258, COM7=0x260, and COM8=0x268.

Note: the Digi PC/8 Interrupt Status Register is at 0x140.

Note: for an AST Fourport, you might need to specify skip_test in
rc.serial.

3.5. Notes For Intelligent Multiport Boards

Read the information that comes with the driver. These boards use
special devices, and not the standard ones. This information varies
depending on your hardware.

4. Interesting Programs You Should Know About

4.1. What is getty ?

getty is a program that handles the login process when you log onto a
Unix box. You will need to use getty if you want to be able to dial
in to your Linux machine with a modem. You do not need to use getty
if you only want to dial out with your modem. There are three
versions that are commonly used with Linux: getty_ps, mgetty and
agetty. The syntax for these programs differs, so be sure to check
and make sure that you are using the correct syntax for
whatever getty you use.

4.1.1. About getty_ps

Most distributions come with the getty_ps package installed. (Debian
used agetty but now uses mgetty.) It contains two programs: getty is
used for console and terminal devices, and uugetty for modems. I use
this version of getty, so that is what I will focus on.

4.1.2. About mgetty

mgetty is a version of getty mainly for use with modems. It may be
used for hard-wired terminals but the documentation is about 99%
related to modems. In addition to allowing dialup logins, mgetty also
provides FAX support and auto PPP detection. The mgetty documentation
(supplied in texinfo format) is good, 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/.

4.1.3. About agetty

agetty is the third variation of getty. It's a simple, completely
functional implementation of getty which is best suited for virtual
consoles or terminals rather than modems.

4.2. What is setserial ?

setserial is a program which allows you to look at and change various
attributes of a serial device, including its port address, its
interrupt, and other serial port options. You can find out what
version you have by running setserial with no arguments. If the
argument is /dev/ttyS1, etc., then you'll see some info about that
port.

When your Linux system boots, only ttyS{0-3} are configured, using the
default IRQs of 4 and 3. So, if you have any other serial ports
provided by other boards or if ttyS{0-3} have a non-standard IRQ, you
must use setserial in order to configure those serial ports. For the
full listing of options, consult the man page.

5. How Do I Dial Out With My Modem?

5.1. Hardware Requirements

You may use either an external or internal modem. The internal costs
less and takes up no space on the desk or table. But the external are
easier to "install" and have lights to give you a clue as to what is
happening. But externals need to be switched off when not in use and
are likely to consume a little electricity even when turned off.

5.1.1. External Modems

First, make sure you have the right cable. Your modem requires a
straight through cable, with no pins crossed. Any computer store
should have these. Make sure you get the correct gender. If you are
using the DB25 serial port, it will always be the male DB25. Do not
confuse it with the parallel port, which is the female DB25. Hook up
your modem to one of your serial ports. Consult your modem manual on
how to do this if you need help.

5.1.2. Internal Modems

For an internal modem, you will not need a cable. An internal modem
does not need a serial port, it has one built in. All you need to do
is configure it to use an interrupt that is not being used, and
configure the port I/O address. Consult your modem manual if you get
stuck. Also, see section ``Can I Use More Than Two Serial Devices?''
if you need help on choosing interrupts or addresses.

On some motherboards you will have to disable the serial port that the
modem is replacing in order to avoid conflicts. This may be done with
jumpers or in the BIOS settings, depending on your motherboard.
Consult your motherboard manual.

If you have an IBM8514 video board, then there is a bug you should
know about. You may encounter problems if you want your internal
modem to be on ttyS3. If Linux does not detect your internal modem on
ttyS3, you can use setserial and the modem will work fine. Internal
modems on ttyS{0-2} should not have any problems being detected.
Linux does not do any autoconfiguration on ttyS3 due to this video
board bug.

5.2. Talking To Your Modem

Use kermit, minicom or some other communications program to test the
setup, before you go jumping into complex things SLIP or PPP. You can
find the latest version of kermit at http://www.columbia.edu/kermit/.
For example, say your modem was on ttyS3, and it's 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:

See section ``Communications Programs'' about communications programs
if you need some pointers.

When you dial out with your modem, set the speed to the highest serial
port speed that your modem supports (provided it's not over 115200
bps). This is not the same as the "advertised" speed of the modem
such as 56K. Versions of Linux with a libc version greater then 5.x
have support for speeds up to 115200 bps. Even higher speeds than
this are in the works for Linux.

5.3. Dial Out Modem Configuration

For dial out use only, you can configure your modem however you want.
If you intend to use your modem for dialin, you must configure your
modem at the same speed that you intend to run getty at. However,
most modems today set their speed automatically to the same speed that
modem commands use. Thus having getty run at 115,200 will
automatically set the modem at this speed when getty sends an init
string to the serial port where the modem is. In general, factory
defaults that enable error correction and hardware flow control are
the best setting for dial out modems, consult your modem manual for
these settings.

5.4. Hardware Flow Control

If your modem supports hardware flow control (RTS/CTS), I highly
recommend you use it. This is particularly important for modems that
support data compression. First, you have to enable RTS/CTS flow
control on the serial port itself. This is best done on startup, like
in /etc/rc.d/rc.local or /etc/rc.d/rc.serial. Make sure that these
files are being run from the main rc.sysinit file! You need to do the
following for each serial port you want to enable hardware flow
control on:
stty crtscts < /dev/ttyS3

If its not enabled by default, you must also enable RTS/CTS flow con�
trol on your modem. Your communication program may enable it. Con�
sult your modem manual if needed and then save your modem configura�
tion if your modem supports stored profiles.

6. How Do I Dial In And Out With My Modem Using getty_ps

Get your modem to dial out correctly. If you haven't read section
``How Do I Dial Out With My Modem'' go read it now! It contains very
important setup information. You do not need to read this section if
you only want to dial out with your modem.

6.1. Dial In And Out Modem Configuration

For dialin and dialout use, you have to set up your modem a certain
way (again, using AT commands on your modem):

E1 command echo ON
Q0 result codes are reported
V1 verbose ON
S0=0 never answer (uugetty handles this with the WAITFOR option)

If you don't set these correctly, your INIT string in your config file
may fail, hosing the whole process. But, more on config files
below...

&C1 DCD is on after connect only
&S0 DSR is always on
DTR on/off resets modem (depends on manufacturer - RTFM)

These affect what your modem does when calls start and end.

If your modem does not support a stored profile, you can set these
through the INIT string in your config file. See below. Some modems
come with DIP switches that affect register settings. Be sure these
are set correctly, too.

I have started a collection of modem setups for different types of
modems. So far, I only have a few of them, if you would like to send
me your working configuration, please do so! You can get them at
ftp://ftp.cc.gatech.edu/pub/people/gregh/modem-configs.

Note: to get my USR Courier V.34 modem to reset correctly when DTR
drops, I 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.

6.2. Installing getty_ps

Get the latest version from sunsite.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.

>From this point on, all references to getty will refer to getty_ps.
References to uugetty will refer to the uugetty that comes with the
getty_ps package. These instructions will not work for mgetty or
agetty.

7. uugetty

7.1. Setting up uugetty

Make sure that you have an outgoing and incoming device for the serial
port your modem is on. If you have your modem on ttyS3 you
will need the /dev/cua3, and /dev/ttyS3 devices. If you don't have
the correct devices, see section ``Creating Devices In <tt>/dev</tt>''
on how to create devices, and create the devices. If you want to be
able to dial out with your modem while uugetty is watching the port
for logins, use the /dev/cuaN device instead of the /dev/ttySN device
[One wouldn't need cua if the software was written to avoid it.]

uugetty does important lock file checking. Update /etc/gettydefs to
include an entry for your modem. When you are done editing
/etc/gettydefs, you can verify that the syntax is correct by doing:

linux# getty -c /etc/gettydefs

7.1.1. 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

7.1.2. 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

7.1.3. Login Banner

If you want, you can make uugetty print interesting things in the
login banner. In my examples, I have 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 getty 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 '@@'.

7.2. 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
getty (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
#
# alternate lock file to check... if this lock file exists, then uugetty is
# restarted so that the modem is re-initialized
ALTLOCK=cua3
ALTLINE=cua3
# line to initialize
INITLINE=cua3
# 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 - config file location, port, speed, and default terminal
type):

S3:456: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 getty(1m) for a
full description. Among other things there is a scheduling feature,
and a ringback feature.

8. How Do I Set Up A Terminal Connected To My PC?

See Text-Terminal-HOWTO for a more detailed (but still incomplete)
discussion of terminals. The instructions in this section will work
for connecting terminals, as well as other computers to the serial
port on your Linux box.

8.1. Hardware Requirements

Make sure you have the right kind of cable. A null modem cable bought
at a computer store may do it, but it probably will not work for
hardware flow control. But it must be a null modem cable! Many
computer stores advertise this kind of cable as a serial printer
cable. Make sure you are using your serial port, the male DB25 or the
DB9, and not your parallel port (female DB25 or centronics).

For a DB25 connector, you need a minimum of:

PC male DB25 Terminal DB25
TxD Transmit Data 2 --> 3 RxD Receive Data
RxD Receive Data 3 <-- 2 TxD Transmit Data
SG Signal Ground 7 --- 7 SG Signal Ground

This null modem cable works OK between two computers that use RTS/CTS
hardware handshaking signals, but many terminals use DTR handshaking
which can't be set in linux with "stty". A workaround is to stty
crtscts and connect terminal pin 20 (DTR) with the PC pin 5 (CTS).

PC male DB25 Terminal DB25
TxD Transmit Data 2 --> 3 RxD Receive Data
RxD Receive Data 3 <-- 2 TxD Transmit Data
RTS Request To Send 4 --> 5 CTS Clear To Send
CTS Clear To Send 5 <-- 4 RTS Request To Send
DSR Data Set Ready 6
|
DCD Carrier Detect 8 <-- 20 DTR Data Terminal Ready
SG Signal Ground 7 --- 7 SG Signal Ground
6 DSR Data Set Ready
|
DTR Data Terminal Ready 20 --> 8 DCD Carrier Detect

If you have a DB9 connector on your serial port, try the following:

PC DB9 Terminal DB25
RxD Receive Data 2 <-- 2 TxD Transmit Data
TxD Transmit Data 3 --> 3 RxD Receive Data
SG Signal Ground 5 --- 7 SG Signal Ground

Alternatively, a full DB9-DB25 null modem cable (Not likely to work
with terminal hardware handshaking; see above):

PC DB9 Terminal DB25
RxD Receive Data 2 <-- 2 TxD Transmit Data
TxD Transmit Data 3 --> 3 RxD Receive Data
6 DSR Data Set Ready
|
DTR Data Terminal Ready 4 --> 8 DCD Carrier Detect
GND Signal Ground 5 --- 7 GND Signal Ground
DCD Carrier Detect 1
|
DSR Data Set Ready 6 <-- 20 DTR Data Terminal Ready
RTS Request To Send 7 --> 5 CTS Clear To Send
CTS Clear To Send 8 <-- 4 RTS Request To Send
(RI Ring Indicator 9 not needed)

(Yes, the pins 2 and 3 really do have the opposite meanings in DB9
connectors than in DB25 connectors!)

If you are not using a full null modem cable, you might have to do the
following trick: on your computer side of the connector, connect RTS
and CTS together, and also connect DSR, DCD and DTR together. This
way, when the computer wants a certain handshaking signal, it will get
it (from itself).

Now that you have the right kind of cable?, connect your terminal to
your computer. If you can, tell the terminal to ignore modem control
signals. Try using 9600 bps, 8 data bits, 1 stop bit, no parity bits
for the terminal's setup (or enable parity if you want to be more
sophisticated).

8.2. Setting up getty (part of getty_ps )

Consult your getty manual for other getty programs such as agetty.
Add entries for getty to use for your terminal in /etc/gettydefs if
there they aren't already there:

# 38400 bps Dumb Terminal entry
DT38400# B38400 CS8 CLOCAL # B38400 SANE -ISTRIP CLOCAL #@S @L login: #DT38400

# 19200 bps Dumb Terminal entry
DT19200# B19200 CS8 CLOCAL # B19200 SANE -ISTRIP CLOCAL #@S @L login: #DT19200

# 9600 bps Dumb Terminal entry
DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #DT9600

If you want, you can make getty print interesting things in the login
banner. In my examples, I have the system name and the serial line
printed. 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 getty 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 '@@'.

When you are done editing /etc/gettydefs, you can verify that the
syntax is correct by doing:

linux# getty -c /etc/gettydefs

Make sure there is no getty or uugetty config file for the serial port
that your terminal is attached to (/etc/default/{uu}getty.ttySN or
/etc/conf.{uu}getty.ttySN), as this will probably interfere with
running getty on a terminal. Remove the file if it exits.

Edit your /etc/inittab file to run getty on the serial port
(substituting in the correct information for your environment - port,
speed, and default terminal type):

S1:456:respawn:/sbin/getty ttyS1 DT9600 vt100

Restart init:

linux# init q

At this point, you should see a login prompt on your terminal. You
may have to hit return to get the terminal's attention.

8.3. Notes On Setting Up A PC As A Terminal

Many people set up other PCs as terminals connected to Linux boxes.
For example, old 8088 or 80286 PCs are perfect for this purpose. All
you need is a DOS boot disk containing a version of DOS suitable for
your terminal-PC, and a communications program for your terminal-PC to
run. kermit works very well for this purpose. You can find pre-
compiled versions of kermit for every OS in existence at
http://www.columbia.edu/kermit/. Other popular DOS comm programs such
as telix and procomm will work equally well. Be sure to input correct
serial port information into your terminal-PC's communications setup.

9. Can I Use More Than Two Serial Devices?

You don't need to read this section, unless you want to use three or
more serial devices... (assuming you don't have a multiport board).

Providing you have another spare serial port, yes you can. The number
of serial ports you can use is limited by the number of interrupts
(IRQs) and port I/O addresses we have to use. This is not a Linux
limitation, but a limitation of the PC bus. Each serial devices must
be assigned it's address and should be assigned it's own interrupt.
If two devices share the same interrupt, it may work OK provided the
two devices are not operating at the same time. (In most cases, the
two programs must not even be running at the same time.) A serial
device can be a serial port, an internal modem, or a multiport serial
board.

Multiport serial boards are specially designed to have multiple serial
ports that share the same IRQ for all serial ports on the board.
Linux gets data from them by using a different I/O address for each
port on the board.

9.1. Choosing Serial Device Interrupts

Your PC will normally come with ttyS0 and ttyS2 at IRQ 4, and ttyS1
and ttyS3 at IRQ 3. You can see what IRQs are in use by typing:
setserial /dev/ttyS2, etc. Looking at /proc/interrupts will show some
of them. To use more than two serial devices, you will have to
reassign an interrupt. A good choice is to reassign an interrupt from
your parallel port. Your PC normally comes with IRQ 5 and IRQ 7 set
up as interrupts for your parallel ports, but few people use two
parallel ports. You can reassign one of the interrupts to a serial
device, and still happily use a parallel port. You will need the
setserial program to do this. In addition, you have to play with the
jumpers on your boards, check the docs for your board. Set the
jumpers to the IRQ you want for each port.

You should set things up so that there is one, and only one interrupt
for each serial device. Here is how Greg set his up in
/etc/rc.d/rc.local - you should do it in a file which runs upon
startup:

/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
IRQ 1 Keyboard
IRQ 2 Cascade for controller 2
IRQ 3 Serial port 2
IRQ 4 Serial port 1
IRQ 5 Parallel port 2
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, or 7 is a good choice. ``not assigned'' means that
currently nothing standard uses these IRQs. Also 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 serial board with a 16-bit bus
connector , you can also use IRQ 10, 11, 12 or 15.

Just make sure you don't use IRQ 0, 1, 6, 8, 13 or 14! These are used
by your mother board. You will make her very unhappy by taking her
IRQs. When you are done, double-check /proc/interrupts and make sure
there are no conflicts.

9.2. Setting Serial Device Addresses

Next, you must set the port address. Check the manual on your board
for the jumper settings. Like interrupts, there can only be one
serial device at each address. Your ports will usually come
configured as follows:

ttyS0 address 0x3f8
ttyS1 address 0x2f8
ttyS2 address 0x3e8
ttyS3 address 0x2e8

Choose which address you want each serial device to have and set the
jumpers accordingly. I have my modem on ttyS3, my mouse on ttyS0, and
my terminal on ttyS2.

When you reboot, Linux should see your serial ports at the address you
set them. The IRQ Linux sees may not correspond to the IRQ you set
with the jumpers. Don't worry about this. Linux does not do any IRQ
detection when it boots, because IRQ detection is dicey and can be
fooled. Use setserial to tell Linux what IRQ the port is using. You
can check /proc/ioports to see what I/O port addresses are in use
after Linux boots.

10. How Do I Set Up My Serial Ports For Higher Speeds? What Speed
Should I Use With My Modem?

This section should help you figure out what speed to use when using
your modem with a communications program, or with a getty program.
Normally you just set the speed by a parameter given to getty or by a
menu in your communications program (or in a configuration file). If
for some reason that doesn't work for high speeds, there is a
workaround by giving special options to setserial: With the spd_hi
or spd_vhi options given to setserial, setting 38400 will actually set
the speed to 57600 (spd_hi) or 115200 (spd_vhi).

Make sure you have at least 16550A UARTs for higher speeds. Here are
some suggested speeds:

� If you have a 28.8K (V.34), 33.6K (V.34), or 56K (V.90) modem use
115200 bps.

� If you have a 14400 bps (V.32bis) modem, with V.42bis data
compression, use 57600 bps.

� If you have a 9600 bps (V.32) modem, with V.42bis data compression,
use 38400 bps.

� If you have something slower than a 9600 bps (V.32) modem, set your
speed to the highest speed your modem supports. For example 300
bps (V.21/Bell 103), 1200 bps (V.22/Bell 212A), or 2400 bps
(V.22bis).

If you use setserial test them on the command line first, and then
when you have them working, put them into /etc/rc.d/rc.serial or
/etc/rc.d/rc.local or /etc/rc.boot/0setserial so that they are run at
startup. Make sure that you are using a valid path for setserial, and
a valid device name. You can check the settings of a serial port by
running:

setserial -a /dev/ttyS3

11. Communications Programs And Utilities

Here is a list of some communication software you can choose from,
available via FTP, if they didn't come with your distribution.

� ecu - a communications program

� C-Kermit <http://www.columbia.edu/kermit/> - portable, scriptable,
serial and TCP/IP communications including file transfer and
character-set translation

� minicom - telix-like communications program

� procomm - procomm-like communications program with zmodem

� seyon - X based communication program

� xc - xcomm communication package

� Other useful programs are term and SLiRP. They offer TCP/IP
functionality using a shell account.

� screen is another multi-session program. This one behaves like the
virtual consoles.

� callback is a program that will have your modem call you back.

� mgetty+fax handles FAX stuff, and provides an alternate ps_getty.

� 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://sunsite.unc.edu/pub/Linux/system/network/serial.

� Other things can be found on
ftp://sunsite.unc.edu/pub/Linux/system/serial and
ftp://sunsite.unc.edu/pub/Linux/apps/serialcomm or one of the many
mirrors. These are the directories where serial programs are kept.

12. Serial Tips And Miscellany

Here are some serial tips you might find helpful...

12.1. kermit and zmodem

To use zmodem with kermit, add the following to your .kermrc:

define rz !rz < /dev/ttyS3 > /dev/ttyS3
define sz !sz \%0 > /dev/ttyS3 < /dev/ttyS3

Be sure to put in the correct port your modem is on. Then, to use it,
just type rz or sz <filename> at the kermit prompt.

12.2. Setting Terminal Types Automagically

To set your terminal type automagically when you log in, add the
terminal type to the entry in /etc/inittab. If I have a vt100
terminal on ttyS1, I would add ``vt100'' to the getty command:

S1:456:respawn:/sbin/getty ttyS1 DT9600 vt100

Other gettys have similar options. You can also use tset, which can
establish terminal characteristics when you log in, and doesn't depend
on any defaults.

12.3. Color ls On Serial Connections

If ls is screwing up your terminal emulation with the color feature,
turn it off. ls --color, and ls --colour all use the color feature.
Some installations have ls set to use color by default. Check
/etc/profile and /etc/csh.cshrc for ls aliases. You can also alias ls
to ls --no-color, if you don't want to change the system defaults.

12.4. Printing To A Printer Connected To A Terminal

There is a program called vtprint, available from
ftp://ftp.sdsu.edu/pub/vtprint, and from http://www.sdsu.edu/~garrett.

Another program that will do this is called xprt. It can be found at
ftp://sunsite.unc.edu/pub/Linux/system/printing.

12.5. Can Linux Configure The Serial Devices Automagically?

Yes. If it's not already set up like this (or close to it) you may
set Linux up to detect and set up the serial devices automatically on
startup. If needed add the line:

/sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig

to your /etc/rc.d/rc.local or /etc/rc.d/rc.serial or
/etc/rc.boot/0setserial file. 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.

12.5.1. Notes For Multiport Boards

For board addresses, and IRQs, look at the rc.serial or
/etc/rc.boot/0setserial that comes with the setserial program. It has
a lot of detail on multiport boards, including I/O addresses and
device names.

12.6. Using A Serial Console

There is an article in issue 36 of the Linux Journal,
http://www.ssc.com/lj/issue36/index.html that explains how to use a
serial console. See Text-Terminal-HOWTO.

12.7. Higher Serial Throughput

If you are seeing slow throughput and serial port overruns on a system
with (E)IDE disk drives, you can get hdparm. This is a utility that
can modify (E)IDE parameters, including unmasking other IRQs during a
disk IRQ. This will improve responsiveness and will help eliminate
overruns. Be sure to read the man page very carefully, since some
drive/controller combinations don't like this and may corrupt the
filesystem.

Also have a look at a utility called irqtune that will change the IRQ
priority of a device, for example the serial port that your modem is
on. This may improve the serial throughput on your system. The
irqtune FAQ is at http://www.best.com/~cae/irqtune.

13. One Step Further...

This section is not required reading, but may give you some further
insight into Unix, and the world of telecommunications.

13.1. What Are Lock Files?

Lock file are simply a file saying that a particular device is in use.
They are kept in /usr/spool/uucp, or /var/lock. Linux lock files are
named LCK..name, where name is either a device name, or a UUCP site
name. Certain processes create these locks so that they can have
exclusive access to devices. For instance if you dial out on your
modem, a lock will appear telling other processes that someone is
using the modem already. Locks mainly contain the PID of the process
that has locked the device. Most programs look at the lock, and try
to determine if that lock is still valid by checking the process table
for the process that has locked the device. If the lock is found to
be valid, the program (should) exit. If not, some programs remove the
stale lock, and use the device, creating their own lock in the
process. Other programs just exit and tell you that the device is in
use.

13.2. ``baud'' Vs. ``bps''

``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!

baud
The baud rate is a measure of how many times per second a
signal, for instance one sent by a modem (modulator-demodulator)
changes. For example, a baud rate of 1200 implies one signal
change every 833 microseconds. Common modem baud rates are 50,
75, 110, 300, 600, 1200, and 2400. Most high speed modems run at
2400 baud. Because of the bandwidth limitations on voice-grade
phone lines, baud rates greater than 2400 are harder to achieve,
and only work under very pristine phone line quality. Multiple
bits can be encoded per baud, to get bit rates that exceed the
baud rate. ``baud'' is named after Emile Baudot, the inventor of
the asynchronous telegraph printer.

bps
The bps rate is a measure of how many bits per second are
transmitted. Common modem bps rates were formerly 50, 75, 110,
300, 1200, 2400, 9600. Today they are 14.4K, 28.8K, 33.6K, and
56K (these do not correspond to the rates over the serial-port-
to-modem cable which are in addition to the old modem bps rates
(50-9600): 19.2K, 38.4K, 57.6K and 115.2K). Using modems with
V.42bis compression (max 4:1 compression), rates up to 115.2K
bps are possible. This is what most people mean when they
misuse the word ``baud''.

So, if high speed modems are running at 2400 baud, how can they send
14400 bps (or higher)? The modems achieve a bps rate greater than
baud rate by encoding many bits in each signal change, or phase
change. Thus, when 2 or more bits are encoded per baud, the bps rate
exceeds the baud rate. If your modem connects at 14400 bps, it's going
to be sending 6 bits per phase change, at 2400 baud.

How did this confusion 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.

13.3. What Are UARTs? How Do They Affect Performance?

UARTs (Universal Asynchronous Receiver Transmitter) are chips on your
PC serial board (if you have one) or on the motherboard. The UART
function may also be done on a chip that does other things as well.
The UART's purpose is to convert data to bits, send the bits down the
serial line, and then rebuild the data again on the other end. UARTs
deal with data in byte sized pieces, which is conveniently also the
size of ASCII characters.

Say you have a terminal hooked up to your PC. When you type a
character, the terminal gives that character to it's transmitter (also
a UART). The transmitter sends that byte out onto the serial line,
one bit at a time, at a specific rate. On the PC end, the receiving
UART takes all the bits and rebuilds the byte and puts it in a buffer.

There are two different types of UARTs. You have probably heard of
dumb UARTs - the 8250 and 16450, and FIFO UARTs - the 16550A. To
understand their differences, first let's examine what happens when a
UART has sent or received a byte.

The UART itself can't do anything with the data, it just sends and
receives it. For the original UARTS, the CPU gets an interrupt from
the serial device every time a byte has been sent or received. The
CPU then moves the received byte out of the UART's buffer and into
memory somewhere, or gives the UART another byte to send. The 8250 and
16450 UARTs only have a 1 byte buffer. That means, that every time 1
byte is sent or received, the CPU is interrupted. At low rates, this
is OK. But, at high transfer rates, the CPU gets so busy dealing with
the UART, that is doesn't have time to tend to other tasks. In some
cases, the CPU does not get around to servicing the interrupt in time,
and the byte is overwritten, because they are coming in so fast.

That's where the 16550A UARTs are useful. These chips come with 16
byte FIFOs. This means that it can receive or transmit up to 14 bytes
before it has to interrupt the CPU. Not only can it wait, but the CPU
then can transfer all 14 bytes at a time. Although the interrupt
threshold is not always set at 14, this is still a significant
advantage over the other UARTs, which only have the 1 byte buffer.
The CPU receives less interrupts, and is free to do other things.
Data is not lost, and everyone is happy. (There is also a 16550 UART,
but it is treated as a 16450 since it is broken.)

In general, the 8250 and 16450 UARTs should be fine for speeds up to
38400 bps. At speeds greater than 38400 bps, you might start seeing
data loss. Other PC operating systems (definition used loosely here),
like DOS aren't multitasking, so they might be able to cope better
with 8250 or 16450s. That's why some people don't see data loss,
until they switch to Linux.

Non-UART, and intelligent multiport boards use DSP chips to do
additional buffering and control, thus relieving the CPU even more.
For example, the Cyclades Cyclom, and Stallion EasyIO boards use a
Cirrus Logic CD1400 RISC UART, and many boards use 80186 CPUs or even
special RISC CPUs, to handle the serial I/O.

Keep in mind that these dumb UART types are not bad, they just aren't
good for high speeds. You should have no problem connecting a
terminal, or a mouse to these UARTs. But, for a high speed modem, the
16550A is definitely a must.

Most newer PC's (486's, Pentiums, or better) come with 16550A's. If
you have something really old you may be able to upgrade it by buying
16550A chips and replacing your existing 16450 UARTs. If the
functionality has been put on another type of chip, you are out of
luck. If the UART is socketed, then upgrading is easy (if you can
find a replacement). The new and old are pin-to-pin compatible. It
may be more feasible to just buy a serial board from the Internet (few
retail stores stock them today).

14. Troubleshooting

14.1. 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.

14.2. When I Try To Dial Out, It Says ``/dev/cua N : Device or
resource busy''

This problem can arise when DCD or DTR are not set correctly. DCD
should only be set when there is an actual connection (ie someone is
dialed in), not when getty is watching the port. Check to make sure
that your modem is configured to only set DCD when there is a
connection. DTR should be set whenever something is using, or
watching the line, like getty, kermit, or some other comm program.

Another common cause of ``device busy'' errors, is that you set up
your serial port with an interrupt already taken by something else.
As each device initializes, it asks Linux for permission to use its
hardware interrupt. Linux keeps track of which interrupt is assigned
to whom, and if your interrupt is already taken, your device won't be
able to initialize properly. The device really doesn't have much of
any way to tell you that this happened, except that when you try to
use it, it will return a ``device-busy'' error. Check the interrupts
on all of your boards (serial, ethernet, SCSI, etc.). Look for IRQ
conflicts.

14.3. I Keep Getting ``Getty respawning too fast: disabled for 5 min�
utes''

Make sure your modem is configured correctly. Look at registers E and
Q. This can occur when your modem is chatting with getty.

Make sure you are calling getty correctly from your /etc/inittab.
Using the wrong syntax or device names will cause serious problems.

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 ``getty Or uugetty Still Doesn't Work''.

14.4. Serial Devices Are Slow, Or Serial Devices Can Only Send In One
Direction

You probably have an IRQ conflict. Make sure there are no IRQs being
shared. Check all your boards (serial, ethernet, SCSI, etc...). Make
sure the jumper settings, and the setserial parameters are correct for
all your serial devices. Also check /proc/ioports and
/proc/interrupts for conflicts.

14.5. 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. I
saw my RD and SD LEDs go crazy when this happened to me. You need to
have your modem reset. Most Hayes compatible modems do this with &D3,
but on my USR Courier, I had to set &D2 and S13=1. Check your modem
manual.

14.6. I Have My Terminal Connected To My PC, But After I Type In A
Login Name, It Just Locks Up

� If you are using getty: You probably don't have CLOCAL in your
/etc/gettydefs entry for the terminal, and you're probably not
using a full null modem cable. You need CLOCAL, which tells Linux
to ignore modem control signals. Here is what it should look like:

# 38400 bps Dumb Terminal entry
DT38400# B38400 CS8 CLOCAL # B38400 SANE -ISTRIP CLOCAL #@S @L login: #DT38400

# 19200 bps Dumb Terminal entry
DT19200# B19200 CS8 CLOCAL # B19200 SANE -ISTRIP CLOCAL #@S @L login: #DT19200

# 9600 bps Dumb Terminal entry
DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #DT9600

Next, kill the getty process so a new one will be spawned with the new
entry.

� If you are using agetty: Add the -L flag to the agetty line in your
/etc/inittab. This will make it ignore modem control signals.
Then restart init by typing init q. The entry should look like
this:

s1:345:respawn:/sbin/agetty -L 9600 ttyS1 vt100

14.7. At High Speeds, My Modem Looses Data

If you are trying to run your modem faster than 38400 bps, and you
don't have 16550A UARTs, you should upgrade them. See section ``What
Are UARTs?'' about UARTs.

14.8. On Startup, Linux Doesn't Report The Serial Devices The Way I
Have Them Configured

This is only partly true. Linux does not do any IRQ detection on
startup, 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 when setserial changes the IRQ's, you should see this on the
startup screen.

So, even though I have my ttyS2 set at IRQ 5, I still see

Jan 23 22:25:28 misfits vmunix: tty02 at 0x03e8 (irq = 4) is a 16550A

at first when Linux boots. You have to use setserial to tell Linux
the IRQ you are using.

14.9. rz And/Or sz Don't Work When I Call My Linux Box On A Modem

If Linux looks for /dev/modem when you try to transfer files, look at
/etc/profile or /etc/csh.cshrc. There may be a bunch of aliases
defined there on some distributions, most notably Slackware. These
aliases mess up the zmodem programs. Take them out, or correct them.

14.10. My Screen Is Displaying Funny Looking Characters

This happens on virtual consoles when you send binary data to your
screen, or sometimes on serial connections. The way to fix this is to
type echo ^v^[c. For the control-character-impaired, thats:

linux% echo <ctrl>v<esc>c

14.11. getty Or 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. Other Sources Of Information

� man pages for: agetty(8), getty(1m), gettydefs(5), init(1),
login(1), mgetty(8), setserial(8)

� Your modem or terminal manual

� NET-3 HOWTO: all about networking, including SLIP, CSLIP, and PPP

� PPP HOWTO: help with PPP

� Printing HOWTO: for setting up a serial printer

� Serial-Programming-HOWTO: for some aspects of serial-port
programming

� Term HOWTO: everything you wanted to know about the term program

� Text-Terminal-HOWTO: how they work and how to install and configure

� UPS HOWTO: setting up UPS sensors connected to your serial port

� UUCP HOWTO: for information on setting up UUCP

� 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.networking
Networking and communications under Linux.
comp.os.linux.setup
Linux installation and system administration.
comp.terminals

� The Linux serial mailing list. To join, 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.

� A white paper discussing serial communications and multiport serial
boards is available from Cyclades at http://www.cyclades.com.

� 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>

� Serial programming: Serial-Programming-HOWTO
Advanced Programming in the UNIX Environment <http://heg-
school.aw.com/cseng/authors/stevens/advanced/advanced.nclk>, by W.
Richard Stevens (ISBN 0-201-56317-7; Addison-Wesley)
<http://www.ora.com/catalog/posix/> name="POSIX Programmer's
Guide">, by Donald Levine (ISBN 0-937175-73-0; O'Reilly)

16. Contributions

Most of the original HOWTO was written by Greg Hankins.
[email protected] He also rewrote many contributions by
others in order to maintain continuity in the writing style and flow.
He wrote: "Thanks to everyone who has contributed or commented, the
list of people has gotten too long to list (somewhere over one
hundred). Special thanks to Ted T'so for answering questions about
the serial drivers, Kris Gleason who used to maintain getty_ps, and
Gert D�ring who maintains mgetty."

END OF SERIAL HOWTO