Serial HOWTO
David S.Lawyer
[email protected] original by Greg Hankins
v2.13 August 2001
This document describes serial port features other than those which
should be covered by Modem-HOWTO, PPP-HOWTO, Serial-Programming-HOWTO,
or Text-Terminal-HOWTO. It does not cover the Universal Serial Bus
(see the kernel documentation for USB). It lists info on multiport
serial cards. It contains technical info about the serial port itself
in more detail than found in the above HOWTOs and should be best for
troubleshooting when the problem is the serial port itself. If you
are dealing with a Modem, PPP (used for Internet access on a phone
line), or a Text-Terminal, those HOWTOs should be consulted first.
______________________________________________________________________
Table of Contents
1. Introduction
1.1 Copyright, Disclaimer, & Credits
1.1.1 Copyright
1.1.2 Disclaimer
1.1.3 Trademarks.
1.1.4 Credits
1.2 New Versions of this Serial-HOWTO
1.3 Related HOWTO's re the Serial Port
1.4 Feedback
1.5 What is a Serial Port?
2. How the Hardware Transfers Bytes
2.1 Transmitting
2.2 Receiving
2.3 The Large Serial Buffers
3. Serial Port Basics
3.1 What is a Serial Port ?
3.1.1 Intro to Serial
3.1.2 Pins and Wires
3.1.3 RS-232 or EIA-232, etc.
3.2 IO Address & IRQ
3.3 Names: ttyS0, ttyS1, etc.
3.4 Interrupts
3.5 Data Flow (Speeds)
3.6 Flow Control
3.6.1 Example of Flow Control
3.6.2 Symptoms of No Flow Control
3.6.3 Hardware vs. Software Flow Control
3.7 Data Flow Path; Buffers
3.8 Complex Flow Control Example
3.9 Serial Driver Module
4. Is the Serial Port Obsolete?
4.1 Introduction
4.2 EIA-232 Cable Is Low Speed & Short Distance
4.3 Inefficient Interface to the Computer
5. Multiport Serial Boards/Cards/Adapters
5.1 Intro to Multiport Serial
5.2 Modem Limitations
5.3 Dumb vs. Smart Cards
5.4 Getting/Enabling a Driver
5.4.1 Introduction
5.4.2 Driver is built into the kernel (mostly dumb boards)
5.4.3 Modules (mostly for smart boards)
5.4.4 Getting info on multiport boards
5.5 Making multiport devices in the /dev directory
5.6 Standard PC Serial Cards
5.7 Dumb Multiport Serial Boards (with standard UART chips)
5.8 Intelligent Multiport Serial Boards
5.9 Unsupported Multiport Boards
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 High-level Configuring: stty, etc.
6.10.1 Configuring Flow Control: Hardware Flow Control is Best
7. Serial Port Devices /dev/ttyS2, etc.
7.1 Devfs (The new Device File System)
7.2 Serial Port Device Names & Numbers
7.3 Universal Serial Bus Ports
7.4 Link ttySN to /dev/modem
7.5 Which Connector on the Back of my PC is ttyS1, etc?
7.5.1 Inspect the connectors
7.5.2 Send bytes to the port
7.5.3 Connect a device to the connector
7.5.4 Missing connectors
7.6 Creating Devices In the /dev directory
8. Interesting Programs You Should Know About
8.1 Serial Monitoring/Diagnostics Programs
8.2 Changing Interrupt Priority
8.3 What is Setserial ?
8.3.1 Introduction
8.3.2 Probing
8.3.3 Boot-time Configuration
8.3.4 Configuration Scripts/Files
8.3.5 Edit a script (required prior to version 2.15)
8.3.6 New configuration method using /etc/serial.conf
8.3.7 IRQs
8.3.8 Laptops: PCMCIA
8.4 Stty
8.4.1 Introduction
8.4.2 Flow control options
8.4.3 Using stty at a "foreign" terminal
8.4.3.1 Old redirection method
8.4.4 Two interfaces at a terminal
8.4.5 Where to put the stty command ?
8.5 What is isapnp ?
8.6 What is slattach?
9. Speed (Flow Rate)
9.1 Can't Set a High Enough Speed
9.1.1 Speeds over 115.2k
9.1.2 How speed is set in hardware: the divisor and baud_base
9.1.3 Work-arounds for setting speed
9.1.4 Crystal frequency is not baud_base
9.2 Higher Serial Throughput
10. Locking Out Others
10.1 Introduction
10.2 Lock-Files
10.3 Change Owners, Groups, and/or Permissions of Device Files
11. Communications Programs And Utilities
11.1 List of Software
11.2 kermit and zmodem
12. Serial Tips And Miscellany
12.1 Serial Module
12.2 Serial Console (console on the serial port)
12.3 Line Drivers
12.4 Stopping the Data Flow when Printing, etc.
12.5 Known Defective Hardware
12.5.1 Avoiding IO Address Conflicts with Certain Video Boards
12.5.2 Problem with AMD Elan SC400 CPU (PC-on-a-chip)
13. Troubleshooting
13.1 Serial Electrical Test Equipment
13.1.1 Breakout Gadgets, etc.
13.1.2 Measuring Voltages
13.1.3 Taste Voltage
13.2 Serial Monitoring/Diagnostics
13.3 (The following subsections are in both the Serial and Modem HOWTOs)
13.4 My Serial Port is Physically There but Can't be Found
13.5 Extremely Slow: Text appears on the screen slowly after long delays
13.6 Somewhat Slow: I expected it to be a few times faster
13.7 The Startup Screen Show Wrong IRQs for the Serial Ports.
13.8 "Cannot open /dev/ttyS?: Permission denied"
13.9 "Operation not supported by device" for ttyS?
13.10 "Cannot create lockfile. Sorry"
13.11 "Device /dev/ttyS? is locked."
13.12 "/dev/tty? Device or resource busy"
13.13 "Input/output error" from setserial or stty
13.14 Overrun errors on serial port
13.15 Port get characters only sporadically
13.16 Troubleshooting Tools
14. Interrupt Problem Details
14.1 Types of interrupt problems
14.2 Symptoms of Mis-set or Conflicting Interrupts
14.3 Mis-set Interrupts
14.4 Interrupt Conflicts
14.5 Resolving Interrupt Problems
15. What Are UARTs? How Do They Affect Performance?
15.1 Introduction to UARTS
15.2 Two Types of UARTs
15.3 FIFOs
15.4 Why FIFO Buffers are Small
15.5 UART Model Numbers
16. Pinout and Signals
16.1 Pinout
16.2 Signals May Have No Fixed Meaning
16.3 Cabling Between Serial Ports
16.4 RTS/CTS and DTR/DSR Flow Control
16.4.1 The DTR and DSR Pins
16.5 Preventing a Port From Opening
17. Voltage Waveshapes
17.1 Voltage for a Bit
17.2 Voltage Sequence for a Byte
17.3 Parity Explained
17.4 Forming a Byte (Framing)
17.5 How "Asynchronous" is Synchronized
18. Other Serial Devices (not async EIA-232)
18.1 Successors to EIA-232
18.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
18.3 EIA-485
18.4 EIA-530
18.5 EIA-612/613
18.6 The Universal Serial Bus (USB)
18.7 Firewire
18.8 Synchronization & Synchronous
18.8.1 Defining Asynchronous vs Synchronous
18.8.2 Synchronous Communication
19. Other Sources of Information
19.1 Books
19.2 Serial Software
19.3 Related Linux Documents
19.4 Usenet newsgroups:
19.5 Serial Mailing List
19.6 Internet
20. Appendix: Obsolete Hardware (prior to 1990) Info
20.1 Replacing obsolete UARTS
______________________________________________________________________
1. Introduction
This HOWTO covers basic info on the Serial Port and multiport serial
cards. Information specific to modems and text-terminals has been
moved to Modem-HOWTO and Text-Terminal-HOWTO. Info on getty (the
program that runs the login process or the like) has been also moved
to these HOWTOs since mgetty and uugetty are best for modems while
agetty is best for text-terminals. If you are dealing with a modem,
text terminal, or printer, then you may not need to consult this
HOWTO. But if you are using the serial port for some other device,
using a multiport serial card, trouble-shooting the serial port
itself, or want to understand more technical details of the serial
port, then you may want to use this HOWTO as well as some of the other
HOWTOs. (See ``Related HOWTO's'') This HOWTO lists info on various
multiport serial cards since they may be used for either modems or
text-terminals. This HOWTO addresses Linux running on PCs (ISA or PCI
buses), although it might be valid for other architectures.
1.1. Copyright, Disclaimer, & Credits
1.1.1. Copyright
Copyright (c) 1993-1997 by Greg Hankins, (c) 1998-2001 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. If it's not a translation: Email a copy of your derivative work (in
a format LDP accepts) to the author(s) and maintainer (could be the
same person). If you don't get a response then email the LDP
(Linux Documentation Project):
[email protected].
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.
1.1.2. 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.
1.1.3. Trademarks.
Any brand names (starts with a capital letter) should be assumed to be
a trademark). Such trademarks belong to their respective owners.
1.1.4. Credits
Most of the original Serial-HOWTO was written by Greg Hankins.
<mailto:
[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 Ts'o for answering questions about
the serial drivers.'' Approximately half of v2.00 was from Greg
Hankins HOWTO and the other half is by David Lawyer. Ted Ts'o has
continued to be helpful.
1.2. New Versions of this Serial-HOWTO
New versions of the Serial-HOWTO will be available to browse and/or
download at LDP mirror sites. For a list of mirror sites see:
<
http://metalab.unc.edu/LDP/mirrors.html>. Various formats are
available. If you only want to quickly check the date of the latest
version look at <
http://www.linuxdoc.org/HOWTO/Serial-HOWTO.html> and
compare it to this version: v2.13 August 2001 . New in recent
versions:
v.2.13 August 2001: fixed typos: done->down and "is is", USRT chip,
synchronous defined better v2.12 July 2001: serial printing under
LPRng
v2.11 May 2001: stty 0 => hangup (was ok in v2.08. ) v2.10 EIA-485,
frame errors on networks, gkermit, firewire
1.3. Related HOWTO's re the Serial Port
Modems, Text-Terminals, some printers, and other peripherals often use
the serial port. Get these HOWTOs from the nearest mirror site as
explained above.
� Modem-HOWTO is about installing and configuring modems
� Printing-HOWTO has info for serial printers using old lpr command
� LPRng-HOWTO (not a LDP HOWTO, may come with software) has info for
serial printing for "Next Generation" lpr
� Serial-Programming-HOWTO helps you write C programs (or parts of
them) that read and write to the serial port and/or check/set its
state. A new version has been written by Vern Hoxie but not
submitted. A copy is at ``Internet''.
� Text-Terminal-HOWTO is about how they work, how to install
configure, and repair them. It includes a section on "Make a
Terminal the Console" which is useful for using a remote terminal
to control a server (via the serial port).
1.4. 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 via email at (David Lawyer).
1.5. What is a Serial Port?
The conventional serial port (not the newer USB port, or HSSI port) is
a very old I/O port. Almost all PC's have them. But Macs (Apple
Computer) after mid 1998 (with colored cases) only have the USB port.
It's possible, however, to put a conventional serial port device on
the USB.
The common specification for the conventional serial port is RS-232
(or EIA-232). The connector for the serial port is often seen as one
or two 9-pin connectors (in some cases 25-pin) on the back of a PC.
But the serial port is more than just that. It includes the
associated electronics which must produce signals conforming to the
EIA-232 specification. See ``Voltage Waveshapes''. One pin is used
to send out data bytes and another to receive data bytes. Another pin
is a common signal ground. The other "useful" pins are used mainly
for signalling purposes with a steady negative voltage meaning "off"
and a steady positive voltage meaning "on".
The UART (Universal Asynchronous Receiver-Transmitter) chip does most
of the work. Today, the functionality of this chip is usually built
into another chip. See ``What Are UARTs?'' These have improved over
time and old models (several years old) are now obsolete.
The serial port was originally designed for connecting modems but it's
used to connect many other devices also such as mice, text-terminals,
some printers, etc. to a computer. You just plug these devices into
the serial port using the correct cable. Many internal modem cards
have a built-in serial port so when you install one inside your PC
it's as if you just installed another serial port in your PC.
2. How the Hardware Transfers Bytes
Below is an introduction to the topic, but for a more advanced
treatment of it see ``FIFOs''.
2.1. Transmitting
Transmitting is sending bytes out of the serial port away from the
computer. Once you understand transmitting, receiving is easy to
understand since it's similar. The first explanation given here will
be grossly oversimplified. Then more detail will be added in later
explanations. When the computer wants to send a byte out the serial
port (to the external cable) the CPU sends the byte on the bus inside
the computer to the I/O address of the serial port. The serial port
takes the byte, and sends it out one bit at a time (a serial bit-
stream) on the transmit pin of the serial cable connector. For what a
bit (and byte) look like electrically see ``Voltage Waveshapes''.
Here's a replay of the above in a little more detail (but still very
incomplete). Most of the work at the serial port is done by the UART
chip (or the like). To transmit a byte, the serial device driver
program (running on the CPU) sends a byte to the serial port"s I/O
address. This byte gets into a 1-byte "transmit shift register" in
the serial port. From this shift register bits are taken from the
byte one-by-one and sent out bit-by-bit on the serial line. Then when
the last bit has been sent and the shift register needs another byte
to send it could just ask the CPU to send it another byte. Thus would
be simple but it would likely introduce delays since the CPU might not
be able to get the byte immediately. After all, the CPU is usually
doing other things besides just handling the serial port.
A way to eliminate such delays is to arrange things so that the CPU
gets the byte before the shift register needs it and stores it in a
serial port buffer (in hardware). Then when the shift register has
sent out its byte and needs a new byte immediately, the serial port
hardware just transfers the next byte from its own buffer to the shift
register. No need to call the CPU to fetch a new byte.
The size of this serial port buffer was originally only one byte, but
today it is usually 16 bytes (more in higher priced serial ports).
Now there is still the problem of keeping this buffer sufficiently
supplied with bytes so that when the shift register needs a byte to
transmit it will always find one there (unless there are no more bytes
to send). This is done by contacting the CPU using an interrupt.
First we'll explain the case of the old fashioned one-byte buffer,
since 16-byte buffers work similarly (but are more complex). When the
shift register grabs the byte out of the buffer and the buffer needs
another byte, it sends an interrupt to the CPU by putting a voltage on
a dedicated wire on the computer bus. Unless the CPU is doing
something very important, the interrupt forces it to stop what it was
doing and start running a program which will supply another byte to
the port's buffer. The purpose of this buffer is to keep an extra
byte (waiting to be sent) queued in hardware so that there will be no
gaps in the transmission of bytes out the serial port cable.
Once the CPU gets the interrupt, it will know who sent the interrupt
since there is a dedicated interrupt wire for each serial port (unless
interrupts are shared). Then the CPU will start running the serial
device driver which checks registers at I/0 addresses to find out what
has happened. It finds out that the serial's transmit buffer is empty
and waiting for another byte. So if there are more bytes to send, it
sends the next byte to the serial port's I/0 address. This next byte
should arrive when the previous byte is still in the transmit shift
register and is still being transmitted bit-by-bit.
In review, when a byte has been fully transmitted out the transmit
wire of the serial port and the shift register is now empty the
following 3 things happen almost simultaneously:
1. The next byte is moved from the transmit buffer into the transmit
shift register
2. The transmission of this new byte (bit-by-bit) begins
3. Another interrupt is issued to tell the device driver to send yet
another byte to the now empty transmit buffer
Thus we say that the serial port is interrupt driven. Each time the
serial port issues an interrupt, the CPU sends it another byte. Once
a byte has been sent to the transmit buffer by the CPU, then the CPU
is free to pursue some other activity until it gets the next
interrupt. The serial port transmits bits at a fixed rate which is
selected by the user (or an application program). It's sometimes
called the baud rate. The serial port also adds extra bits to each
byte (start, stop and perhaps parity bits) so there are often 10 bits
sent per byte. At a rate (also called speed) of 19,200 bits per
second (bps), there are thus 1,920 bytes/sec (and also 1,920
interrupts/sec).
Doing all this is a lot of work for the CPU. This is true for many
reasons. First, just sending one 8-bit byte at a time over a 32-bit
data bus (or even 64-bit) is not a very efficient use of bus width.
Also, there is a lot of overhead in handing each interrupt. When the
interrupt is received, the device driver only knows that something
caused an interrupt at the serial port but doesn't know that it's
because a character has been sent. The device driver has to make
various checks to find out what happened. The same interrupt could
mean that a character was received, one of the control lines changed
state, etc.
A major improvement has been the enlargement of the buffer size of the
serial port from 1-byte to 16-bytes. This means that when the CPU
gets an interrupt it gives the serial port up to 16 new bytes to
transmit. This is fewer interrupts to service but data must still be
transferred one byte at a time over a wide bus. The 16-byte buffer is
actually a FIFO (First In First Out) queue and is often called a FIFO.
See ``FIFOs'' for details about the FIFO along with a repeat of some
of the above info.
2.2. Receiving
Receiving bytes by a serial port is similar to sending them only it's
in the opposite direction. It's also interrupt driven. For the
obsolete type of serial port with 1-byte buffers, when a byte is fully
received from the external cable it goes into the 1-byte receive
buffer. Then the port gives the CPU an interrupt to tell it to pick
up that byte so that the serial port will have room for storing the
next byte which is currently being received. For newer serial ports
with 16-byte buffers, this interrupt (to fetch the bytes) may be sent
after 14 bytes are in the receive buffer. The CPU then stops what it
was doing, runs the interrupt service routine, and picks up 14 to 16
bytes from the port. For an interrupt sent when the 14th byte has
been received, there could be 16 bytes to get if 2 more bytes have
arrived since the interrupt. But if 3 more bytes should arrive
(instead of 2), then the 16-byte buffer will overrun. It also may
pick up less than 14 bytes by setting it that way or due to timeouts.
See ``FIFOs'' for more details.
2.3. The Large Serial Buffers
We've talked about small 16-byte serial port hardware buffers but
there are also much larger buffers in main memory. When the CPU takes
some bytes out of the receive buffer of the hardware, it puts them
into a much larger (say 8k-byte) receive buffer in main memory. Then
a program that is getting bytes from the serial port takes the bytes
it's receiving out of that large buffer (using a "read" statement in
the program). A similar situation exists for bytes that are to be
transmitted. When the CPU needs to fetch some bytes to be transmitted
it takes them out of a large (8k-byte) transmit buffer in main memory
and puts them into the small 16-byte transmit buffer in the hardware.
3. Serial Port Basics
You don't have to understand the basics to use the serial port But
understanding it may help to determine what is wrong if you run into
problems. This section not only presents new topics but also repeats
some of what was said in the previous section ``How the Hardware
Transfers Bytes'' but in greater detail.
3.1. What is a Serial Port ?
3.1.1. Intro to Serial
The serial port is an I/O (Input/Output) device.
An I/O device is just a way to get data into and out of a computer.
There are many types of I/O devices such as serial ports, parallel
ports, disk drive controllers, ethernet boards, universal serial
buses, etc. 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?'' 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.
3.1.2. 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 section ``Pinout and
Signals'' for more details.
3.1.3. RS-232 or EIA-232, etc.
The serial port (not the USB) is usually a RS-232-C, EIA-232-D, or
EIA-232-E. These three are almost the same thing. The original RS
(Recommended Standard) prefix became EIA (Electronics Industries
Association) and later EIA/TIA after EIA merged with TIA
(Telecommunications Industries Association). The EIA-232 spec
provides also for synchronous (sync) communication but the hardware to
support sync is almost always missing on PC's. The RS designation is
obsolete but is still widely used. EIA will be used in this howto.
Some documents use the full EIA/TIA designation. For info on other
(non-EIA-232) serial ports see the section ``Other Serial Devices (not
async EIA-232)''
3.2. 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.
3.3. 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>
3.4. Interrupts
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. There is no
``Flow Control'' to prevent this.
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.
3.5. Data Flow (Speeds)
Data (bytes representing letters, pictures, etc.) flows into and out
of your serial port. Flow rates (such as 56k (56000) bits/sec) are
(incorrectly) called "speed". But almost everyone says "speed"
instead of "flow rate".
It's important to understand that the average speed is often less than
the specified speed. 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 between bytes. If the device on
the serial port (such as a modem) can't accept the full serial port
speed, then the average speed must be reduced.
3.6. Flow Control
Flow control means the ability to slow down 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.
3.6.1. 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.
3.6.2. Symptoms of No Flow Control
Understanding flow-control theory can be of practical use. The
symptom of no flow control is chunks of data missing from files sent
without the benefit of flow control. This is because when overflow
happens, it's usually more than just a few bytes that overflow and are
lost. Often hundreds or even thousands of bytes get lost, and all in
contiguous chunks.
3.6.3. 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.
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.
3.7. 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
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 or other device connected to the serial
port 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 device connected to the serial port
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".
3.8. Complex Flow Control Example
For many situations, there is a transmit path involving several links,
each with its own flow control. For example, I type at a text-
terminal connected to a PC with a modem to access a BBS. For this I
use the application program "minicom" which deals with 2 serial ports:
one connected to a modem and another connected to the text-terminal.
What I type at the text terminal goes into the first serial port to
minicom, then from minicom out the second serial port to the modem,
and then onto the telephone line to the BBS. The text-terminal has a
limit to the speed at which bytes can be displayed on its screen and
issues a flow control "stop" from time to time to slow down the flow.
What happens when such a "stop" is issued? Let's consider a case
where the "stop" is long enough to get thru to the BBS and stop the
program at the BBS which is sending out the bytes.
Let's trace out the flow of this "stop" (which may be "hardware" on
some links and "software" on others). First, suppose I'm "capturing"
a long file from the BBS which is being sent simultaneously to both my
text-terminal and a to file on my hard-disk. The bytes are coming in
faster than the terminal can handle them so it sends a "stop" out its
serial port to the first serial port on my PC. The device driver
detects it and stops sending bytes from the 8k serial buffer (in main
memory) to the terminal. Now minicom still keeps sending out bytes
for the terminal into this 8k buffer.
When this 8k transmit buffer (on the first serial port) is full,
minicom must stop writing to it. Minicom stops and waits. But this
also causes minicom to stop reading from the 8k receive buffer on the
2nd serial port connected to the modem. Flow from the modem continues
until this 8k buffer too fills up and sends a different "stop" to the
modem. Now the modem's buffer ceases to send to the serial port and
also fills up. The modem (assuming error correction is enabled) sends
a "stop signal" to the other modem at the BBS. This modem stops
sending bytes out of its buffer and when its buffer gets fill, another
stop signal is sent to the serial port of the BBS. At the BBS, the
8-k (or whatever) buffer fills up and the program at the BBS can't
write to it anymore and thus temporarily halts.
Thus a stop signal from a text terminal has halted a programs on a BBS
computer. What a complex sequence of events! Note that the stop
signal passed thru 4 serial ports, 2 modems, and one application
program (minicom). Each serial port has 2 buffers (in one direction
of flow): the 8k one and the hardware 16-byte one. The application
program may have a buffer in its C_code. This adds up to 11 different
buffers the data is passing thru. Note that the small serial hardware
buffers do not participate directly in flow control.
If the terminal speed limitation is the bottleneck in the flow from
the BBS to the terminal, then its flow control "stop" is actually
stopping the program that is sending from the BBS as explained above.
But you may ask: How can a "stop" last so long that 11 buffers (some
of them large) all get filled up? It can actually happen this way if
all the buffers were near their upper limits when the terminal sent
out the "stop".
But if you were to run a simulation on it you would discover that it's
usually more complicated than this. At an instant of time some links
are flowing and others are stopped (due to flow control). A "stop"
from the terminal seldom propagates back to the BBS neatly as
described above. It may take a few "stops" from the terminal to
result in one "stop" at the BBS. To understand what is going on you
really need to observe a simulation which can be done for a simple
case with coins on a table. Use only a few buffers and set the upper
level for each buffer at only a few coins.
Does one really need to understand all this? Well, understanding this
explained to me why capturing text from a BBS was loosing text. The
situation was exactly the above example but modem-to-modem flow
control was disabled. Chunks of captured text that were supposed to
also get to my hard-disk never got there because of an overflow at my
modem buffer due to flow control "stops" from the terminal. Even
though the BBS had a flow path to the hard-disk without bottlenecks,
the overflow due to the terminal happened on this path and chunks of
text were lost and never even made it to the hard-disk. Note that the
flow to the hard-disk passed thru my modem and since the overflow
happened there, bytes intended for the hard-disk were lost.
3.9. 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'' See
``What is Setserial'' for more info on setserial.
4. Is the Serial Port Obsolete?
4.1. Introduction
The answer is yes, but ... The serial port is somewhat obsolete but
it's still needed, especially for Linux. The serial port has many
shortcomings but almost all new PC's seem to come with them them.
Linux supports ordinary telephone modems only if they work thru a
serial port.
The serial port must pass data between the computer and the external
cable. Thus it has two interfaces and both of these interfaces are
slow. First we'll consider the interface via external cable to the
outside world.
4.2. EIA-232 Cable Is Low Speed & Short Distance
The conventional EIA-232 serial port is inherently low speed and is
severely limited in distance. Ads often read "high speed" but it can
only work at "high speed" over very short distances such as to a modem
located right next to the computer. Compared to a network card, even
this "high speed" is low speed. All of the serial cable wires use a
common ground return wire so that twisted-pair technology (needed for
high speeds) can't be used without additional hardware. More modern
interfaces for serial ports exist but they are not standard on PC's
like the EIA-232 is. See ``Successors to EIA-232''. Some multiport
serial cards support them.
It is somewhat tragic that the RS-232 standard from 1969 did not use
twisted pair technology which could operate about a hundred times
faster. Twisted pairs have been used in telephone cables since the
late 1800's. In 1888 (over 110 years ago) the "Cable Conference"
reported its support of twisted-pair (for telephone systems) and
pointed out its advantages. But over 80 years after this approval by
the "Cable Conference", RS-232 failed to utilize it. Since RS-232
was originally designed for connecting a terminal to a low speed modem
located nearby, the need for high speed and longer distance
transmission was apparently not recognized.
4.3. Inefficient Interface to the Computer
To communicate with the computer, any I/O device needs to have an
address so that the computer can write to it and read from it. For
this purpose many I/O devices (such as serial ports) use a special
type of address known as an I/O addresses (sometimes called an I/O
port). It's actually a range of addresses and the lower address in
this range is the base address. If someone only says (or writes)
"address" it likely really means "base address"
Instead of using I/O, addresses some I/O devices read and write
directly from/to main memory. This provides more bandwidth since the
conventional serial I/O system only moves a byte at a time. There are
various ways to read/write directly to main memory. One way is called
shared memory I/O (where the shared memory is usually on the same card
as the I/O device). Other methods are DMA (direct memory access) on
the ISA bus and what is about the same as DMA (only much faster):
"bus mastering" on the PCI bus. These methods are a lot faster than
those used for the serial port. Thus the conventional serial port
with its interrupt driven (every 14 bytes) interface and single bytes
transfers on a bus which could accommodate 4 (or 8) bytes at a time is
not suited for very high speed I/O.
5. Multiport Serial Boards/Cards/Adapters
5.1. Intro to Multiport Serial
Multiport serial cards install in slots in a PC on the ISA or PCI bus.
Instead of being called "... cards" they are also called "...
adapters" or "... boards". Each such card provides you with many
serial ports. Today they are commonly used for the control of
external devices (including automation for both industry and the
home). They can connect to computer servers for the purpose of
monitoring/controlling the server from a remote location. They were
once mainly used for connecting up many terminals and/or modems to
serial ports. They are still used this way but today most modems are
internal digital modems (often called something else) and they don't
usually use multiport serial cards.
Each multiport card has a number of external connecters (DB-25 or
RJ45) so that one may connect up a number of devices (modems,
terminals, etc.). Each such physical device would then be connected
to its own serial port. Since the space on the external-facing part
of the card is limited there is often not enough room for all the
serial port connectors. To solve this problem, the connectors may be
on the ends of cables which come out (externally) from the card
(octopus cable). Or they may be on an external box (possibly rack
mountable) which is connected by a cable to a multiport card.
5.2. Modem Limitations
For a modem to transmit at nearly 56k requires that it be a special
digital modem and have a digital connection to a digital phone line
(such as a T1 line). Modem banks that connect to multiport cards do
exist, and some have a card that can access multiplexed digital phone
lines. Thus one can use a multiport card with a few 56k digital
modems.
For both analog and digital modem there is one modem on each serial
port so there needs to be an external cable (modem bank to multiport)
for each modem. This can lead to a large number of cables. So it's
less clutter (and cheaper) to use internal modems without a multiport
card. It's somewhat analogous to the lower cost of an internal modem
for a desktop PC as compared to the higher cost (and more cabling) for
an external modem. See Modem-HOWTO: Modem Pools, Digital Modems.
5.3. Dumb vs. Smart Cards
Dumb multiport cards are not too much different than ordinary serial
ports. They are interrupt driven and the CPU of the computer does
most all the work servicing them. They usually have a system of
sharing a single interrupt for all the ports. This doesn't decrease
the load on the CPU since the single interrupt will be sent to the CPU
each time any one port needs servicing. Such devices usually require
special drivers that you must put into the kernel or activate by
putting a #define in the source code (or the like).
Smart boards may use ordinary UARTs but handle most interrupts from
the UARTs internally within the board. This frees the CPU from the
burden of handling all these interrupts. The board may save up bytes
in its large internal FIFOs and transfer perhaps 1k bytes at a time to
the serial buffer in main memory. It may use the full bus width of 32
bits for making data transfers to main memory (instead of transferring
only 8-bit bytes like dumb serial cards do). Not all "smart" boards
are equally efficient. Many boards today are Plug-and-Play.
5.4. Getting/Enabling a Driver
5.4.1. Introduction
For a multiport board to work, a special driver for it must be used.
This driver may either be built into the kernel source code or
supplied as a module. Support for dumb boards is likely to the built
into the kernel while smart boards usually need a module.
5.4.2. Driver is built into the kernel (mostly dumb boards)
A pre-compiled kernel is not likely to have multiport support built in
(especially after kernel 2.4). So you probably need to compile it
yourself. In kernel 2.4 you should select "CONFIG_SERIAL_EXTENDED
when configuring the kernel (just before you compile). If you select
this there will be still more choices presented to you. Even after
you do this you may need to edit the resulting source code a little
(depending on the card).
5.4.3. Modules (mostly for smart boards)
A pre-compiled kernel may come with a pre-compiled module for the
board so that you don't need to recompile the kernel. This module
must be loaded in order to use it, but the kernel may automatically do
this for you if a program is trying to use a device on the smart board
(provided there exists a table showing which module to load for the
device). This table may be in /etc/modules.conf and/or be internal to
the kernel. Also certain parameters may need to be passed to the
driver (via lilo's append command or via /etc/modules.conf). For
kernel 2.4 the modules should be found in
/lib/modules/.../kernel/drivers/char.
5.4.4. Getting info on multiport boards
The board's manufacturer should have info on their website.
Unfortunately, info for old boards is sometimes not there but might be
found somewhere else on the Internet (including discussion groups).
You might also want to look at the kernel documentation in
/usr/share/kernel-doc... For configuring the kernel or modules prior
to compiling see: Configure.help and search for "serial", etc. There
are also kernel documentation files for certain boards including
computone, hayes-esp, moxa-smartio, riscom8, specialix, stallion, and
sx (specialix).
5.5. Making multiport devices in the /dev directory
The serial ports your multiport board uses depends on what kind of
board you have. Some have their own device names like /dev/ttyE27 or
/dev/ttyD2, etc. Ones that use the standard names like /dev/ttyS14
may be listed in detail in rc.serial or in 0setserial. These files
may be included in a setserial or serial package. You may need to
create these devices (unless an installation script does it for you).
Either use the mknod command, or the MAKEDEV script. Devices (in the
/dev directory) for ttyS type serial ports are made by adding ``64 +
port number''. So, if you wanted to create devices for ttyS17, you
would type:
linux# mknod -m 666 /dev/ttyS17 c 4 81
Note the "major" number is always 4 for ttyS devices (and 5 for the
obsolete cua devices). Also ``64 + 17 = 81''. Using the MAKEDEV
script, you would type:
linux# cd /dev
linux# ./MAKEDEV ttyS17
For the names and numbers of other types of serial ports other than
ttyS.. see devices.txt in the kernel documentation. Besides the
listing of various brands of multiports found in this HOWTO there is
Gary's Encyclopedia - Serial Cards
<
http://members.aa.net/~swear/pedia/serialcards.html>. It's not as
complete, but may have some different links.
5.6. Standard PC Serial Cards
In olden days PCs came with a serial card installed. Later on the
serial function was put on the hard-drive interface card. Today, one
or two serial ports are usually built into the motherboard. Most of
them (as of 2001) use a 16550 but some use 16650 (32-byte FIFOs). But
one may still buy the old PC serial cards if they need 1-4 more serial
ports. These are for ttyS0-ttyS3 (COM1 - COM4). They can be used to
connect external serial devices (modems, serial mice, etc...). Only a
tiny percentage of retail computer stores carry such cards. But one
can purchase them on the Internet. Before getting a PCI one, make
sure Linux supports it.
Here's a list of a few popular brands:
� Byte Runner (may order directly, shows prices)
<
http://www.byterunner.com>
� SIIG <
http://www.siig.com/io>
� Dolphin <
http://www.dolphinfast.com/sersol/>
Note: due to address conflicts, you may not be able to use COM4 and
IBM8514 video card (or some others) simultaneously. See ``Avoiding IO
Address Conflicts with Certain Video Boards''
5.7. Dumb Multiport Serial Boards (with standard UART chips)
They are also called "serial adapters". They often have a special
method of sharing interrupts which requires that you compile support
for them into the kernel.
* => The file that ran setserial in Debian shows some details of
configuring # => See note below for this board
� 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; See
the Boca mini-howto revised in 2001) * #
� Boca IOAT66 or? ATIO66 (6 ports, Linux doesn't support its IRQ
sharing ?? Uses odd-ball 10-cond RJ45-like connectors)
� Boca 2by4 (4 serial ports, 2 parallel ports)
� Byte Runner <
http://www.byterunner.com>
� Computone ValuePort V4-ISA (AST FourPort compatible) *
� Digi PC/8 (8 ports) #
� Dolphin <
http://www.dolphinfast.com/sersol/>
� Globetek <
http://www.globetek.com/>
� GTEK BBS-550 (8 ports; See the mini-howto)
� Hayes ESP (after kernel 2.1.15)
� HUB-6 See Bell Technologies.
� Longshine LCS-8880, Longshine LCS-8880+ (AST FourPort compatible) *
� Moxa C104, Moxa C104+ (AST FourPort compatible) *
� NI-SERIAL
<
http://digital.natinst.com/manuals.nsf/web%2Fbyproductcurrent?OpenView&Start=1&Count=500&Expand=15.1#15.1>
by National Instruments
� 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) *
� VScom (uses same driver as ByteRunner)
In general, Linux will support any serial board which uses a 8250,
16450, 16550, 16550A, 16650, etc. UART. See the latest man page for
"setserial" for a more complete list.
Notes:
AST Fourport: You might need to specify skip_test in rc.serial.
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.
Digi PC/8 Interrupt Status Register is at 0x140.
SIIG IO1812 manual for the listing for COM5-COM8 is wrong. They
should be COM5=0x250, COM6=0x258, COM7=0x260, and COM8=0x268.
5.8. Intelligent Multiport Serial Boards
Make sure that a Linux-compatible driver is available and read the
information that comes with it. These boards use special devices (in
the /dev directory), and not the standard ones. This information
varies depending on your hardware. If you have updated info which
should be shown here please email it to me.
Names of Linux driver modules are *.o but these may not work for all
models shown. Also, parameters (such as the io and irq often need to
be given to the module so you need to find instructions on this
(possibly in the source code tree). To check on the latest serial
driver go to Linux Serial Driver home page
<
http://serial.sourceforge.net/>
There are many different brands, each of which often offers many
different cards. No attempt is currently being made to list all the
cards here (and many listed are obsolete). But all major brands and
websites should be shown here so it something is missing let me know.
Go the the webpage shown for more information. These websites often
also have info (ads) on related hardware such as modem pools, remote
access servers (RASs), and terminal servers. Where there is no
webpage, the cards are likely obsolete. If you would like to put
together a better list, let me know.
� Chase Research (UK based, ISA/PCI cards)
webpage: <www.chaser.com>
driver status: for 2.2 kernel. Supported by Chase.
� Comtrol RocketPort (36MHz ASIC; 4, 8, 16, 32, up to 128 ports)
webpage:
http://www.comtrol.com
driver status: supported by Comtrol. rocket.o
driver location:
ftp://tsx-11.mit.edu/pub/linux/packages/comtrol
� Computone IntelliPort II (ISA, PCI and EISA busses up to 64 ports)
webpage: <
http://www.computone.com>
driver location:
<
ftp://ftp.computone.com/PUB/Products/IntelliPortII/Linux/>, patch
at <
http://www.wittsend.com/computone/linux-2.2.10-ctone.patch.gz>
mailing list: <mailto:
[email protected]> with
"subscribe linux-computone" in body
note: Old ATvantage and Intelliport cards are not supported by
Computone
� Connecttech
website: <
http://www.connecttech.com/porducts/products.html>
driver location: <
ftp://ftp.connecttech.com/pub/linux/>
� Cyclades
Cyclom-Y (Cirrus Logic CD1400 UARTs; 8 - 32 ports),
Cyclom-Z (MIPS R3000; 8 - 64 ports)
website: <
http://www.cyclades.com/products.html>
driver status: supported by Cyclades
driver location:
ftp://ftp.cyclades.com/pub/cyclades and included
in Linux kernel since version 1.1.75: cyclades.o
� Decision PCCOM (2-8 ports; ISA and PCI; AKA PC COM)
ISA:
contact: <mailto:
[email protected]>
driver location:
ftp://ftp.signum.se/pub/pccom8
PCI:
drivers: <
http://www.decision.com.tw> Click "download"
driver status: Support in serial driver 5.03. For an earlier
driver, there exists a patch for kernel 2.2.16 at
<
http://www.qualica.com/serial/>
� 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)
website: <
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. epca.o
� Digi COM/Xi (10MHz 80188; 4 or 8 ports)
contact: Simon Park,
[email protected]
driver status: ?
note: Simon is often away from email for months at a time due to
his job. Mark Hatle, <mailto:
[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)
website:
http://www.equinox.com
driver status: supported by Equinox
driver location:
ftp://ftp.equinox.com/library/sst
� Globetek
website: <
http://www.globetek.com/products.shtml>
driver location:
<
http://www.globetek.com/media/files/linux.tar.gz>
� 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)
website:
http://www.gtek.com
driver status: supported by GTEK
driver location:
ftp://ftp.gtek.com/pub
� Hayes ESP (COM-bic; 1 - 8 ports)
website:
http://www.nyx.net/~arobinso
driver status: Supported by Linux kernel (1998) since v. 2.1.15.
esp.o. Setserial 2.15+ supports. Also supported by author
driver location:
http://www.nyx.net/~arobinso
� Intelligent Serial Interface by Multi-Tech Systems
PCI: 4 or 8 port. ISA 8 port. DTE speed 460.8k
webpage: <
http://www.multitech.com/products/>
� Maxpeed SS (Toshiba; 4, 8 and 16 ports)
website:
http://www.maxpeed.com
driver status: supported by Maxpeed
driver location:
ftp://maxpeed.com/pub/ss
� Microgate SyncLink ISA and PCI high speed multiprotocol serial.
Intended for synchronous HDLC.
website: <
http://ww/microgate.com/products/sllinux/hdlcapi.htm>
driver status: supported by Microgate: synclink.o
� Moxa C218 (12MHz 80286; 8 ports),
Moxa C320 (40MHz TMS320; 8 - 32 ports)
website:
http://www.moxa.com
driver status: supported by Moxa
driver locations:
<
http://www.moxa.com/support/download/download.php3>>
<
ftp://ftp.moxa.com/drivers/linux> (also from Taiwan at
www.moxa.com.tw/...) where ... is the same as above)
� SDL RISCom/8 (Cirrus Logic CD180; 8 ports)
website:
http://www.sdlcomm.com
driver status: supported by SDL
driver location:
ftp://ftp.sdlcomm.com/pub/drivers
� Specialix SX (25MHz T225; 8? - 32 ports),
SIO/XIO (20 MHz Zilog Z280; 4 - 32 ports)
webpage: <www.specialix.com/products/io/serialio.htm>
driver status: Supported by Specialix
driver location: <
http://www.BitWizard.nl/specialix/>
old driver location:
<
ftp://metalab.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
� System Base website: <
http://www.sysbas.com/>
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.
5.9. Unsupported Multiport Boards
The following boards don't mention any Linux support as of 1 Jan.
2000. Let me know if this changes.
� Aurora (PCI only) <www.auroratech.com>
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 modem card you should be looking at Modem-HOWTO and not this
Serial-HOWTO. If you just have a PCI serial port card (with no modem
on the card) 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. 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 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.
Using "scanport" will probe all I/O ports and will indicate what it
thinks may be serial port. After this you could try probing with
setserial using the "autoconfig" option. You'll need to guess the
addresses to probe at (using clues from "scanport"). See ``What is
Setserial''. If your serial port is is ISA Plug-and-Play or PCI 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 a PCI serial port, use the "lspci" command (for kernels <2.2 look
at /proc/pci).
For an ISA Plug-and-Play (PnP) port 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
(or may not) imply how it's under Linux. Windows stores its
configuration info in its Registry which is not used by Linux so they
are not necessarily configured the same. 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 starting
Linux) then Linux should use whatever configuration is in the BIOS's
non-volatile memory. Windows also makes use of the same non-volatile
memory but doesn't necessarily configure it that way.
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 devices
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. High-level Configuring: stty, etc.
As a rule, your application program will do most (or all) of this.
The command which does it is stty. See ``Stty''
6.10.1. Configuring Flow Control: Hardware Flow Control is Best
See ``Flow Control'' for an explanation of it. It's usually better to
use hardware flow control rather than software flow control using
Xon/Xoff. To use full hardware flow control you must normally have
two wires for it in the cable between the serial port and the device.
If the device is on a card, then it should always be possible to use
hardware flow control.
Many applications (and the getty program) give you an option regarding
flow control and will set it for you. It might even set hardware flow
control by default. Like the IRQ and IO address, it must be set both
in the serial driver and the hardware connected to the serial port.
How it's set into the hardware is hardware dependent. Often there is
a certain "init string" you send to the hardware device via the serial
port from your PC. For a modem, the communication program should set
it in both places.
If a program you use doesn't set flow control in the serial driver,
then you may do it yourself using the stty command. Since the driver
doesn't remember the setting after you stop Linux, you could put the
stty command in a file that runs at start-up or when you login (such
as /etc/profile for the bash shell). Here's what you would add for
hardware flow control for port ttyS2:
stty crtscts < /dev/ttyS2
or for stty version >= 1.17:
stty -F /dev/ttyS2 crtscts
crtscts stands for a Control setting to use the RTS and CTS pins of
the serial port for hardware flow control. The upper case letters of
the last sentence spell: crtscts.
7. Serial Port Devices /dev/ttyS2, etc.
For creating devices in the device directory see:
``Creating Devices In the /dev directory''
7.1. Devfs (The new Device File System)
This is a new type of device interface to Linux. It's optional
starting with kernel 2.4. It's more efficient than the conventional
interface and makes it easy to deal with a huge number of devices.
The device names have all changed as well. But there's an option to
continue using the old names. For a detailed description of it see:
<
http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html>
The name changes (if used) are: ttyS2 becomes tts/2 (Serial port),
tty3 becomes vc/3 (Virtual Console), ptyp1 becomes pty/m1 (PTY
master), ttyp2 becomes pty/s2 (PTY slave). "tts" looks like a
directory which contains devices "files": 0, 1, 2, etc. All of these
new names should still be in the /dev directory although optionally
one may put them elsewhere.
7.2. Serial Port Device Names & Numbers
Devices in Linux have major and minor numbers (unless you use the new
devfs). The serial port ttySx (x=0,1,2, etc.) has major number 4.
You may see this (and the minor numbers too) by typing: "ls -l ttyS*"
in the /dev directory.
There formerly was an alternate name for each serial port. For
example, ttyS2 would have cua2 as an alternate name. You may still
have the cua devices in your /dev directory but they are now
deprecated. Their drivers behave slightly different than for the ttyS
ones. Device Obsolete">."') See the Modem-HOWTO section: "cua Device
Obsolete">.
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 devfs
dos major minor major minor address name
COM1 /dev/ttyS0 4, 64; /dev/cua0 5, 64 3F8 /dev/tts/0
COM2 /dev/ttyS1 4, 65; /dev/cua1 5, 65 2F8 /dev/tts/1
COM3 /dev/ttyS2 4, 66; /dev/cua2 5, 66 3E8 /dev/tts/2
COM4 /dev/ttyS3 4, 67; /dev/cua3 5, 67 2E8 /dev/tts/3
7.3. Universal Serial Bus Ports
Although the USB is not covered in this HOWTO, the serial ports on the
USB are: /dev/ttyUSB0 /dev/ttyUSB1, etc.
7.4. Link ttySN to /dev/modem
On some installations, two extra devices will be created, /dev/modem
for your modem and /dev/mouse for a mouse. Both of these are symbolic
links to the appropriate serial device in /dev which you specified
during the installation Except if you have a bus mouse, then
/dev/mouse will point to the bus mouse device).
Formerly (in the 1990s) the use of /dev/modem was discouraged since
lock files might not realize that it was really say /dev/ttyS2. The
newer lock file system doesn't fall into this trap so it's now OK to
use such links.
7.5. Which Connector on the Back of my PC is ttyS1, etc?
7.5.1. Inspect the connectors
Inspecting the connectors may give some clues but is often not
definitive. The serial connectors on the back side of a PC are
usually DB connectors with male pins. 9-pin is the most common but
some are 25-pin (especially older PCs like 486s). There may be one
9-pin (perhaps ttyS0 ??) and one 25-pin (perhaps ttyS1 ??). For two
9-pin ones the top one might be ttyS0.
If you only have one serial port connector on the back of your PC,
this may be easy. If you also have an internal modem, a program like
wvdial may be able to tell you what port it's on (unless it's a PnP
that hasn't been PnP configured yet). A report from setserial (at
boot-time or run by you from the command line) should help you
identify the non-modem port.
If you have two serial connectors it may be more difficult. First
check manuals (if any) for your computer. Look at the connectors for
meaningful labels. You might even want to take off the PC's cover and
see if there are any meaningful labels on the card where the internal
ribbon cables plug in. Labels (if any) are likely to say something
like "serial 1", "serial 2" or A, B. Which com port it actually is
will depend on jumper or PnP settings (sometimes shown in a CMOS setup
menu). But 1 or A are more likely to be ttyS0 with 2 or B ttyS1.
7.5.2. Send bytes to the port
Labels are not apt to be definitive so here's another method. If the
serial ports have been configured correctly per setserial, then you
may send some bytes out a port and try to detect which connector (if
any) it's coming out of. One way to send such a signal is to copy a
long text file to the port using a command like: cp my_file_name
/dev/ttyS1. A voltmeter connected to the DTR pin (see Serial-HOWTO
for Pinout) will display positive as soon as you give the copy
command.
The transmit pin should go from several volts negative to a voltage
fluctuating around zero after you start sending the bytes. If it
doesn't (but the DTR went positive) then you've got the right port but
it's blocked from sending. This may be due to a wrong IRQ, -clocal
being set, etc. The command "stty -F /dev/ttyS1 -a" should show
clocal (and not -clocal). If not, change it to clocal.
Another test is to jumper the transmit and receive pins (pins 2 and 3
of either the 25-pin or 9-pin connector) of a test serial port. Then
send something to each port (from the PCs keyboard) and see if it gets
sent back. If it does it's likely the port with the jumper on it.
Then remove the jumper and verify that nothing gets sent back. Note
that if "echo" is set (per stty) then a jumper creates an infinite
loop. Bytes that pass thru the jumper go into the port and come right
back out of the other pin back to the jumper. Then they go back in
and out again and again. Whatever you send to the port repeats itself
forever (until you interrupt it by removing the jumper, etc.). This
may be a good way to test it as the repeating test messages halt when
the jumper is removed.
As a jumper you could use a mini (or micro) jumper cable and perhaps
use a scrap of paper to prevent the mini clips from accidentally
touching the metal of the connector. Whatever you use as a jumper
take care not to bend or excessively scratch the pins. To receive
something from a port, you can go to a virtual terminal (Alt-F2 for
example) and type something like "cp /dev/ttyS2 /dev/tty". Then at
another virtual terminal you may send something to ttyS2 (or whatever)
by "echo test_message > /dev/ttyS2". Then go back to the receive
virtual terminal and look for the test_message. See ``Serial
Electrical Test Equipment'' for more info.
7.5.3. Connect a device to the connector
Another way to try to identify a serial port is to connect some
physical serial device to it and see if it works. But a problem here
is that it might not work because it's not configured right. A serial
mouse might get detected if connected.
7.5.4. Missing connectors
If the software shows that you have more serial ports than you have
connectors for (including an internal modem which counts as a serial
port) then you may have a serial port that has no connector. Some
motherboards come with a serial port with no cable or serial DB
connector. Someone may build a PC from this and omit the connector.
There may be a "serial" label on the motherboard but no ribbon cable
connects to the pins next to this label. To use this port you must
get a ribbon cable/connector. I've seen different wiring arrangements
for such ribbon cables so beware.
7.6. Creating Devices In the /dev directory
If you don't have a device "file" that you need, you will have to
create it with the mknod command or with the MAKEDEV shell script.
Example, suppose you needed to create ttyS0:
linux# mknod -m 666 /dev/ttyS0 c 4 64
You can use the MAKEDEV script, which lives in /dev. See the man page
for it. 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 and should set the correct permis�
sions. For making multiport devices see ``Making multiport devices in
the /dev directory''.
8. Interesting Programs You Should Know About
Most info on getty has been moved to Modem-HOWTO with a little info on
the use of getty with directly connected terminals now found in Text-
Terminal-HOWTO.
8.1. Serial Monitoring/Diagnostics Programs
A few Linux programs (and one "file") will monitor various modem
control lines and indicate if they are positive (1 or green) or
negative (0 or red).
� The "file": /proc/tty/driver/serial lists those that are positive
� modemstat (Only works correctly on Linux PC consoles. Status
monitored in a tiny window. Color-coded and compact. Must kill it
(a process) to quit.
� statserial (Info displayed on entire screen)
� serialmon (Doesn't monitor RTS, CTS, DSR but logs other functions)
You may already have them. If not, download them from Serial
Software <
http://metalab.unc.edu/pub/Linux/system/serial/>. As of
June 1998, I know of no diagnostic program in Linux for the serial
port.
8.2. Changing Interrupt Priority
� irqtune will give serial port interrupts higher priority to improve
performance.
� hdparm for hard-disk tuning may help some more.
8.3. 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.
8.3.1. Introduction
If you have a Laptop (PCMCIA) don't use setserial until you read
``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 may
do this).
If you only have one or two built-in serial ports, they will usually
get set up correctly without using setserial. Otherwise, if you add
more serial ports (such as a modem card) 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 setserial man page.
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 displayed (at the top of the screen) 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 your data is correct 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 may get automatically saved to a
configuration file. In older versions, the configuration file only
changes if you edit it manually so the configuration always remains
the same from boot to boot. See ``Configuration Scripts/Files''
8.3.2. Probing
Prior to probing with "setserial", one may run the "scanport" command
to check all possible ports in one scan. It makes crude guesses as to
what is on some ports but doesn't determine the IRQ. But it's a fast
first start. It may hang your PC but so far it's worked fine for me.
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 for example :
setserial /dev/ttyS2 -v autoconfig
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 could be automatically 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.
8.3.3. 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''.
8.3.4. 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.
8.3.5. 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
8.3.6. 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 so that you don't need to edit any file in order to
set up (or change) what setserial does 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 that 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
it 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
should contain some comments to explain this.
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".
8.3.7. 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".
8.3.8. Laptops: PCMCIA
If you have a Laptop, read PCMCIA-HOWTO for info on the serial
configuration. For serial ports on the motherboard, setserial is used
just like it is for a desktop. But for PCMCIA cards (such as a modem)
it's a different story. The configuring of the PCMCIA system should
automatically run setserial so you shouldn't need to run it. If you
you do run it (by a script file or by /etc/serial.conf) it might be
different and cause trouble. The autosave feature for serial.conf
shouldn't save anything for PCMCIA cards (but Debian did until
2.15-7). Of course, it's always OK to use setserial to find out how
the driver is configured for PCMCIA cards.
8.4. Stty
8.4.1. Introduction
stty does much of the configuration of the serial port but since
application programs (and the getty program) often handle it, you may
not need to use it much. It's handy if your having problems or want
to see how the port is set up. Try typing ``stty -a'' at your
terminal/console to see how it's now set. Also try typing it without
the -a (all) for a short listing which shows how it's set different
than normal. Don't try to learn all the setting unless you want to
become a serial guru. Most of the defaults should work OK and some of
the settings are needed only for certain obsolete dumb terminals made
in the 1970's.
stty is documented in the man pages with a more detailed account in
the info pages. Type "man stty" or "info stty".
Whereas setserial only deals with actual serial ports, stty is used
both for serial ports and for virtual terminals such as the standard
Linux text interface at a PC monitor. For the PC monitor, many of the
stty settings are meaningless. Changing the baud rate, etc. doesn't
appear to actually do anything.
Here are some of the items stty configures: speed (bits/sec), parity,
bits/byte, # of stop bits, strip 8th bit?, modem control signals, flow
control, break signal, end-of-line markers, change case, padding, beep
if buffer overrun?, echo what you type to the screen, allow background
tasks to write to terminal?, define special (control) characters (such
as what key to press for interrupt). See the stty man or info page
for more details. Also see the man page: termios which covers the
same options set by stty but (as of mid 1999) covers features which
the stty man page fails to mention.
With some implementations of getty (getty_ps package), the commands
that one would normally give to stty are typed into a getty
configuration file: /etc/gettydefs. Even without this configuration
file, the getty command line may be sufficient to set things up so
that you don't need stty."')
One may write C programs which change the stty configuration, etc.
Looking at some of the documentation for this may help one better
understand the use of the stty command (and its many possible
arguments). Serial-Programming-HOWTO is useful. The manual page:
termios contains a description of the C-language structure (of type
termios) which stores the stty configuration in computer memory. Many
of the flag names in this C-structure are almost the same (and do the
same thing) as the arguments to the stty command.
8.4.2. Flow control options
To set hardware flow control use "crtscts". For software flow control
there are 3 settings: ixon, ixoff, and ixany.
ixany: Mainly for terminals. Hitting any key will restarts the flow
after a flow-control stop. If you stop scrolling with the "stop
scroll" key (or the like) then hitting any key will resume scrolling.
It's seldom needed since hitting the "scroll lock" key again will do
the same thing.
ixon: Enables the port to listen for Xoff and to stop transmitting
when it gets an Xoff. Likewise, it will resume transmitting if it
gets an Xon.
ixoff: enables the port to send the Xoff signal out the transmit line
when its buffers in main memory are nearly full. It protects the
device where the port is located from being overrun.
For a slow dumb terminal (or other slow device) connected to a fast
PC, it's unlikely the the PC's port will be overrun. So you seldom
actually need to enable ixoff. But it's often enabled "just in case".
8.4.3. Using stty at a "foreign" terminal
Using stty to configure the terminal that you are currently using is
easy. Doing it for a different (foreign) terminal or serial port may
be impossible. For example, let's say you are at the PC monitor
(tty1) and want to use stty to deal with the serial port ttyS2. Prior
to about 2000 you needed to use the redirection operator "<". After
2000 (provided your version of setserial is >= 1.17 and stty >= 2.0)
there is a better method using the -F option. This will work when the
old redirection method fails. Even with the latest versions be warned
that if there is a terminal on ttyS2 and a shell is running on that
terminal, then what you see will likely be deceptive and trying to set
it will not work. See ``Two interfaces at a terminal'' to understand
it.
The new method is ``stty -F /dev/ttyS2 ...'' (or --file instead of F).
If ... is -a it displays all the stty settings. The old redirection
method (which still works in later versions) is to type ``stty ...
</dev/ttyS2''. If the new method works but the old one hangs, it
implies that the port is hung due to a modem control line not being
asserted. Thus the old method is still useful for troubleshooting.
See the following subsection for details.
8.4.3.1. Old redirection method
Here's a problem with the old redirection operator (which doesn't
happen if you use the newer -F option instead). Sometimes when trying
to use stty, the command hangs and nothing happens (you don't get a
prompt for a next command even after hitting <return>). This is
likely due to the port being stuck because it's waiting for one of the
modem control lines to be asserted. For example, unless you've set
"clocal" to ignore modem control lines, then if no CD signal is
asserted the port will not open and stty will not work for it (unless
you use the newer -F option). A similar situation seems to exist for
hardware flow control. If the cable for the port doesn't even have a
conductor for the pin that needs to be asserted then there is no easy
way to stop the hang.
One way to try to get out of the above hang is to use the newer -F
option and set "clocal" and/or "crtscts" as needed. If you don't have
the -F option then you may try to run some program (such a minicom) on
the port that will force it to operate even if the control lines say
not to. Then hopefully this program might set the port so it doesn't
need the control signal in the future in order to open: clocal or
-crtscts. To use "minicom" to do this you likely will have to
reconfigure minicom and then exit it and restart it. Instead of all
this bother, it may be simpler to just reboot the PC.
The old redirection method makes ttyS2 the standard input to stty.
This gives the stty program a link to the "file" ttyS2 so that it may
"read" it. But instead of reading the bytes sent to ttyS2 as one
might expect, it uses the link to find the configuration settings of
the port so that it may read or change them. Some people tried to use
``stty ... > /dev/ttyS2'' to set the terminal. This will not do it.
Instead, it takes the message normal displayed by the stty command for
the terminal you are on (say tty1) and sends this message to ttyS2.
But it doesn't change any settings for ttyS2.
8.4.4. Two interfaces at a terminal
When using a shell (such as bash) with command-line-editing enabled
there are two different terminal interfaces (what you see when you
type stty -a). When you type in modern shells at the command line you
have a temporary "raw" interface (or raw mode) where each character is
read by the command-line-editor as you type it. Once you hit the
<return> key, the command-line-editor is exited and the terminal
interface is changed to the nominal "cooked" interface (cooked mode)
for the terminal. This cooked mode lasts until the next prompt is
sent to the terminal (which is only a small fraction of a second).
Note that one never gets to type anything to this cooked mode but what
was typed in raw mode gets executed while in cooked mode.
When a prompt is sent to the terminal, the terminal goes from "cooked"
to "raw" mode (just like it does when you start an editor since you
are starting the command-line editor). The settings for the "raw"
mode are based only on the basic settings taken from the "cooked"
mode. Raw mode keeps these setting but changes several other settings
in order to change the mode to "raw". It is not at all based on the
settings used in the previous "raw" mode. Thus if one uses stty to
change settings for the raw mode, such settings will be permanently
lost as soon as one hits the <return> key at the terminal that has
supposedly been "set".
Now when one types stty to look at the terminal interface, one may
either get a view of the cooked mode or the raw mode. You need to
figure out which one you're looking at. It you use stty from another
(foreign) terminal then you will see the raw mode settings. Any
changes made will only be made to the raw mode and will be lost when
someone presses <return> at the terminal you tried to "set". But if
you type a stty command at your terminal (without the -F option or
redirection) and then hit <return> it's a different story. The
<return> puts the terminal in cooked mode. Your changes are saved and
will still be there when the terminal goes back into raw mode (unless
of course it's a setting not allowed in raw mode).
This situation can create problems. For example, suppose you corrupt
your terminal interface. To restore it you go to another terminal and
"stty -F dev/ttyS1 sane" (or the like). It will not work! Of course
you can try to type "stty sane ..." at the terminal that is corrupted
but you can't see what you typed. All the above not only applies to
dumb terminals but to virtual terminals used on a PC Monitor as well
as to the terminal windows in X. In other words, it applies to almost
everyone who uses Linux.
Luckily, when you start up Linux, any file that runs stty at boot-time
will likely deal with a terminal (or serial port with no terminal)
that has no shell running on it so there's no problem for this special
case.
8.4.5. Where to put the stty command ?
Should you need to have stty set up the serial interface each time the
computer starts up then you need to put the stty command in a file
that will be executed each time the computer is started up (Linux
boots). It should be run before the serial port is used (including
running getty on the port). There are many possible places to put it.
If it gets put in more than one place and you only know about (or
remember) one of those places, then a conflict is likely. So make
sure to document what you do.
One place to put it would be in the same file that runs setserial when
the system is booted. The location is distribution and version
dependent. It would seem best to put it after the setserial command
so that the low level stuff is done first. If you have directories in
the /etc tree where every file in them is executed at boot-time
(System V Init) then you could create a file named "stty" for this
purpose.
8.5. 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. 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.
8.6. What is slattach?
It's "serial line attach". It puts the serial line into a networking
mode. You can thus network two computers together via a serial line
using, for example, the slip protocol. But for the ppp protocol, you
need to start pppd on the serial line.
9. Speed (Flow Rate)
By "speed" we really mean the "data flow rate" but almost everybody
incorrectly calls it speed. The speed is measured in bits/sec (or
baud). Speed is set using the "stty" command or by a program which
uses the serial port. See ``Stty''
9.1. Can't Set a High Enough Speed
9.1.1. Speeds over 115.2k
The top speed of 115.2k has been the standard speed since the mid
1990's. By the year 2000, many serial ports supported higher speeds
but Linux seldom used them due to lack of drivers. These high-speed
ports by default only support 115.2k and must have special software to
enable the higher speeds. Today (2001) almost all new serial ports
support speeds of 230.4k and 460.8k. Some also support 921.6k.
Unfortunately, to get these speeds you need to compile the kernel with
a special patch and it seems the patch doesn't support the 2.4 kernels
yet.
For internal modems, only a minority of them advertise that they
support speeds of over 115.2k for their built-in serial ports. Will
shsmod support these ??
The patch to support high-speed is called shsmod (Super High Speed).
There are both Windows and Linux versions of this patch. See
<
http://www.devdrv.com/shsmod>. For Linux, much of the documentation
is only in Japanese. There is also a module for the VIA VT82C686
chip <www.kati.fi/viahss/>.
9.1.2. 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.
9.1.3. 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''
9.1.4. 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.
9.2. 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
<
http://www.best.com/~cae/irqtune>
10. Locking Out Others
10.1. Introduction
When you are using a serial port, you may want to prevent others from
using it at the same time. However there may be cases where you do
want others to use it, such as sending you an important message if you
are using a text-terminal.
There are various ways of preventing others (or other processes) from
using your serial port when you are using it (locking). This should
all happen automatically but it's important to know about this if it
gives you trouble. If a program is abnormally exited or the PC is
abruptly turned off (by pulling the plug, etc.) your serial port might
wind up locked. Even if the lock remains, it's usually automatically
removed when you want to use the serial port again. But in rare cases
it isn't. That's when you need to understand what happened.
One way to implement locking is to design the kernel to handle it but
Linux thus far has shunned this solution (with an exception involving
the cua device which is now obsolete). Two solutions used by Linux is
to:
1. create lock-files
2. modify the permissions and/or owners of devices such as /dev/ttyS2
10.2. Lock-Files
A lock-file is simply a file created to mean that a particular device
is in use. They are kept in /var/lock. Formerly they were in
/usr/spool/uucp. Linux lock-files are usually named LCK..name, where
name may be a device name, a process id number, a device's major and
minor numbers, or a UUCP site name. Most processes (an exception is
getty) create these locks so that they can have exclusive access to
devices. For instance if you dial out on your modem, some lockfiles
will appear to tell other processes that someone else is using the
modem. In older versions (in the 1990s) there was usually only one
lockfile per process. Lock files contain the PID of the process that
has locked the device. Note that if a process insists on using a
device that is locked, it may ignore the lockfile and use the device
anyway. This is useful in sending a message to a text-terminal, etc.
When a program wants to use a serial port but finds it locked with
lock-files it should check to see if the lock-file's PID is still in
use. If it's not it means that the lock is stale and it's OK to go
ahead and use the port anyway (after removing the stale lock-files).
Unfortunately, there may be some programs that don't do this and give
up by telling you that a device is already in use when it really
isn't.
When there were only lockfiles with device names, the following
problem could arise: If the same device has two different names then
two different processes could each use a differnet name for the same
device. This results in lockfiles with different names that actually
are the same device. Formerly each physical serial port was known by
two different device names: ttyS0 and cua0. To solve this lockfile
alias problem, 3 methods have been used. It may be overkill since any
one of these methods would have fixed the problem.
1. The lock checking software was made aware of ttyS vs. cua.
2. The device cua was deprecated
3. Additional locks were created which use unique device numbers
instead of names.
Using alternate names such as /dev/modem for /dev/ttyS2 may cause
problems with older versions. For dumb terminals, lockfiles are not
used since this would not permit someone else to send a message to
your terminal using the write or talk program.
10.3. Change Owners, Groups, and/or Permissions of Device Files
In order to use a device, you (or the program you run if you have "set
user id") needs to have permission to read and write the device "file"
in the /dev directory. So a logical way to prevent others from using
a device is to make yourself the temporary owner of the device and set
permissions so that no one else can use it. A program may do this for
you. A similar method can be used with the group of the device file.
While lock files prevent other process from using the device, changing
device file owners/permissions restricts other users (or the group)
from using it. One case is where the group is permitted to write to
the port, but not to read from it. Writing to the port might just
mean a message sent to a text-terminal while reading means destructive
reading. The original process that needs to read the data may find
data missing if another process has already read that data. Thus a
read can do more harm that a write since a read causes loss of data
while a write only adds extra data. That's a reason to allow writes
but not reads. This is exactly the opposite of the case for ordinary
files where you allow others to read the file but not write (modify)
it. Use of a port normally requires both read and write permissions.
A program that changes the device file attributes should undo these
changes when it exits. But if the exit is abnormal, then a device
file may be left in such a condition that it gives the error
"permission denied" when one attempts to use it again.
11. Communications Programs And Utilities
11.1. List of Software
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,
character-set translation, and zmodem support
� gkermit Tiny GPLed kermit run only from the command line. Can't
connect to another computer
� minicom - telix-like communications program
� seyon - X based communication program
� xc - xcomm communication package
� term and SLiRP offer TCP/IP functionality using a shell account.
� screen is another multi-session program. This one behaves like the
virtual consoles.
� 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).
� 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://metalab.unc.edu/pub/Linux/system/network/serial.
11.2. kermit and zmodem
For use of kermit with modems see the Modem-HOWTO. One can run zmodem
within the kermit program. To do this (for ttyS3), add the following
to your .kermrc file:
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. Serial Tips And Miscellany
12.1. Serial Module
Often the serial driver is provided as a module. Parameters may be
supplied to certain modules in /etc/modules.conf. Since kernel 2.2
you don't edit this file but use the program update-modules to change
it. The info that is used to update modules.conf is put in
/etc/modutils/. The Debian/GNU Linux has a file here named
/etc/modutils/setserial which runs the serial script in /etc/init.d/
every time the serial module is loaded or unloaded. When the serial
module is unloaded this script will save the state of the module in
/var/run/setserial.conf. Then if the module loads again this saved
state is restored. When the serial module first loads at boot-time,
there's nothing in /var/run/setserial.conf so the state is obtained
from /etc/serial.conf. So there are two files that save the state.
Other distributions may do something similar.
One may modify the serial driver by editing the source code. Much of
the serial driver is found in the file serial.c. For info regarding
writing of programs for the serial port see Serial-Programming-HOWTO
(revised in 1999 by Vern Hoxie but not at LDP. Get it from
<scicom.alphacdc.com/pub/linux>)
12.2. Serial Console (console on the serial port)
See the kernel documentation in: Documentation/serial-console.txt.
Kernel 2.4+ has better documentation. See also "Serial Console" in
Text-Terminal-HOWTO.
12.3. Line Drivers
For a text terminal, the EIA-232 speeds are fast enough but the usable
cable length is often too short. Balanced technology could fix this.
The common method of obtaining balanced communication with a text
terminal is to install 2@ line drivers in the serial line to convert
unbalanced to balanced (and conversely). They are a specialty item
and are expensive if purchased new.
12.4. Stopping the Data Flow when Printing, etc.
Normally flow control and/or application programs stop the flow of
bytes when its needed. But sometimes they don't. One example is
printing to printer on the serial port. If you want to instantly stop
printing you may try turning off the printer. With older versions of
the serial driver, the printer would attempt to resume printing if you
turned the printer back on again (before the time specified by
closing_wait of setserial had expired). The attempt to resume would
happen even if you used a command to stop the printing. The problem
was that once the printer software sent bytes to the large serial
buffer to be printed, these bytes were not removed from this buffer
when the print job was canceled. One way to remove them (for newer
serial drivers) is to simply turn off the printer. This will drop all
modem control signals from the printer and empty the buffer. Modern
printers have large buffers and often a button on the printer to empty
the buffer.
12.5. Known Defective Hardware
12.5.1. Avoiding IO Address Conflicts with Certain Video Boards
The IO address of the IBM 8514 video board (and others) is allegedly
0x?2e8 where ? is 2, 4, 8, or 9. This may conflict (but shouldn't if
the serial port is well designed) with the IO address of ttyS3 at
0x02e8 if the serial port ignores the leading 0 hex digit when it
decodes the address (many do). That is bad news if you try to use
ttyS3 at this IO address. Another story is that Linux will not detect
your internal modem on ttyS3 but that you can use setserial to put
ttyS3 at this address and the modem will work fine.
12.5.2. Problem with AMD Elan SC400 CPU (PC-on-a-chip)
This has a race condition between an interrupt and a status register
of the UART. An interrupt is issued when the UART transmitter
finishes the transmission of a byte and the UART transmit buffer
becomes empty (waiting for the next byte). But a status register of
the UART doesn't get updated fast enough to reflect this. As a
result, the interrupt service routine rapidly checks and determines
(erroneously) that nothing has happened. Thus no byte is sent to the
port to be transmitted and the UART transmitter waits in vain for a
byte that never arrives. If the interrupt service routine had waited
just a bit longer before checking the status register, then it would
have been updated to reflect the true state and all would be OK.
There is a proposal to fix this by patching the serial driver. But
Should linux be patched to accommodate defective hardware, especially
if this patch may impair performance of good hardware?
13. Troubleshooting
See Modem-HOWTO for troubleshooting related to modems or getty for
modems. For a Text-Terminal much of the info here will be of value as
well as the troubleshooting info in Text-Terminal-HOWTO.
13.1. Serial Electrical Test Equipment
13.1.1. Breakout Gadgets, etc.
While a multimeter (used as a voltmeter) may be all that you need for
just a few serial ports, simple special test equipment has been made
for testing serial port lines. Some are called "breakout ... " where
breakout means to break out conductors from a cable. These gadgets
have a couple of connectors which connect to serial port connectors
(either at the ends of serial cables or at the back of a PC). Some
have test points for connecting a voltmeter. Others have LED lamps
which light when certain modem control lines are asserted (turned on).
The color of the light may indicate the polarity of the signal
(positive or negative voltage). Still others have jumpers so that you
can connect any wire to any wire. Some have switches.
Radio Shack sells (in 1998) a "RS-232 Troubleshooter" or "RS-232 Line
Tester" which checks TD, RD, CD, RTS, CTS, DTR, and DSR. A green
light means on (+12 v) while red means off (-12 v). They also sell a
"RS-232 Serial Jumper Box" which permits connecting the pins anyway
you choose.
13.1.2. Measuring Voltages
Any voltmeter or multimeter, even the cheapest that sells for about
$10, should work fine. Trying to use other methods for checking
voltage is tricky. Don't use a LED unless it has a series resistor to
reduce the voltage across the LED. A 470 ohm resistor is used for a
20 ma LED (but not all LED's are 20 ma). The LED will only light for
a certain polarity so you may test for + or - voltages. Does anyone
make such a gadget for automotive circuit testing?? Logic probes may
be damaged if you try to use them since the TTL voltages for which
they are designed are only 5 volts. Trying to use a 12 V incandescent
light bulb is not a good idea. It won't show polarity and due to
limited output current of the UART it probably will not even light up.
To measure voltage on a female connector you may plug in a bent paper
clip into the desired opening. The paper clip's diameter should be no
larger than the pins so that it doesn't damage the contact. Clip an
alligator clip (or the like) to the paper clip to connect up. Take
care not to touch two pins at the same time with any metal object.
13.1.3. Taste Voltage
As a last resort, if you have no test equipment and are willing to
risk getting shocked (or even electrocuted) you can always taste the
voltage. Before touching one of the test leads with your tongue, test
them to make sure that there is no high voltage on them. Touch both
leads (at the same time) to one hand to see if they shock you. Then
if no shock, wet the skin contact points by licking and repeat. If
this test gives you a shock, you certainly don't want to use your
tongue.
For the test for 12 V, Lick a finger and hold one test lead in it.
Put the other test lead on your tongue. If the lead on your tongue is
positive, there will be a noticeable taste. You might try this with
flashlight batteries first so you will know what taste to expect.
13.2. Serial Monitoring/Diagnostics
A few Linux programs will monitor the modem control lines and indicate
if they are positive (1) or negative (0). See section ``Serial
Monitoring/Diagnostics''
13.3. (The following subsections are in both the Serial and Modem
HOWTOs)
13.4. 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''.
13.5. 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
``Interrupt Problem Details'' and/or ``Interrupt Conflicts'' and/or
``Mis-set Interrupts''. 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.
13.6. 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.
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?'' 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.
13.7. 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.
13.8. "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.
13.9. "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.
13.10. "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 ``What Are Lock Files''
13.11. "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).
13.12. "/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.
13.13. "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.
13.14. Overrun errors on serial port
This is an overrun of the hardware FIFO buffer and you can't increase
its size. See
13.15. Port get characters only sporadically
There could be some other program running on the port. Use "top"
(provided you've set it to display the port number) or "ps -alxw".
Look at the results to see if the port is being used by another
program. Be on the lookout for the gpm mouse program which often runs
on a serial port.
13.16. 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 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).
14. Interrupt Problem Details
While the section ``Troubleshooting'' lists problems by symptom, this
section explains what will happen if interrupts are set incorrectly.
This section helps you understand what caused the symptom, what other
symptoms might be due to the same problem, and what to do about it.
14.1. Types of interrupt problems
The "setserial" program will show you how serial driver thinks the
interrupts are set. If the serial driver (and setserial) has it right
then everything regarding interrupts should be OK. Of course a
/dev/ttyS must exist for the device and Plug-and-Play (or jumpers)
must have set an address and IRQ in the hardware. Linux will not
knowingly permit an interrupt conflict and you will get a "Device or
resource busy" error message if you attempt to do something that would
create a conflict.
Since the kernel tries to avoid interrupt conflicts and gives you the
"resource busy" message if you try to create a conflict, how can
interrupt conflicts happen? Easy. "setserial" may have it wrong and
erroneously predicts no conflict when there will actually be a real
conflict based on what is set in the hardware. When this happens
there will be no "... busy" message but a conflict will physically
happen. Performance is likely to be extremely slow. Both devices
will send identical interrupt signals on the same wire and the CPU
will erroneously think that the interrupts only come from one device.
This will be explained in detail in the following sections.
Linux doesn't complain when you assign two devices the same IRQ
provided that neither device is in use. As each device starts up
(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 in use, you'll see this "... busy"
error message. Thus if two devices use the same IRQ and you start up
only one of the devices, everything is OK. But when you next try to
start the second device (without quitting the first device) you get
"... busy" error message.
14.2. Symptoms of Mis-set or Conflicting Interrupts
The symptoms depend on whether or not you have a modern serial port
with FIFO buffers or an obsolete serial port without FIFO buffers.
It's important to understand the symptoms for the obsolete ones also
since sometimes modern ports seem to behave that way.
For the obsolete serial ports, only one character gets thru every
several seconds. This is so slow that it seems almost like nothing is
working (especially if the character that gets thru is invisible (such
a space or newline). For the modern ports with FIFO buffers you will
likely see bursts of up to 16 characters every several seconds.
If you have a modem on the port and dial a number, it seemingly may
not connect since the CONNECT message may not make it thru. But after
a long wait it may finally connect and you may see part of a login
message (or the like). The response from your side of the connection
may be so delayed that the other side gives up and disconnects you,
resulting in a NO CARRIER message.
If you use minicom, a common test to see if things are working is to
type the simplest "AT" command and see if the modem responds. Typing
just at<enter> should normally (if interrupts are OK) result in an
immediate "OK" response from the modem. With bad interrupts you type
at<enter> and may see nothing. But then after 10 seconds or so you
see the cursor drop down one line. What is going on is that the FIFO
is behaving like it can only hold one byte. The "at" you typed caused
it to overrun and both letters were lost. But the final <enter>
eventually got thru and you "see" this invisible character by noticing
that the cursor jumped down one line. If you were to type a single
letter and then wait about 10 seconds, you should see it echo back to
the screen. This is fine if your typing speed is less that one word
per minute :-)
14.3. Mis-set Interrupts
If you don't understand what an interrupt does see ``Interrupts''. If
a serial port has one IRQ set in the hardware but a different one set
in the device driver, the device driver will not catch any interrupts
sent by the serial port. Since the serial port uses interrupts to
call its driver to service the port (fetching bytes from its 16-byte
receive buffer or putting another 16-bytes in its transmit buffer) one
might expect that the serial port would not work at all.
But it still may work anyway --sort of. Why? Well, besides the
interrupt method of servicing the port there's a slow polling method
that doesn't need interrupts. The way it works is that every so often
the device driver checks the serial port to see if it needs anything
such as if it has some bytes that need fetching from its receive
buffer. If interrupts don't work, the serial driver falls back to
this polling method. But this polling method was not intended to be
used a substitute for interrupts. It's so slow that it's not
practical to use and may cause buffer overruns. Its purpose may have
been to get things going again if just one interrupt is lost or fails
to do the right thing. It's also useful in showing you that
interrupts have failed. Don't confuse this slow polling method with
the fast polling method that operates on ports that have their IRQs
set to 0.
For the 16-byte transmit buffer, 16 bytes will be transmitted and then
it will wait until the next polling takes place (several seconds
later) before the next 16 bytes are sent out. Thus transmission is
very slow and in small chunks. Receiving is slow too since bytes that
are received by the receive buffer are likely to remain there for
several seconds until it is polled.
This explains why it takes so long before you see what you typed.
When you type say AT to a modem, the AT goes out the serial port to
the modem. The modem then echos the AT back thru the serial port to
the screen. Thus the AT characters have to pass twice thru the serial
port. Normally this happens so fast that AT seems to appear on the
screen at the same time you hit the keys on the keyboard. With slow
polling delays at the serial port, you don't see what you typed until
many seconds later.
What about overruns of the 16-byte receive buffer? This will happen
with an external modem since the modem just sends to the serial port
at high speed which is likely to overrun the 16-byte buffer. But for
an internal modem, the serial port is on the same card and it's likely
to check that this receive buffer has room for more bytes before
putting received bytes into it. In this case there will be no overrun
of this receive buffer, but text will just appear on your screen in
16-byte chunks spaced at intervals of several seconds.
Even with an external modem you might not get overruns. If just a few
characters (under 16) are sent you don't get overruns since the buffer
likely has room for them. But attempts to send a larger number of
bytes from your modem to your screen may result in overruns. However,
more than 16 (with no gaps) can get thru without overruns if the
timing is right. For example, suppose a burst of 32 bytes is sent
into the port from the external cable. The polling might just happen
after the first 16 bytes came in so it would pick up these 16 bytes
OK. Then there would be space for the next 16 bytes so that entire 32
bytes gets thru OK. While this scenario is not very likely, similar
cases where 17 to 31 bytes make thru are more likely. But it's even
more likely that only an occasional 16-byte chunk will get thru with
possible loss of data.
If you have an obsolete serial port with only a 1-byte buffer (or it's
been incorrectly set to work like a 1-byte buffer) then the situation
will be much worse than described above and only one character will
occasionally make it thru the port. Every character received causes
an overrun (and is lost) except for the last character received. This
character is likely to be just a line-feed since this is often the
last character to be transmitted in a burst of characters sent to your
screen. Thus you may type AT<return> to the modem but never see AT on
the screen. All you see several seconds later is that the cursor
drops down one line (a line feed). This has happened to me with a
16-byte FIFO buffer that was behaving like a 1-byte buffer.
When a communication program starts up, it expects interrupts to be
working. It's not geared to using this slow polling-like mode of
operation. Thus all sorts of mistakes may be made such as setting up
the serial port and/or modem incorrectly. It may fail to realize when
a connection has been made. If a script is being used for login, it
may fail (caused by timeout) due to the polling delays.
14.4. Interrupt Conflicts
When two devices have the same IRQ number it's called sharing
interrupts. Under some conditions this sharing works out OK.
Starting with kernel version 2.2, ISA serial ports may, if the
hardware is designed for this, share interrupts with other serial
ports. Devices on the PCI bus may share the same IRQ interrupt with
other devices on the PCI bus (provided the software supports this).
In other cases where there is potential for conflict, there should be
no problem if no two devices with the same IRQ are ever "in use" at
the same time. More precisely, "in use" really means "open" (in
programmer jargon). In cases other than the exceptions mentioned
above (unless special software and hardware permit sharing), sharing
is not allowed and conflicts arise if sharing is attempted.
Even if two processes with conflicting IRQs run at the same time, one
of the devices will likely have its interrupts caught by its device
driver and may work OK. The other device will not have its interrupts
caught by the correct driver and will likely behave just like a
process with mis-set interrupts. See ``Mis-set Interrupts'' for more
details.
14.5. Resolving Interrupt Problems
If you are getting a very slow response as described above, then one
test is to change the IRQ to 0 (uses fast polling instead of
interrupts) and see if the problem goes away. Note that the polling
due to IRQ=0 is orders of magnitude faster than the slow "polling" due
to bad interrupts. If IRQ=0 seems to fix the problem, then there was
likely something wrong with the interrupts. Using IRQ=0 is very
resource intensive and is only a temporary fix. You should try to
find the cause of the interrupt problem and not permanently use IRQ=0.
Check /proc/interrupts to see if the IRQ is currently in use by
another process. If it's in use by another serial port you could try
"top" (type f and then enable the TTY display) or "ps -e" to find out
which serial ports are in use. If you suspect that setserial has a
wrong IRQ then see ``What is the current IO address and IRQ of my
Serial Port ?''
15. What Are UARTs? How Do They Affect Performance?
15.1. Introduction to UARTS
UARTs (Universal Asynchronous Receiver Transmitter) are serial chips
on your PC motherboard (or on an internal modem card). The UART
function may also be done on a chip that does other things as well.
On older computers like many 486's, the chips were on the disk IO
controller card. Still older computer have dedicated serial boards.
The UART's purpose is to convert bytes from the PC's parallel bus to a
serial bit-stream. The cable going out of the serial port is serial
and has only one wire for each direction of flow. The serial port
sends out a stream of bits, one bit at a time. Conversely, the bit
stream that enters the serial port via the external cable is converted
to parallel bytes that the computer can understand. 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 its 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 (parallel) byte and puts it
in a buffer.
Along with converting between serial and parallel, the UART does some
other things as a byproduct (side effect) of its primary task. The
voltage used to represent bits is also converted (changed). Extra
bits (called start and stop bits) are added to each byte before it is
transmitted. See the Serial-HOWTO section, ``Voltage Waveshapes'' for
details. Also, while the flow rate (in bytes/sec) on the parallel bus
inside the computer is very high, the flow rate out the UART on the
serial port side of it is much lower. The UART has a fixed set of
rates (speeds) which it can use at its serial port interface.
15.2. Two Types of UARTs
There are two basic types of UARTs: dumb UARTS and FIFO UARTS. Dumb
UARTs are the 8250, 16450, early 16550, and early 16650. They are
obsolete but if you understand how they work it's easy to understand
how the modern ones work with FIFO UARTS ( late 16550, 16550A, 16c552,
late 16650, 16750, and 16C950).
There is some confusion regarding 16550. Early models had a bug and
worked properly only as 16450's (no FIFO). Later models with the bug
fixed were named 16550A but many manufacturers did not accept the name
change and continued calling it a 16550. Most all 16550's in use
today are like 16550A's. Linux will report it as being a 16550A even
though your hardware manual (or a label note) says it's a 16550. A
similar situation exists for the 16650 (only it's worse since the
manufacturer allegedly didn't admit anything was wrong). Linux will
report a late 16650 as being a 16650V2. If it reports it as 16650 it
is bad news and only is used as if it had a one-byte buffer.
15.3. FIFOs
To understand the differences between dumb and FIFO (First In, First
Out queue discipline) 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 passing thru it, it just receives and sends it. For the obsolete
dumb 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 obsolete 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 transfer 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 adequately 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. This is called an "overrun" or "overflow".
FIFO UARTs help solve this problem. The 16550A (or 16550) FIFO chip
comes with 16 byte FIFO buffers. This means that it can receive up to
14 bytes (or send 16 bytes) before it has to interrupt the CPU. Not
only can it wait for more bytes, but the CPU then can transfer all (14
to 16) bytes at a time. This is a significant advantage over the
obsolete UARTs, which only had 1 byte buffers. The CPU receives less
interrupts, and is free to do other things. Data is rarely lost.
Note that the interrupt threshold of FIFO buffers (trigger level) may
be set at less than 14. 1, 4 and 8 are other possible choices. As of
late 2000 there was no way the Linux user could set these directly
(setserial can't do it). While many PC's only have a 16550 with
16-byte buffers, better UARTS have even larger buffers.
Note that the interrupt is issued slightly before the buffer gets full
(at say a "trigger level" of 14 bytes for a 16-byte buffer). This
allows room for a couple more bytes to be received before the
interrupt service routine is able to actually fetch all these bytes.
The trigger level may be set to various permitted values by kernel
software. A trigger level of 1 will be almost like an obsolete UART
(except that it still has room for 15 more bytes after it issues the
interrupt).
Now consider the case where you're on the Internet. It's just sent
you a short webpage of text. All of this came in thru the serial
port. If you had a 16-byte buffer on the serial port which held back
characters until it had 14 of them, some of the last several
characters on the screen might be missing as the FIFO buffer waited to
get the 14th character. But the 14th character doesn't arrive since
you've been sent the entire page (over the phone line) and there are
no more characters to send to you. It could be that these last
characters are part of the HTML formatting, etc. and are not
characters to display on the screen but you don't want to lose format
either.
There is a "timeout" to prevent the above problem. The "timeout"
works like this for the receive UART buffer: If characters arrive one
after another, then an interrupt is issued only when say the 14th
character reaches the buffer. But if a character arrives and the next
character doesn't arrive soon thereafter, then an interrupt is issued
anyway. This results in fetching all of the characters in the FIFO
buffer, even if only a few (or only one) are present. There is also
"timeout" for the transmit buffer as well.
15.4. Why FIFO Buffers are Small
You may wonder why the FIFO buffers are not larger. After all, memory
is cheap and it wouldn't cost much more to use buffers in the kilo-
byte range. The reason is flow control. Flow control stops the flow
of data (bytes) on serial line when necessary. If a stop signal is
sent to serial port, then the stop request is handled by software
(even if the flow control is "hardware"). The serial port hardware
knows nothing about flow control.
If the serial port buffer contains 64 bytes ready to send when it
receives a flow control signal to stop sending, it will send out the
64 bytes anyway in violation of the stop request. There is no
stopping it since it doesn't know about flow control. If the buffer
was large, then many more bytes would be sent in violation of flow
control's request to stop.
15.5. UART Model Numbers
Here's a list of UARTs. TL is Trigger Level
� 8250, 16450, early 16550: Obsolete with 1-byte buffers
� 16550, 16550A, 16c552: 16-byte buffers, TL=1,4,8,14
� 16650: 32-byte buffers. Speed up to 460.8 kbps
� 16750: 64-byte buffer for send, 56-byte for receive. Speed up to
921.6 kbps
� Hayes ESP: 1k-byte buffers.
The obsolete ones are only good for modems no higher than 14.4k (DTE
speeds up to 38400 bps). For modern modems you need at least a 16550
(and not an early 16550). For V.90 56k modems, it may be a several
percent faster with a 16650 (especially if you are downloading large
uncompressed files). The main advantage of the 16650 is its larger
buffer size as the extra speed isn't needed unless the modem
compression ratio is high. Some 56k internal modems may come with a
16650 ??
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 IO.
Many 486 PCs (old) and all Pentiums (or the like) should have 16550As
(usually called just 16550's) with FIFOs. Some better motherboards
today (2000) even have 16650s. For replacing obsolete UARTs with
newer ones in pre 1990 hardware see the Appendix: Obsolete ...
16. Pinout and Signals
16.1. Pinout
PINOUT of the SERIAL PORT (--> direction is out of PC)
(Note DCD is sometimes labeled CD)
Pin # Pin # Acronym Full-Name Direction What-it-May-Do/Mean
9-pin 25-pin
3 2 TxD Transmit Data --> Transmits bytes out of PC
2 3 RxD Receive Data <-- Receives bytes into PC
7 4 RTS Request To Send --> RTS/CTS flow control
8 5 CTS Clear To Send <-- RTS/CTS flow control
6 6 DSR Data Set Ready <-- I'm ready to communicate
4 20 DTR Data Terminal Ready--> I'm ready to communicate
1 8 DCD Data Carrier Detect<-- Modem connected to another
9 22 RI Ring Indicator <-- Telephone line ringing
5 7 SG Signal Ground
16.2. Signals May Have No Fixed Meaning
Only 3 of the 9 pins have a fixed assignment: transmit, receive and
signal ground. This is fixed by the hardware and you can't change it.
But the other signal lines are controlled by software and may do (and
mean) almost anything at all. However they can only be in one of two
states: asserted (+12 volts) or negated (-12 volts). Asserted is "on"
and negated is "off". For example, Linux software may command that
DTR be negated and the hardware only carries out this command and puts
-12 volts on the DTR pin. A modem (or other device) that receives
this DTR signal may do various things. If a modem has been configured
a certain way it will hang up the telephone line when DTR is negated.
In other cases it may ignore this signal or do something else when DTR
is negated (turned off).
It's like this for all the 6 signal lines. The hardware only sends
and receives the signals, but what action (if any) they perform is up
to the Linux software and the configuration/design of devices that you
connect to the serial port. However, most pins have certain functions
which they normally perform but this may vary with the operating
system and the device driver configuration. Under Linux, one may
modify the source code to make these signal lines behave differently
(some people have).
16.3. Cabling Between Serial Ports
A cable from a serial port always connects to another serial port. An
external modem or other device that connects to the serial port has a
serial port built into it. For modems, the cable is always straight
thru: pin 2 goes to pin 2, etc. The modem is said to be DCE (Data
Communications Equipment) and the computer is said to be DTE (Data
Terminal Equipment). Thus for connecting DTE-to-DCE you use straight-
thru cable. For connecting DTE-to-DTE you must use a null-modem cable
(also called a crossover cable). There are many ways to wire such
cable (see examples in Text-Terminal-HOWTO subsection: "Direct Cable
Connection")
There are good reasons why it works this way. One reason is that the
signals are unidirectional. If pin 2 sends a signal out of it (but is
unable to receive any signal) then obviously you can't connect it to
pin 2 of the same type of device. If you did, they would both send
out signals on the same wire to each other but neither would be able
to receive any signal. There are two ways to deal with this
situation. One way is to have a two different types of equipment
where pin 2 of the first type sends the signal to pin 2 of the second
type (which receives the signal). That's the way it's done when you
connect a PC (DTE) to a modem (DCE). There's a second way to do this
without having two different types of equipment: Connect pin sending
pin 2 to a receiving pin 3 on same type of equipment. That's the way
it's done when you connect 2 PCs together or a PC to a terminal (DTE-
to-DTE). The cable used for this is called a null-modem cable since
it connects two PCs without use of a modem. A null-modem cable may
also be called a cross-over cable since the wires between pins 2 and 3
cross over each other (if you draw them on a sheet of paper). The
above example is for a 25 pin connector but for a 9-pin connector the
pin numbers are just the opposite.
The serial pin designations were originally intended for connecting a
dumb terminal to a modem. The terminal was DTE (Data Terminal
Equipment) and the modem was DCE (Data Communication Equipment).
Today the PC is usually used as DTE instead of a terminal (but real
terminals may still be used this way). The names of the pins are the
same on both DTE and DCE. The words: "receive" and "transmit" are
from the "point of view" of the PC (DTE). The transmit pin from the
PC transmits to the "transmit" pin of the modem (but actually the
modem is receiving the data from this pin so from the point of view of
the modem it would be a receive pin).
The serial port was originally intended to be used for connecting DTE
to DCE which makes cabling simple: just use a straight-thru cable.
Thus when one connects a modem one seldom needs to worry about which
pin is which. But people wanted to connect DTE to DTE (for example a
computer to a terminal) and various ways were found to do this by
fabricating various types of special null-modem cables. In this case
what pin connects to what pin becomes significant.
16.4. RTS/CTS and DTR/DSR Flow Control
This is "hardware" flow control. Flow control was previously
explained in the ``Flow Control'' subsection but the pins and voltage
signals were not. Linux only supports RTS/CTS flow control at present
(but a special driver may exist for a specific application which
supports DTR/DSR flow control). Only RTS/CTS flow control will be
discussed since DTR/DSR flow control works the same way. To get
RTS/CTS flow control one needs to either select hardware flow control
in an application program or use the command:
stty crtscts < /dev/ttyS2 (or the like). This enables RTS/CTS
hardware flow control in the Linux device driver.
Then when a DTE (such as a PC) wants to stop the flow into it, it
negates RTS. Negated "Request To Send" (-12 volts) means "request NOT
to send to me" (stop sending). When the PC is ready for more bytes it
asserts RTS (+12 volts) and the flow of bytes to it resumes. Flow
control signals are always sent in a direction opposite to the flow of
bytes that is being controlled. DCE equipment (modems) works the same
way but sends the stop signal out the CTS pin. Thus it's RTS/CTS flow
control using 2 lines.
On what pins is this stop signal received? That depends on whether we
have a DCE-DTE connection or a DTE-DTE connection. For DCE-DTE it's a
straight-thru connection so obviously the signal is received on a pin
with the same name as the pin it's sent out from. It's RTS-->RTS (PC
to modem) and CTS<--CTS (modem to PC). For DTE-to-DTE the connection
is also easy to figure out. The RTS pin always sends and the CTS pin
always receives. Assume that we connect two PCs (PC1 and PC2)
together via their serial ports. Then it's RTS(PC1)-->CTS(PC2) and
CTS(PC1)<--RTS(PC2). In other words RTS and CTS cross over. Such a
cable (with other signals crossed over as well) is called a "null
modem" cable. See ``Cabling Between Serial Ports''
What is sometimes confusing is that there is the original use of RTS
where it means about the opposite of the previous explanation above.
This original meaning is: I Request To Send to you. This request was
intended to be sent from a terminal (or computer) to a modem which, if
it decided to grant the request, would send back an asserted CTS from
its CTS pin to the CTS pin of the computer: You are Cleared To Send to
me. Note that in contrast to the modern RTS/CTS bi-directional flow
control, this only protects the flow in one direction: from the
computer (or terminal) to the modem. This original use appears to be
little used today on modern equipment (including modems).
16.4.1. The DTR and DSR Pins
Just like RTS and CTS, these pins are paired. For DTE-to-DTE
connections they are likely to cross over. There are two ways to use
these pins. One way is to use them as a substitute for RTS/CTS flow
control. The DTR pin is just like the RTS pin while the DSR pin
behaves like the CTS pin. Although Linux doesn't support DTR/DSR flow
control, it can be obtained by connecting the RTS/CTS pins at the PC
to the DSR/DTR pins at the device that uses DTR/DSR flow control. DTR
flow control is the same as DTR/DSR flow control but it's only one-way
and only uses the DTR pin at the device. Many text terminals and some
printers use DTR/DSR (or just DTR) flow control. In the future, Linux
may support DTR/DSR flow control. The software has already been
written but it's not clear when (or if) it will incorporated into the
serial driver.
The normal use of DTR and DSR (not for flow control) is as follows: A
device asserting DTR says that its powered on and ready to operate.
For a modem, the meaning of a DTR signal from the PC depends on how
the modem is configured. Negating DTR is sometimes called "hanging
up" but it doesn't always do this. One way to "hang up" (negate DTR)
is to set the baud rate to 0 using the command "stty 0". Trying to do
this from a "foreign" terminal may not work due to the two-interface
problem. See ``Two interfaces at a terminal''. For internal modem-
serial_ports it worked OK with a port using minicom but didn't work if
the port was using wvdial. Why?
16.5. Preventing a Port From Opening
If "stty -clocal" (or getty is used with the "local" flag negated)
then a serial port can't open until DCD gets an assert (+12 volts)
signal.
17. Voltage Waveshapes
17.1. Voltage for a Bit
At the EIA-232 serial port, voltages are bipolar (positive or negative
with respect to ground) and should be about 12 volts in magnitude
(some are 5 or 10 volts). For the transmit and receive pins +12
volts is a 0-bit (sometimes called "space") and -12 volts is a 1-bit
(sometimes called "mark"). This is known as inverted logic since
normally a 0-bit is both false and negative while a one is normally
both true and positive. Although the receive and transmit pins are
inverted logic, other pins (modem control lines) are normal logic with
a positive voltage being true (or "on" or "asserted") and a negative
voltage being false (or "off" or "negated"). Zero voltage has no
meaning (except it usually means that the unit is powered off).
A range of voltages is allowed. The specs say the magnitude of a
transmitted signal should be between 5 and 15 volts but must never
exceed 25 V. Any voltage received under 3 V is undefined (but some
devices will accept a lower voltage as valid). One sometimes sees
erroneous claims that the voltage is commonly 5 volts (or even 3
volts) but it's usually 11-12 volts. If you are using a EIA-422 port
on a Mac computer as an EIA-232 (requires a special cable) or EIA-423
then the voltage will actually be only 5 V. The discussion here
assumes 12 V.
Note that normal computer logic normally is just a few volts (5 volts
was once the standard) so that if you try to use test equipment
designed for testing 3-5 volt computer logic (TTL) on the 12 volts of
a serial port, it may damage the test equipment.
17.2. Voltage Sequence for a Byte
The transmit pin (TxD) is held at -12 V (mark) at idle when nothing is
being sent. To start a byte it jumps to +12 V (space) for the start
bit and remains at +12 V for the duration (period) of the start bit.
Next comes the low-order bit of the data byte. If it's a 0-bit
nothing changes and the line remains at +12 V for another bit-period.
If it's a 1-bit the voltage jumps from +12 to -12 V. After that comes
the next bit (-12 V if a 1 or +12 V if a 0), etc., etc. After the
last data bit a parity bit may be sent and then a -12 V (mark) stop
bit. Then the line remains at -12 V (idle) until the next start bit.
Note that there is no return to 0 volts and thus there is no simple
way (except by a synchronizing signal) to tell where one bit ends and
the next one begins for the case where 2 consecutive bits are the same
polarity (both zero or both one).
A 2nd stop bit would also be -12 V, just the same as the first stop
bit. Since there is no signal to mark the boundaries between these
bits, the only effect of the 2nd stop bit is that the line must remain
at -12 V idle twice as long. The receiver has no way of detecting the
difference between a 2nd stop bit and a longer idle time between
bytes. Thus communications works OK if one end uses one stop bit and
the other end uses 2 stop bits, but using only one stop bit is
obviously faster. In rare cases 1 1/2 stop bits are used. This means
that the line is kept at -12 V for 1 1/2 time periods (like a stop bit
50% wider than normal).
17.3. Parity Explained
Characters are normally transmitted with either 7 or 8 bits of data.
An additional parity bit may (or may not) be appended to this
resulting in a byte length of 7, 8 or 9 bits. Some terminal emulators
and older terminals do not allow 9 bits. Some prohibit 9 bits if 2
stop bits are used (since this would make the total number of bits too
large: 12 bits total after adding the start bit).
The parity may be set to odd, even or none (mark and space parity may
be options on some terminals or other serial devices). With odd
parity, the parity bit is selected so that the number of 1-bits in a
byte, including the parity bit, is odd. If a such a byte gets
corrupted by a bit being flipped, the result is an illegal byte of
even parity. This error will be detected and if it's an incoming byte
to the terminal an error-character symbol will appear on the screen.
Even parity works in a similar manner with all legal bytes (including
the parity bit) having an even number of 1-bits. During set-up, the
number of bits per character usually means only the number of data
bits per byte (7 for true ASCII and 8 for various ISO character sets).
A "mark" is a 1-bit (or logic 1) and a "space" is a 0-bit (or logic
0). For mark parity, the parity bit is always a one-bit. For space
parity it's always a zero-bit. Mark or space parity (also known as
"sticky parity") only wastes bandwidth and should be avoided if
feasible. The stty command can't set sticky parity but it's supported
by serial hardware and can be dealt with by programming in C. "No
parity" means that no parity bit is added. For terminals that don't
permit 9 bit bytes, "no parity" must be selected when using 8 bit
character sets since there is no room for a parity bit.
17.4. Forming a Byte (Framing)
In serial transmission of bytes via EIA-232 ports, the low-order bit
is always sent first. Serial ports on PC's use asynchronous
communication where there is a start bit and a stop bit to mark the
beginning and end of a byte. This is called framing and the framed
byte is sometimes called a frame. As a result a total of 9, 10, or 11
bits are sent per byte with 10 being the most common. 8-N-1 means 8
data bits, No parity, 1 stop bit. This adds up to 10 bits total when
one counts the start bit. One stop bit is almost universally used.
At 110 bits/sec (and sometimes at 300 bits/sec) 2 stop bits were once
used but today the 2nd stop bit is used only in very unusual
situations (or by mistake since it still works OK that way but wastes
bandwidth).
Don't confuse this type of framing with the framing used for a packet
of bytes on a network. The serial port just frames every byte. For a
network many bytes are framed into a packet (sometimes called a
frame). For a network frame, instead of a start bit, there is a
sequence of bytes called a header. On a network that uses serial
ports (with modems), a report of a frame error usually refers to a
multi-byte frame and not the serial port frame of a single byte.
17.5. How "Asynchronous" is Synchronized
The EIA-232 serial port as implemented on PC is asynchronous which in
effect means that there is no "clock" signal sent with "ticks" to mark
when each bit is sent.. There are only two states of the transmit (or
receive) wire: mark (-12 V) or space (+12 V). There is no state of 0
V. Thus a sequence of 1-bits is transmitted by just a steady -12 V
with no markers of any kind between bits. For the receiver to detect
individual bits it must always have a clock signal which is in
synchronization with the transmitter clock. Such a clock would
generate a "tick" in synchronization with each transmitted (or
received) bit.
For asynchronous transmission, synchronization is achieved by framing
each byte with a start bit and a stop bit (done by hardware). The
receiver listens on the negative line for a positive start bit and
when it detects one it starts its clock ticking. It uses this clock
tick to time the reading of the next 7, 8 or 9 bits. (It actually is
a little more complex than this since several samples of a bit are
normally taken and this requires additional timing ticks.) Then the
stop bit is read, the clock stops and the receiver waits for the next
start bit. Thus async is actually synchronized during the reception
of a single byte but there is no synchronization between one byte and
the next byte.
18. Other Serial Devices (not async EIA-232)
18.1. Successors to EIA-232
A number of EIA standards have been established for higher speeds and
longer distances using twisted-pair (balanced) technology. Balanced
transmission can sometimes be a hundred times faster than unbalanced
EIA-232. For a given speed, the distance (maximum cable length) may
be many times longer with twisted pair. But PC-s keep being made with
the "obsolete" EIA-232 since it works OK with modems and mice since
the cable length is short. If this appears in the latest version of
this HOWTO, please let me know if any of the non-EIA-232 listed below
are supported by Linux.
18.2. EIA-422-A (balanced) and EIA-423-A (unbalanced)
EIA-423 is just like the unbalanced EIA-232 except that the voltage is
only 5 volts. Since this falls within EIA-232 specs it can be
connected to a EIA-232 port. Its specs call for somewhat higher
speeds than the EIA-232 (but this may be of little help on a long run
where it's the unbalance that causes interference). Since EIA-423 is
not much of an improvement over EIA-232, it is not popular except on
old Mac computers.
EIA-422 is twisted pair (known as "balanced" or "differential) and is
(per specs) exactly 100 times as fast as EIA-423 (which in turn is
somewhat faster than EIA-232). Apple's Mac computer prior to mid-1998
with its EIA-232/EIA-422 Port uses it. The Mac used a small round
"mini-DIN-8" connector. It also provided conventional EIA-232 but at
only at 5 volts (which is still legal EIA-232). To make it work like
at EIA-232 one must use a special cable which (signal) grounds RxD+
(one side of a balanced pair) and use RxD- as the receive pin. While
TxD- is used as the transmit pin, for some reason TxD+ should not be
grounded. See Macintosh Communications FAQ
<
http://www.modemshop.com/csm-comm-faq.html>. However, due to the
fact that Macs (and upgrades for them) cost more than PC's, they are
not widely as host computers for Linux.
18.3. EIA-485
This is like EIA-422 (balanced = differential). It is half-duplex.
It's not just point-to-point but is like ethernet or the USB since all
devices (nodes) on it share the same "bus". It may be used for a
multidrop LAN (up to 32 nodes or more). Since many nodes share the
same twisted pair the need to use the electrical tri-state mode where
besides the 0 and 1 binary states there is also an open circuit state
to permit other nodes to uses the twisted pair line. Instead of a
transmitter keeping a 1-state voltage on the line during line idle,
the line is open circuited and all nodes just listen (receive mode).
The most common architecture is master/slave. The master polls the
slaves to see if they have anything to send. A slave can only
transmit just after it's been polled.
There is an alternative implementation where two pair of wires are
used for sending data. One pair is only for the Master to send to the
Slaves. Since no one transmits on this line except the master, there
is no need for it to be tri-state. Thus the Master may just be
EIA-232 but the slaves must still be EIA-485. See
<
http://www.hw.cz/english/docs/rs485/rs485.html> for more details.
18.4. EIA-530
EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s
(balanced) was intended to be a replacement for EIA-232 but few have
been installed. It uses the same 25-pin connector as EIA-232.
18.5. EIA-612/613
The High Speed Serial Interface ( HSSI = EIA-612/613) uses a 50-pin
connector and goes up to about 50 Mbits/s but the distance is limited
to only several meters. For Linux there are PCI cards supporting
HSSI. The companies that sell the cards often provide (or point you
to) a Linux driver. A howto or the like is needed for this topic.
18.6. The Universal Serial Bus (USB)
The Universal Serial Bus (USB) is being built into PCI chips. Newer
PC's have them. It is 12 Mbps (with 200 Mbps planned) over a twisted
pair with a 4-pin connector (2 wires are power supply). It also is
limited to short distances of at most 5 meters (depends on
configuration). Linux supports the bus, although not all devices that
can plug into the bus are supported.
It is synchronous and transmits in special packets like a network.
Just like a network, it can have several devices attached to it. Each
device on it gets a time-slice of exclusive use for a short time. A
device can also be guaranteed the use of the bus at fixed intervals.
One device can monopolize it if no other device wants to use it. It's
not simple to describe in detail.
For documentation, see the USB directory in /usr/share/doc/kernel ...
It would be nice to have a HOWTO on the USB. See also
<
http://www.linux-usb.org> and/or <
http://.www.qbik.ch/usb/>.
18.7. Firewire
Firewire (IEEE 1394) is something like the USB only faster (800 Mbps
is planned). The protocol on the bus is claimed to be more efficient
than USB's. It uses two twisted pair for data plus two power
conductors (6 conductors in all). A variants uses only 4 conductors.
You may compile firewire support into the Linux kernel. Like USB,
it's also limited to short distances.
18.8. Synchronization & Synchronous
Beside the asynchronous EIA-232 (and others) there are a number of
synchronous serial port standards. In fact EIA-232 includes
synchronous specifications but they aren't normally implemented for
serial ports on PC's. But first we'll explain what a synchronous
means.
18.8.1. Defining Asynchronous vs Synchronous
Asynchronous (async) means "not synchronous". In practice, an async
signal is what the async serial port sends and receives which is a
stream of bytes with each byte framed by a start and stop bit.
Synchronous (sync) is most everything else. But this doesn't explain
the basic concepts.
In theory, synchronous means that bytes are sent out at a constant
rate one after another in step with a clock signal tick. There is
often a separate wire or channel for sending the clock signal. The
clock signal might also be embedded in the transmitted bytes.
Asynchronous bytes may be sent out erratically with various time
intervals between bytes (like someone typing characters at a
keyboard).
When a file is being sent thru the async serial port, the flow of
bytes will likely be at the speed of the port (say 115.2k) which is a
constant rate. This flow may frequently start and stop due to flow
control. Is this sync or async? Ignoring the flow control stops, it
might seem like sync since it's a steady flow. But it's not because
there is no clock signal and the bytes could have been sent
erratically since they are framed by start/stop bits.
Another case is where data bytes (without any start-stop bits) are put
into packets with possible erratic spacing between one packet and the
next. This is called sync since the bytes within each packet are
transmitted synchronously.
18.8.2. Synchronous Communication
Did you ever wonder what all the unused pins are for on a 25-pin
connector for the serial port? Most of them are for use in
synchronous communication which is seldom implemented in chips for
PC's. There are pins for sync timing signals as well as for a sync
reverse channel. The EIA-232 spec provides for both sync and async
but PC's use a UART (Universal Asynchronous Receiver/Transmitter) chip
such as a 16450, 16550A, or 16650 and can't deal with sync. For sync
one needs a USRT chip or the equivalent where the "S" stands for
Synchronous. A USART chip supports both synchronous and asynchronous.
Since sync is a niche market, a sync serial port is likely to be quite
expensive.
SCC stands for "Serial Communication Controller" or "Serial Controller
Chip". It's likely old terminology and since it doesn't say "sync" or
"async" it might support both.
Besides the sync part of the EIA-232, there are various other EIA
synchronous standards. For EIA-232, 3 pins of the connector are
reserved for clock (or timing) signals. Sometimes it's a modem's task
to generate some timing signals making it impossible to use
synchronous communications without a synchronous modem (or without a
device called a "synchronous modem eliminator" which provides the
timing signals).
Although few serial ports are sync, synchronous communication does
often take place over telephone lines using modems which use V.42
error correction. This strips off the start/stop bits and puts the
data bytes in packets resulting in synchronous operation over the
phone line.
19. Other Sources of Information
19.1. Books
1. Axleson, Jan: Serial Port Complete, Lakeview Research, Madison, WI,
1998.
2. Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
Computer Society Press, Los Alamitos, CA, 1996.
3. Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.
4. Campbell, Joe: C Programmer's Guide to Serial Communications, 2nd
ed., Unknown Publisher, 1993.
5. Levine, Donald: POSIX Programmer's Guide
<
http://www.ora.com/catalog/posix/>, O'Reilly, 1991.
6. Nelson, Mark: Serial Communications Developer's Guide, 2nd ed.,
Hungry Minds, 2000.
7. Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.
8. Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall, 1991.
9. Stevens, Richard W.: Advanced Programming in the UNIX Environment
<
http://heg-
school.aw.com/cseng/authors/stevens/advanced/advanced.nclk>, (ISBN
0-201-56317-7; Addison-Wesley)
10.
Tischert, Michael & Bruno Jennrich: PC Intern, Abacus 1996.
Chapter 7: Serial Ports
Notes re books:
1. "... Complete" has hardware details (including register) but the
programming aspect is Window oriented.
2. "Physical Layer ..." covers much more than just EIA-232.
19.2. Serial Software
It's best to use the nearest mirror site, but here's the main sites:
Serial Software <
ftp://metalab.unc.edu/pub/Linux/system/serial/> for
Linux software for the serial ports including getty and port monitors.
Serial Communications
<
ftp://metalab.unc.edu/pub/Linux/apps/serialcomm> for communication
programs.
� irqtune will give serial port interrupts higher priority to improve
performance. Using hdparm for hard-disk tuning may help some more.
� modemstat and statserial show the current state of various modem
control lines. See ``Serial Monitoring/Diagnostics''
19.3. Related Linux Documents
� man pages for: setserial and stty
� Low-Level Terminal Interface
<www.gnu.org/manual/glibc/html_chapter/libc_12.html> part of "GNU C
Library Reference manual" (in libc (or glibc) docs package). It
covers the detailed meaning of "stty" commands, etc.
� Modem-HOWTO: modems on the serial port
� PPP-HOWTO: help with PPP (using a modem on the serial port)
� Printing-HOWTO: for setting up a serial printer
� Serial-Programming-HOWTO: for some aspects of serial-port
programming
� 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
19.4. Usenet newsgroups:
� comp.os.linux.answers
� 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.
19.5. Serial Mailing List
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.
19.6. Internet
� Linux Serial Driver home page <
http://serial.sourceforge.net/>
Includes info about PCI support.
� 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. He also has a Serial-
Programming-HOWTO (not yet available from the Linux Documentation
Project). Your browser should automatically log you in but if you
do it manually login as "anonymous" and use your full e-mail
address as the password.
� A white paper discussing serial communications and multiport serial
boards was available from Cyclades at
http://www.cyclades.com.
20. Appendix: Obsolete Hardware (prior to 1990) Info
20.1. Replacing obsolete UARTS
Many 486 PCs (old) and all Pentiums (or the like) should have modern
16550As (usually called just 16550's) with FIFOs. If you have
something really old the chip may unplug so that you may be able to
upgrade by buying a 16550A chip and replacing your existing 16450
UART. If the functionality has been built into 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 new serial card on
the Internet (few retail stores stock them today) or find a used one.
END OF Serial-HOWTO
