Serial-Programming-HOWTO(1).pdf

Serial Programming HOWTO

Gary Frerking

gary@frerking.org

Peter Baumann

Revision History

Revision 1.01

2001−08−26

Revised by: glf

New maintainer, converted to DocBook

Revision 1.0

1998−01−22

Revised by: phb

Initial document release

This document describes how to program communications with devices over a serial port on a Linux box.

Serial Programming HOWTO

Table of Contents

1. Introduction. ....................................................................................................................................................1

1.1. Copyright Information. .....................................................................................................................1

1.2. Disclaimer. ........................................................................................................................................1

1.3. New Versions. ...................................................................................................................................2

1.4. Credits. ..............................................................................................................................................2

1.5. Feedback. ..........................................................................................................................................2

2. Getting started. ................................................................................................................................................3

2.1. Debugging. ........................................................................................................................................3

2.2. Port Settings. .....................................................................................................................................3

2.3. Input Concepts for Serial Devices. ...................................................................................................4

2.3.1. Canonical Input Processing. .............................................................................................4

2.3.2. Non−Canonical Input Processing. ....................................................................................4

2.3.3. Asynchronous Input. .........................................................................................................4

2.3.4. Waiting for Input from Multiple Sources. ........................................................................4

3. Program Examples.. ........................................................................................................................................6

3.1. Canonical Input Processing. ..............................................................................................................6

3.2. Non−Canonical Input Processing .....................................................................................................8

3.3. Asynchronous Input. .........................................................................................................................9

3.4. Waiting for Input from Multiple Sources. ......................................................................................11

4. Other Sources of Information. .....................................................................................................................13

1. Introduction

This is the Linux Serial Programming HOWTO. All about how to program communications with other

devices / computers over a serial line under Linux. Different techniques are explained: Canonical I/O (only

complete lines are transmitted/received), asyncronous I/O, and waiting for input from multiple sources.

This is the first update to the initial release of the Linux Serial Programming HOWTO. The primary purpose

of this update is to change the author information and convert the document to DocBook format. In terms of

technical content, very little if anything has changed at this time. Sweeping changes to the technical content

aren't going to happen overnight, but I'll work on it as much as time allows.

If you've been waiting in the wings for someone to take over this HOWTO, you've gotten your wish. Please

send me any and all feedback you have, it'd be very much appreciated.

All examples were tested using a i386 Linux Kernel 2.0.29.

1.1. Copyright Information

terms of the Linux Documentation Project (LDP) license, stated below.

Unless otherwise stated, Linux HOWTO documents are copyrighted by their respective authors. Linux

HOWTO documents may be reproduced and distributed in whole or in part, in any medium physical or

electronic, as long as this copyright notice is retained on all copies. Commercial redistribution is allowed and

encouraged; however, the author would like to be notified of any such distributions.

All translations, derivative works, or aggregate works incorporating any Linux HOWTO documents must be

covered under this copyright notice. That is, you may not produce a derivative work from a HOWTO and

impose additional restrictions on its distribution. Exceptions to these rules may be granted under certain

conditions; please contact the Linux HOWTO coordinator at the address given below.

In short, we wish to promote dissemination of this information through as many channels as possible.

However, we do wish to retain copyright on the HOWTO documents, and would like to be notified of any

plans to redistribute the HOWTOs.

If you have any questions, please contact < linux−howto@metalab.unc.edu >

1.2. Disclaimer

No liability for the contents of this documents can be accepted. Use the concepts, examples and other content

at your own risk. As this is a new edition of this document, there may be errors and inaccuracies, that may of

course be damaging to your system. Proceed with caution, and although this is highly unlikely, the author(s)

do not take any responsibility for that.

All copyrights are held by their by their respective owners, unless specifically noted otherwise. Use of a term

in this document should not be regarded as affecting the validity of any trademark or service mark.

1. Introduction

Serial Programming HOWTO

Naming of particular products or brands should not be seen as endorsements.

You are strongly recommended to take a backup of your system before major installation and backups at

regular intervals.

1.3. New Versions

As previously mentioned, not much is new in terms of technical content yet.

1.4. Credits

The original author thanked Mr. Strudthoff, Michael Carter, Peter Waltenberg, Antonino Ianella, Greg

Hankins, Dave Pfaltzgraff, Sean Lincolne, Michael Wiedmann, and Adrey Bonar.

1.5. Feedback

Feedback is most certainly welcome for this document. Without your submissions and input, this document

wouldn't exist. Please send your additions, comments and criticisms to the following email address :

< gary@frerking.org > .

1.3. New Versions

2. Getting started

2.1. Debugging

The best way to debug your code is to set up another Linux box, and connect the two computers via a

null−modem cable. Use miniterm (available from the LDP programmers guide

( ftp://sunsite.unc.edu/pub/Linux/docs/LDP/programmers−guide/lpg−0.4.tar.gz in

the examples directory) to transmit characters to your Linux box. Miniterm can be compiled very easily and

will transmit all keyboard input raw over the serial port. Only the define statement #define

MODEMDEVICE "/dev/ttyS0" has to be checked. Set it to ttyS0 for COM1, ttyS1 for COM2, etc.. It

is essential for testing, that all characters are transmitted raw (without output processing) over the line. To

test your connection, start miniterm on both computers and just type away. The characters input on one

computer should appear on the other computer and vice versa. The input will not be echoed to the attached

screen.

To make a null−modem cable you have to cross the TxD (transmit) and RxD (receive) lines. For a description

of a cable see sect. 7 of the Serial−HOWTO.

It is also possible to perform this testing with only one computer, if you have two unused serial ports. You

can then run two miniterms off two virtual consoles. If you free a serial port by disconnecting the mouse,

remember to redirect /dev/mouse if it exists. If you use a multiport serial card, be sure to configure it

correctly. I had mine configured wrong and everything worked fine as long as I was testing only on my

computer. When I connected to another computer, the port started loosing characters. Executing two

programs on one computer just isn't fully asynchronous.

2.2. Port Settings

The devices /dev/ttyS* are intended to hook up terminals to your Linux box, and are configured for this

use after startup. This has to be kept in mind when programming communication with a raw device. E.g. the

ports are configured to echo characters sent from the device back to it, which normally has to be changed for

data transmission.

All parameters can be easily configured from within a program. The configuration is stored in a structure

struct termios , which is defined in <asm/termbits.h> :

#define NCCS 19

struct termios {

tcflag_t c_iflag; /* input mode flags */

tcflag_t c_oflag; /* output mode flags */

tcflag_t c_cflag; /* control mode flags */

tcflag_t c_lflag; /* local mode flags */

cc_t c_line; /* line discipline */

cc_t c_cc[NCCS]; /* control characters */

};

This file also includes all flag definitions. The input mode flags in c_iflag handle all input processing,

which means that the characters sent from the device can be processed before they are read with read .

Similarly c_oflag handles the output processing. c_cflag contains the settings for the port, as the

2. Getting started

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