Serial Programming Guide for POSIX Operating Systems.

Serial Programming Guide for POSIX

Operating Systems

5th Edition

Michael R. Sweet

Copyright 1994-1999, All Rights Reserved.

Table of Contents

Serial Programming Guide for POSIX Operating Systems.................................................................1

Introduction..........................................................................................................................................2

RXD - Received Data.............................................................................................................4

DCD - Data Carrier Detect.....................................................................................................4

DTR - Data Terminal Ready...................................................................................................4

CTS - Clear To Send...............................................................................................................4

RTS - Request To Send...........................................................................................................4

Asynchronous Communications......................................................................................................5

What Are Full Duplex and Half Duplex?....................................................................................5

Flow Control...............................................................................................................................5

What Is a Break?.........................................................................................................................6

Synchronous Communications........................................................................................................6

Closing a Serial Port...................................................................................................................8

Chapter 2, Configuring the Serial Port.................................................................................................8

The POSIX Terminal Interface........................................................................................................8

Control Options...........................................................................................................................9

Setting the Baud Rate...........................................................................................................10

Setting the Character Size....................................................................................................11

Setting Parity Checking........................................................................................................11

Setting Hardware Flow Control...........................................................................................12

Local Options............................................................................................................................12

Choosing Canonical Input....................................................................................................12

Choosing Processed Output..................................................................................................15

Choosing Raw Output..........................................................................................................15

Control Characters....................................................................................................................15

Setting Software Flow Control Characters...........................................................................16

Setting Read Timeouts..........................................................................................................16

Chapter 3, MODEM Communications...............................................................................................16

What Is a MODEM?......................................................................................................................16

Communicating With a MODEM..................................................................................................16

Standard MODEM Commands.................................................................................................18

ATD - Dial A Number..........................................................................................................18

ATZ - Reset MODEM..........................................................................................................18

Getting the Number of Bytes Available....................................................................................20

Selecting Input from a Serial Port..................................................................................................21

The SELECT System Call........................................................................................................21

Using the SELECT System Call...............................................................................................21

Using SELECT with the X Intrinsics Library...........................................................................22

Appendix A, Pinouts...........................................................................................................................22

RS-232 Pinouts..............................................................................................................................23

RS-422 Pinouts..............................................................................................................................23

RS-574 (IBM PC/AT) Pinouts.......................................................................................................24

SGI Pinouts....................................................................................................................................24

functions and should work with very few modifications under IRIX®, HP-UX, SunOS®, Solaris®,

Digital UNIX®, Linux®, and most other UNIX operating systems. The biggest difference between

operating systems that you will find is the filenames used for serial port device and lock files.

This guide is organized into the following chapters and appendices:

MODEMs, and terminals.

When doing serial communications each word (i.e. byte or character) of data you send or receive is

sent one bit at a time. Each bit is either on or off. The terms you'll hear sometimes are mark for the

on state and space for the off state.

The speed of the serial data is most often expressed as bits-per-second ("bps") or baudot rate

("baud"). This just represents the number of ones and zeroes that can be sent in one second. Back at

the dawn of the computer age, 300 baud was considered fast, but today computers can handle RS-

232 speeds as high as 430,800 baud! When the baud rate exceeds 1,000, you'll usually see the rate

shown in kilo baud, or kbps (e.g. 9.6k, 19.2k, etc). For rates above 1,000,000 that rate is shown in

megabaud, or Mbps (e.g. 1.5Mbps).

Industries Association ("EIA"). RS-232 actually comes in 3 different flavors (A, B, and C) with

each one defining a different voltage range for the on and off levels. The most commonly used

variety is RS-232C, which defines a mark (on) bit as a voltage between -3V and -12V and a space

(off) bit as a voltage between +3V and +12V. The RS-232C specification says these signals can go

about 25 feet (8m) before they become unusable. You can usually send signals a bit farther than this

as long as the baud is low enough.

Besides wires for incoming and outgoing data, there are others that provide timing, status, and

handshaking:

TXD -

Data

GND -

Ground

Secondary

DCD

Receiver

Clock

RXD -

Received

Data

8

DCD - Data

Carrier

Detect

13

Secondary

CTS

18 Unassigned 23

Data Rate

Select

9 Reserved 14

19

24

DTR - Data

Ready

Two standards for serial interfaces you may also see are RS-422 and RS-574. RS-422 uses lower

voltages and differential signals to allow cable lengths up to about 1000ft (300m). RS-574 defines

the 9-pin PC serial connector and voltages.

Signal Definitions

The RS-232 standard defines some 18 different signals for serial communications. Of these, only

six are generally available in the UNIX environment.

GND - Logic Ground

Technically the logic ground is not a signal, but without it none of the other signals will operate.