Serial Programming Guide
POSIX Operating Systems

5th Edition, 3rd Revision
Copyright 1994-2003 by Michael R. Sweet

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in Appendix C, GNU Free Documentation License.

Table of Contents

Introduction Chapter 1, Basics of Serial Communications Chapter 2, Configuring the Serial Port Chapter 3, MODEM Communications Chapter 4, Advanced Serial Programming Appendix A, Pinouts Appendix B, ASCII Control Codes Appendix C, GNU Free Documentation License

Appendix D, Change History


The Serial Programming Guide for POSIX Operating Systems will teach you how to successfully, efficiently, and portably program the serial ports on your UNIX® workstation or PC. Each chapter provides programming examples that use the POSIX (Portable Standard for UNIX) terminal control 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.


Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in Appendix C, GNU Free Documentation License.


This guide is organized into the following chapters and appendices:

Chapter 1, Basics of Serial Communications

This chapter introduces serial communications, RS-232 and other standards that are used on most computers as well as how to access a serial port from a C program.

What Are Serial Communications?

Computers transfer information (data) one or more bits at a time. Serial refers to the transfer of data one bit at a time. Serial communications include most network devices, keyboards, mice, 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).

When referring to serial devices or ports, they are either labeled as Data Communications Equipment ("DCE") or Data Terminal Equipment ("DTE"). The difference between these is simple - every signal pair, like transmit and receive, is swapped. When connecting two DTE or two DCE interfaces together, a serial null-MODEM cable or adapter is used that swaps the signal pairs.

What Is RS-232?

RS-232 is a standard electrical interface for serial communications defined by the Electronic 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:

Table 1 - RS-232 Pin Assignments
Pin Description Pin Description Pin Description Pin Description Pin Description
1 Earth Ground 6 DSR - Data Set Ready 11 Unassigned 16 Secondary RXD 21 Signal Quality Detect
2 TXD - Transmitted Data 7 GND - Logic Ground 12 Secondary DCD 17 Receiver Clock 22 Ring Detect
3 RXD - Received Data 8 DCD - Data Carrier Detect 13 Secondary CTS 18 Unassigned 23 Data Rate Select
4 RTS - Request To Send 9 Reserved 14 Secondary TXD 19 Secondary RTS 24 Transmit Clock
5 CTS - Clear To Send 10 Reserved 15 Transmit Clock 20 DTR - Data Terminal Ready 25 Unassigned

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. Basically, the logic ground acts as a reference voltage so that the electronics know which voltages are positive or negative.

TXD - Transmitted Data

The TXD signal carries data transmitted from your workstation to the computer or device on the other end (like a MODEM). A mark voltage is interpreted as a value of 1, while a space voltage is interpreted as a value of 0.

RXD - Received Data

The RXD signal carries data transmitted from the computer or device on the other end to your workstation. Like TXD, mark and space voltages are interpreted as 1 and 0, respectively.

DCD - Data Carrier Detect

The DCD signal is received from the computer or device on the other end of your serial cable. A space voltage on this signal line indicates that the computer or device is currently connected or on line. DCD is not always used or available.

DTR - Data Terminal Ready

The DTR signal is generated by your workstation and tells the computer or device on the other end that you are ready (a space voltage) or not-ready (a mark voltage). DTR is usually enabled automatically whenever you open the serial interface on the workstation.

CTS - Clear To Send

The CTS signal is received from the other end of the serial cable. A space voltage indicates that is alright to send more serial data from your workstation.

CTS is usually used to regulate the flow of serial data from your workstation to the other end.

RTS - Request To Send

The RTS signal is set to the space voltage by your workstation to indicate that more data is ready to be sent.

Like CTS, RTS helps to regulate the flow of data between your workstation and the computer or device on the other end of the serial cable. Most workstations leave this signal set to the space voltage all the time.

Asynchronous Communications

For the computer to understand the serial data coming into it, it needs some way to determine where one character ends and the next begins. This guide deals exclusively with asynchronous serial data.

In asynchronous mode the serial data line stays in the mark (1) state until a character is transmitted. A start bit preceeds each character and is followed immediately by each bit in the character, an optional parity bit, and one or more stop bits. The start bit is always a space (0) and tells the computer that new serial data is available. Data can be sent or received at any time, thus the name asynchronous.

Figure 1 - Asynchronous Data Transmission

The optional parity bit is a simple sum of the data bits indicating whether or not the data contains an even or odd number of 1 bits. With even parity, the parity bit is 0 if there is an even number of 1's in the character. With odd parity, the parity bit is 0 if there is an odd number of 1's in the data. You may also hear the terms space parity, mark parity, and no parity. Space parity means that the parity bit is always 0, while mark parity means the bit is always 1. No parity means that no parity bit is present or transmitted.

The remaining bits are called stop bits. There can be 1, 1.5, or 2 stop bits between characters and they always have a value of 1. Stop bits traditionally were used to give the computer time to process the previous character, but now only serve to synchronize the receiving computer to the incoming characters.

Asynchronous data formats are usually expressed as "8N1", "7E1", and so forth. These stand for "8 data bits, no parity, 1 stop bit" and "7 data bits, even parity, 1 stop bit" respectively.

What Are Full Duplex and Half Duplex?

Full duplex means that the computer can send and receive data simultaneously - there are two separate data channels (one coming in, one going out).

Half duplex means that the computer cannot send or receive data at the same time. Usually this means there is only a single data channel to talk over. This does not mean that any of the RS-232 signals are not used. Rather, it usually means that the communications link uses some standard other than RS-232 that does not support full duplex operation.

Flow Control

It is often necessary to regulate the flow of data when transferring data between two serial interfaces. This can be due to limitations in an intermediate serial communications link, one of the serial interfaces, or some storage media. Two methods are commonly used for asynchronous data.

The first method is often called "software" flow control and uses special characters to start (XON or DC1, 021 octal) or stop (XOFF or DC3, 023 octal) the flow of data. These characters are defined in the American Standard Code for Information Interchange ("ASCII"). While these codes are useful when transferring textual information, they cannot be used when transferring other types of information without special programming.

The second method is called "hardware" flow control and uses the RS-232 CTS and RTS signals instead of special characters. The receiver sets CTS to the space voltage when it is ready to receive more data and to the mark voltage when it is not ready. Likewise, the sender sets RTS to the space voltage when it is ready to send more data. Because hardware flow control uses a separate set of signals, it is much faster than software flow control which needs to send or receive multiple bits of information to do the same thing. CTS/RTS flow control is not supported by all hardware or operating systems.

What Is a Break?

Normally a receive or transmit data signal stays at the mark voltage until a new character is transferred. If the signal is dropped to the space voltage for a long period of time, usually 1/4 to 1/2 second, then a break condition is said to exist.

A break is sometimes used to reset a communications line or change the operating mode of communications hardware like a MODEM. Chapter 3, Talking to MODEMs covers these applications in more depth.

Synchronous Communications

Unlike asynchronous data, synchronous data appears as a constant stream of bits. To read the data on the line, the computer must provide or receive a common bit clock so that both the sender and receiver are synchronized.

Even with this synchronization, the computer must mark the beginning of the data somehow. The most common way of doing this is to use a data packet protocol like Serial Data Link Control ("SDLC") or High-Speed Data Link Control ("HDLC").

Each protocol defines certain bit sequences to represent the beginning and end of a data packet. Each also defines a bit sequence that is used when there is no data. These bit sequences allow the computer see the beginning of a data packet.

Because synchronous protocols do not use per-character synchronization bits they typically provide at least a 25% improvement in performance over asynchronous communications and are suitable for remote networking and configurations with more than two serial interfaces.

Despite the speed advantages of synchronous communications, most RS-232 hardware does not support it due to the extra hardware and software required.

Accessing Serial Ports

Like all devices, UNIX provides access to serial ports via device files. To access a serial port you simply open the corresponding device file.

Serial Port Files

Each serial port on a UNIX system has one or more device files (files in the /dev directory) associated with it:

Table 2 - Serial Port Device Files
System Port 1 Port 2
IRIX® /dev/ttyf1 /dev/ttyf2
HP-UX /dev/tty1p0 /dev/tty2p0
Solaris®/SunOS® /dev/ttya /dev/ttyb
Linux® /dev/ttyS0 /dev/ttyS1
Digital UNIX® /dev/tty01 /dev/tty02

Opening a Serial Port

Since a serial port is a file, the open(2) function is used to access it. The one hitch with UNIX is that device files are usually not accessable by normal users. Workarounds include changing the access permissions to the file(s) in question, running your program as the super-user (root), or making your program set-userid so that it runs as the owner of the device file (not recommended for obvious security reasons...)

For now we'll assume that the file is accessable by all users. The code to open serial port 1 on a PC running Linux is show in Listing 1.

Listing 1 - Opening a serial port.

    #include <stdio.h>   /* Standard input/output definitions */
    #include <string.h>  /* String function definitions */
    #include <unistd.h>  /* UNIX standard function definitions */
    #include <fcntl.h>   /* File control definitions */
    #include <errno.h>   /* Error number definitions */
    #include <termios.h> /* POSIX terminal control definitions */

     * 'open_port()' - Open serial port 1.
     * Returns the file descriptor on success or -1 on error.

      int fd; /* File descriptor for the port */

      fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
      if (fd == -1)
	* Could not open the port.

	perror("open_port: Unable to open /dev/ttyS0 - ");
	fcntl(fd, F_SETFL, 0);

      return (fd);

Other systems would require the corresponding device file name, but otherwise the code is the same.

Open Options

You'll notice that when we opened the device file we used two other flags along with the read+write mode:

    fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);

The O_NOCTTY flag tells UNIX that this program doesn't want to be the "controlling terminal" for that port. If you don't specify this then any input (such as keyboard abort signals and so forth) will affect your process. Programs like getty(1M/8) use this feature when starting the login process, but normally a user program does not want this behavior.

The O_NDELAY flag tells UNIX that this program doesn't care what state the DCD signal line is in - whether the other end of the port is up and running. If you do not specify this flag, your process will be put to sleep until the DCD signal line is the space voltage.

Writing Data to the Port

Writing data to the port is easy - just use the write(2) system call to send data it:

    n = write(fd, "ATZ\r", 4);
    if (n < 0)
      fputs("write() of 4 bytes failed!\n", stderr);

The write function returns the number of bytes sent or -1 if an error occurred. Usually the only error you'll run into is EIO when a MODEM or data link drops the Data Carrier Detect (DCD) line. This condition will persist until you close the port.

Reading Data from the Port

Reading data from a port is a little trickier. When you operate the port in raw data mode, each read(2) system call will return however many characters are actually available in the serial input buffers. If no characters are available, the call will block (wait) until characters come in, an interval timer expires, or an error occurs. The read function can be made to return immediately by doing the following:

    fcntl(fd, F_SETFL, FNDELAY);

The FNDELAY option causes the read function to return 0 if no characters are available on the port. To restore normal (blocking) behavior, call fcntl() without the FNDELAY option:

    fcntl(fd, F_SETFL, 0);

This is also used after opening a serial port with the O_NDELAY option.

Closing a Serial Port

To close the serial port, just use the close system call:


Closing a serial port will also usually set the DTR signal low which causes most MODEMs to hang up.

Chapter 2, Configuring the Serial Port

This chapter discusses how to configure a serial port from C using the POSIX termios interface.

The POSIX Terminal Interface

Most systems support the POSIX terminal (serial) interface for changing parameters such as baud rate, character size, and so on. The first thing you need to do is include the file <termios.h>; this defines the terminal control structure as well as the POSIX control functions.

The two most important POSIX functions are tcgetattr(3) and tcsetattr(3). These get and set terminal attributes, respectively; you provide a pointer to a termios structure that contains all of the serial options available:

Table 3 - Termios Structure Members
Member Description
c_cflag Control options
c_lflag Line options
c_iflag Input options
c_oflag Output options
c_cc Control characters
c_ispeed Input baud (new interface)
c_ospeed Output baud (new interface)

Control Options

The c_cflag member controls the baud rate, number of data bits, parity, stop bits, and hardware flow control. There are constants for all of the supported configurations.
Table 4 - Constants for the c_cflag Member
Constant Description
CBAUD Bit mask for baud rate
B0 0 baud (drop DTR)
B50 50 baud
B75 75 baud
B110 110 baud
B134 134.5 baud
B150 150 baud
B200 200 baud
B300 300 baud
B600 600 baud
B1200 1200 baud
B1800 1800 baud
B2400 2400 baud
B4800 4800 baud
B9600 9600 baud
B19200 19200 baud
B38400 38400 baud
B57600 57,600 baud
B76800 76,800 baud
B115200 115,200 baud
EXTA External rate clock
EXTB External rate clock
CSIZE Bit mask for data bits
CS5 5 data bits
CS6 6 data bits
CS7 7 data bits
CS8 8 data bits
CSTOPB 2 stop bits (1 otherwise)
CREAD Enable receiver
PARENB Enable parity bit
PARODD Use odd parity instead of even
HUPCL Hangup (drop DTR) on last close
CLOCAL Local line - do not change "owner" of port
LOBLK Block job control output
Enable hardware flow control (not supported on all platforms)

The c_cflag member contains two options that should always be enabled, CLOCAL and CREAD. These will ensure that your program does not become the 'owner' of the port subject to sporatic job control and hangup signals, and also that the serial interface driver will read incoming data bytes.

The baud rate constants (CBAUD, B9600, etc.) are used for older interfaces that lack the c_ispeed and c_ospeed members. See the next section for information on the POSIX functions used to set the baud rate.

Never initialize the c_cflag (or any other flag) member directly; you should always use the bitwise AND, OR, and NOT operators to set or clear bits in the members. Different operating system versions (and even patches) can and do use the bits differently, so using the bitwise operators will prevent you from clobbering a bit flag that is needed in a newer serial driver.

Setting the Baud Rate

The baud rate is stored in different places depending on the operating system. Older interfaces store the baud rate in the c_cflag member using one of the baud rate constants in table 4, while newer implementations provide the c_ispeed and c_ospeed members that contain the actual baud rate value.

The cfsetospeed(3) and cfsetispeed(3) functions are provided to set the baud rate in the termios structure regardless of the underlying operating system interface. Typically you'd use the code in Listing 2 to set the baud rate.

Listing 2 - Setting the baud rate.

    struct termios options;

     * Get the current options for the port...

    tcgetattr(fd, &options);

     * Set the baud rates to 19200...

    cfsetispeed(&options, B19200);
    cfsetospeed(&options, B19200);

     * Enable the receiver and set local mode...

    options.c_cflag |= (CLOCAL | CREAD);

     * Set the new options for the port...

    tcsetattr(fd, TCSANOW, &options);

The tcgetattr(3) function fills the termios structure you provide with the current serial port configuration. After we set the baud rates and enable local mode and serial data receipt, we select the new configuration using tcsetattr(3). The TCSANOW constant specifies that all changes should occur immediately without waiting for output data to finish sending or input data to finish receiving. There are other constants to wait for input and output to finish or to flush the input and output buffers.

Most systems do not support different input and output speeds, so be sure to set both to the same value for maximum portability.

Table 5 - Constants for tcsetattr
Constant Description
TCSANOW Make changes now without waiting for data to complete
TCSADRAIN Wait until everything has been transmitted
TCSAFLUSH Flush input and output buffers and make the change

Setting the Character Size

Unlike the baud rate, there is no convienience function to set the character size. Instead you must do a little bitmasking to set things up. The character size is specified in bits:

    options.c_cflag &= ~CSIZE; /* Mask the character size bits */
    options.c_cflag |= CS8;    /* Select 8 data bits */

Setting Parity Checking

Like the character size you must manually set the parity enable and parity type bits. UNIX serial drivers support even, odd, and no parity bit generation. Space parity can be simulated with clever coding.

Setting Hardware Flow Control

Some versions of UNIX support hardware flow control using the CTS (Clear To Send) and RTS (Request To Send) signal lines. If the CNEW_RTSCTS or CRTSCTS constants are defined on your system then hardware flow control is probably supported. Do the following to enable hardware flow control:

    options.c_cflag |= CNEW_RTSCTS;    /* Also called CRTSCTS */

Similarly, to disable hardware flow control:

    options.c_cflag &= ~CNEW_RTSCTS;

Local Options

The local modes member c_lflag controls how input characters are managed by the serial driver. In general you will configure the c_lflag member for canonical or raw input.

Table 6 - Constants for the c_lflag Member
Constant Description
ICANON Enable canonical input (else raw)
XCASE Map uppercase \lowercase (obsolete)
ECHO Enable echoing of input characters
ECHOE Echo erase character as BS-SP-BS
ECHOK Echo NL after kill character
NOFLSH Disable flushing of input buffers after interrupt or quit characters
IEXTEN Enable extended functions
ECHOCTL Echo control characters as ^char and delete as ~?
ECHOPRT Echo erased character as character erased
ECHOKE BS-SP-BS entire line on line kill
FLUSHO Output being flushed
PENDIN Retype pending input at next read or input char
TOSTOP Send SIGTTOU for background output

Choosing Canonical Input

Canonical input is line-oriented. Input characters are put into a buffer which can be edited interactively by the user until a CR (carriage return) or LF (line feed) character is received.

When selecting this mode you normally select the ICANON, ECHO, and ECHOE options:

    options.c_lflag |= (ICANON | ECHO | ECHOE);

Choosing Raw Input

Raw input is unprocessed. Input characters are passed through exactly as they are received, when they are received. Generally you'll deselect the ICANON, ECHO, ECHOE, and ISIG options when using raw input:

    options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);

A Note About Input Echo

Never enable input echo (ECHO, ECHOE) when sending commands to a MODEM or other computer that is echoing characters, as you will generate a feedback loop between the two serial interfaces!

Input Options

The input modes member c_iflag controls any input processing that is done to characters received on the port. Like the c_cflag field, the final value stored in c_iflag is the bitwise OR of the desired options.

Table 7 - Constants for the c_iflag Member
Constant Description
INPCK Enable parity check
IGNPAR Ignore parity errors
PARMRK Mark parity errors
ISTRIP Strip parity bits
IXON Enable software flow control (outgoing)
IXOFF Enable software flow control (incoming)
IXANY Allow any character to start flow again
IGNBRK Ignore break condition
BRKINT Send a SIGINT when a break condition is detected
IUCLC Map uppercase to lowercase
IMAXBEL Echo BEL on input line too long

Setting Input Parity Options

You should enable input parity checking when you have enabled parity in the c_cflag member (PARENB). The revelant constants for input parity checking are INPCK, IGNPAR, PARMRK , and ISTRIP. Generally you will select INPCK and ISTRIP to enable checking and stripping of the parity bit:

    options.c_iflag |= (INPCK | ISTRIP);

IGNPAR is a somewhat dangerous option that tells the serial driver to ignore parity errors and pass the incoming data through as if no errors had occurred. This can be useful for testing the quality of a communications link, but in general is not used for practical reasons.

PARMRK causes parity errors to be 'marked' in the input stream using special characters. If IGNPAR is enabled, a NUL character (000 octal) is sent to your program before every character with a parity error. Otherwise, a DEL (177 octal) and NUL character is sent along with the bad character.

Setting Software Flow Control

Software flow control is enabled using the IXON, IXOFF, and IXANY constants:

    options.c_iflag |= (IXON | IXOFF | IXANY);

To disable software flow control simply mask those bits:

    options.c_iflag &= ~(IXON | IXOFF | IXANY);

The XON (start data) and XOFF (stop data) characters are defined in the c_cc array described below.

Output Options

The c_oflag member contains output filtering options. Like the input modes, you can select processed or raw data output.

Table 8 - Constants for the c_oflag Member
Constant Description
OPOST Postprocess output (not set = raw output)
OLCUC Map lowercase to uppercase
NOCR No CR output at column 0
ONLRET NL performs CR function
OFILL Use fill characters for delay
OFDEL Fill character is DEL
NLDLY Mask for delay time needed between lines
NL0 No delay for NLs
NL1 Delay further output after newline for 100 milliseconds
CRDLY Mask for delay time needed to return carriage to left column
CR0 No delay for CRs
CR1 Delay after CRs depending on current column position
CR2 Delay 100 milliseconds after sending CRs
CR3 Delay 150 milliseconds after sending CRs
TABDLY Mask for delay time needed after TABs
TAB0 No delay for TABs
TAB1 Delay after TABs depending on current column position
TAB2 Delay 100 milliseconds after sending TABs
TAB3 Expand TAB characters to spaces
BSDLY Mask for delay time needed after BSs
BS0 No delay for BSs
BS1 Delay 50 milliseconds after sending BSs
VTDLY Mask for delay time needed after VTs
VT0 No delay for VTs
VT1 Delay 2 seconds after sending VTs
FFDLY Mask for delay time needed after FFs
FF0 No delay for FFs
FF1 Delay 2 seconds after sending FFs

Choosing Processed Output

Processed output is selected by setting the OPOST option in the c_oflag member:

    options.c_oflag |= OPOST;

Of all the different options, you will only probably use the ONLCR option which maps newlines into CR-LF pairs. The rest of the output options are primarily historic and date back to the time when line printers and terminals could not keep up with the serial data stream!

Choosing Raw Output

Raw output is selected by resetting the OPOST option in the c_oflag member:

    options.c_oflag &= ~OPOST;

When the OPOST option is disabled, all other option bits in c_oflag are ignored.

Control Characters

The c_cc character array contains control character definitions as well as timeout parameters. Constants are defined for every element of this array.

Table 9 - Control Characters in the c_cc Member
Constant Description Key
VINTR Interrupt CTRL-C
VERASE Erase Backspace (BS)
VKILL Kill-line CTRL-U
VEOF End-of-file CTRL-D
VEOL End-of-line Carriage return (CR)
VEOL2 Second end-of-line Line feed (LF)
VMIN Minimum number of characters to read -
VTIME Time to wait for data (tenths of seconds) -

Setting Software Flow Control Characters

The VSTART and VSTOP elements of the c_cc array contain the characters used for software flow control. Normally they should be set to DC1 (021 octal) and DC3 (023 octal) which represent the ASCII standard XON and XOFF characters.

Setting Read Timeouts

UNIX serial interface drivers provide the ability to specify character and packet timeouts. Two elements of the c_cc array are used for timeouts: VMIN and VTIME. Timeouts are ignored in canonical input mode or when the NDELAY option is set on the file via open or fcntl.

VMIN specifies the minimum number of characters to read. If it is set to 0, then the VTIME value specifies the time to wait for every character read. Note that this does not mean that a read call for N bytes will wait for N characters to come in. Rather, the timeout will apply to the first character and the read call will return the number of characters immediately available (up to the number you request).

If VMIN is non-zero, VTIME specifies the time to wait for the first character read. If a character is read within the time given, any read will block (wait) until all VMIN characters are read. That is, once the first character is read, the serial interface driver expects to receive an entire packet of characters (VMIN bytes total). If no character is read within the time allowed, then the call to read returns 0. This method allows you to tell the serial driver you need exactly N bytes and any read call will return 0 or N bytes. However, the timeout only applies to the first character read, so if for some reason the driver misses one character inside the N byte packet then the read call could block forever waiting for additional input characters.

VTIME specifies the amount of time to wait for incoming characters in tenths of seconds. If VTIME is set to 0 (the default), reads will block (wait) indefinitely unless the NDELAY option is set on the port with open or fcntl.

Chapter 3, MODEM Communications

This chapter covers the basics of dialup telephone Modulator/Demodulator (MODEM) communications. Examples are provided for MODEMs that use the defacto standard "AT" command set.

What Is a MODEM?

MODEMs are devices that modulate serial data into frequencies that can be transferred over an analog data link such as a telephone line or cable TV connection. A standard telephone MODEM converts serial data into tones that can be passed over the phone lines; because of the speed and complexity of the conversion these tones sound more like loud screeching if you listen to them.

Telephone MODEMs are available today that can transfer data across a telephone line at nearly 53,000 bits per second, or 53kbps. In addition, most MODEMs use data compression technology that can increase the bit rate to well over 100kbps on some types of data.

Communicating With a MODEM

The first step in communicating with a MODEM is to open and configure the port for raw input as shown in Listing 3 .

Listing 3 - Configuring the port for raw input.

    int            fd;
    struct termios options;

    /* open the port */
    fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
    fcntl(fd, F_SETFL, 0);

    /* get the current options */
    tcgetattr(fd, &options);

    /* set raw input, 1 second timeout */
    options.c_cflag     |= (CLOCAL | CREAD);
    options.c_lflag     &= ~(ICANON | ECHO | ECHOE | ISIG);
    options.c_oflag     &= ~OPOST;
    options.c_cc[VMIN]  = 0;
    options.c_cc[VTIME] = 10;

    /* set the options */
    tcsetattr(fd, TCSANOW, &options);

Next you need to establish communications with the MODEM. The best way to do this is by sending the "AT" command to the MODEM. This also allows smart MODEMs to detect the baud you are using. When the MODEM is connected correctly and powered on it will respond with the response "OK".

Listing 4 - Initializing the MODEM.

    int                  /* O - 0 = MODEM ok, -1 = MODEM bad */
    init_modem(int fd)   /* I - Serial port file */
      char buffer[255];  /* Input buffer */
      char *bufptr;      /* Current char in buffer */
      int  nbytes;       /* Number of bytes read */
      int  tries;        /* Number of tries so far */

      for (tries = 0; tries < 3; tries ++)
       /* send an AT command followed by a CR */
	if (write(fd, "AT\r", 3) < 3)

       /* read characters into our string buffer until we get a CR or NL */
	bufptr = buffer;
	while ((nbytes = read(fd, bufptr, buffer + sizeof(buffer) - bufptr - 1)) > 0)
	  bufptr += nbytes;
	  if (bufptr[-1] == '\n' || bufptr[-1] == '\r')

       /* nul terminate the string and see if we got an OK response */
	*bufptr = '\0';

	if (strncmp(buffer, "OK", 2) == 0)
	  return (0);

      return (-1);

Standard MODEM Commands

Most MODEMs support the "AT" command set, so called because each command starts with the "AT" characters. Each command is sent with the "AT" characters starting in the first column followed by the specific command and a carriage return (CR, 015 octal). After processing the command the MODEM will reply with one of several textual messages depending on the command.

ATD - Dial A Number

The ATD command dials the specified number. In addition to numbers and dashes you can specify tone ("T") or pulse ("P") dialing, pause for one second (","), and wait for a dialtone ("W"):

    ATDT 555-1212
    ATDT 18008008008W1234,1,1234
    ATD T555-1212WP1234

The MODEM will reply with one of the following messages:

    CONNECT baud

ATH - Hang Up

The ATH command causes the MODEM to hang up. Since the MODEM must be in "command" mode you probably won't use it during a normal phone call.

Most MODEMs will also hang up if DTR is dropped; you can do this by setting the baud to 0 for at least 1 second. Dropping DTR also returns the MODEM to command mode.

After a successful hang up the MODEM will reply with "NO CARRIER". If the MODEM is still connected the "CONNECT" or "CONNECT baud" message will be sent.


The ATZ command resets the MODEM. The MODEM will reply with the string "OK".

Common MODEM Communication Problems

First and foremost, don't forget to disable input echoing. Input echoing will cause a feedback loop between the MODEM and computer.

Second, when sending MODEM commands you must terminate them with a carriage return (CR) and not a newline (NL). The C character constant for CR is "\r".

Finally, when dealing with a MODEM make sure you use a baud that the MODEM supports. While many MODEMs do auto-baud detection, some have limits (19.2kbps is common on older MODEMs) that you must observe.

Chapter 4, Advanced Serial Programming

This chapter covers advanced serial programming techniques using the ioctl(2) and select(2) system calls.

Serial Port IOCTLs

In Chapter 2, Configuring the Serial Port we used the tcgetattr and tcsetattr functions to configure the serial port. Under UNIX these functions use the ioctl(2) system call to do their magic.

The ioctl system call takes three arguments:

    int ioctl(int fd, int request, ...);

The fd argument specifies the serial port file descriptor. The request argument is a constant defined in the <termios.h> header file and is typically one of the constants listed in Table 10.

Table 10 - IOCTL Requests for Serial Ports
Request Description POSIX Function
TCGETS Gets the current serial port settings. tcgetattr
TCSETS Sets the serial port settings immediately. tcsetattr(fd, TCSANOW, &options)
TCSETSF Sets the serial port settings after flushing the input and output buffers. tcsetattr(fd, TCSAFLUSH, &options)
TCSETSW Sets the serial port settings after allowing the input and output buffers to drain/empty. tcsetattr(fd, TCSADRAIN, &options)
TCSBRK Sends a break for the given time. tcsendbreak, tcdrain
TCXONC Controls software flow control. tcflow
TCFLSH Flushes the input and/or output queue. tcflush
TIOCMGET Returns the state of the "MODEM" bits. None
TIOCMSET Sets the state of the "MODEM" bits. None
FIONREAD Returns the number of bytes in the input buffer. None

Getting the Control Signals

The TIOCMGET ioctl gets the current "MODEM" status bits, which consist of all of the RS-232 signal lines except RXD and TXD, listed in Table 11.

To get the status bits, call ioctl with a pointer to an integer to hold the bits, as shown in Listing 5 .

Listing 5 - Getting the MODEM status bits.

    #include <unistd.h>
    #include <termios.h>

    int fd;
    int status;

    ioctl(fd, TIOCMGET, &status);
Table 11 - Control Signal Constants
Constant Description
TIOCM_LE DSR (data set ready/line enable)
TIOCM_DTR DTR (data terminal ready)
TIOCM_RTS RTS (request to send)
TIOCM_ST Secondary TXD (transmit)
TIOCM_SR Secondary RXD (receive)
TIOCM_CTS CTS (clear to send)
TIOCM_CAR DCD (data carrier detect)
TIOCM_DSR DSR (data set ready)

Setting the Control Signals

The TIOCMSET ioctl sets the "MODEM" status bits defined above. To drop the DTR signal you can use the code in Listing 6.

Listing 6 - Dropping DTR with the TIOCMSET ioctl.

    #include <unistd.h>
    #include <termios.h>

    int fd;
    int status;

    ioctl(fd, TIOCMGET, &status);

    status &= ~TIOCM_DTR;

    ioctl(fd, TIOCMSET, &status);

The bits that can be set depend on the operating system, driver, and modes in use. Consult your operating system documentation for more information.

Getting the Number of Bytes Available

The FIONREAD ioctl gets the number of bytes in the serial port input buffer. As with TIOCMGET you pass in a pointer to an integer to hold the number of bytes, as shown in Listing 7.

Listing 7 - Getting the number of bytes in the input buffer.

    #include <unistd.h>
    #include <termios.h>

    int fd;
    int bytes;

    ioctl(fd, FIONREAD, &bytes);

This can be useful when polling a serial port for data, as your program can determine the number of bytes in the input buffer before attempting a read.

Selecting Input from a Serial Port

While simple applications can poll or wait on data coming from the serial port, most applications are not simple and need to handle input from multiple sources.

UNIX provides this capability through the select(2) system call. This system call allows your program to check for input, output, or error conditions on one or more file descriptors. The file descriptors can point to serial ports, regular files, other devices, pipes, or sockets. You can poll to check for pending input, wait for input indefinitely, or timeout after a specific amount of time, making the select system call extremely flexible.

Most GUI Toolkits provide an interface to select; we will discuss the X Intrinsics ("Xt") library later in this chapter.

The SELECT System Call

The select system call accepts 5 arguments:

    int select(int max_fd, fd_set *input, fd_set *output, fd_set *error,
               struct timeval *timeout);

The max_fd argument specifies the highest numbered file descriptor in the input, output, and error sets. The input, output, and error arguments specify sets of file descriptors for pending input, output, or error conditions; specify NULL to disable monitoring for the corresponding condition. These sets are initialized using three macros:

    FD_SET(fd, fd_set);
    FD_CLR(fd, fd_set);

The FD_ZERO macro clears the set entirely. The FD_SET and FD_CLR macros add and remove a file descriptor from the set, respectively.

The timeout argument specifies a timeout value which consists of seconds (timeout.tv_sec) and microseconds (timeout.tv_usec ). To poll one or more file descriptors, set the seconds and microseconds to zero. To wait indefinitely specify NULL for the timeout pointer.

The select system call returns the number of file descriptors that have a pending condition, or -1 if there was an error.

Using the SELECT System Call

Suppose we are reading data from a serial port and a socket. We want to check for input from either file descriptor, but want to notify the user if no data is seen within 10 seconds. To do this we'll need to use the select system call, as shown in Listing 8.

Listing 8 - Using SELECT to process input from more than one source.

    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/time.h>
    #include <sys/select.h>

    int            n;
    int            socket;
    int            fd;
    int            max_fd;
    fd_set         input;
    struct timeval timeout;

    /* Initialize the input set */
    FD_SET(fd, input);
    FD_SET(socket, input);

    max_fd = (socket > fd ? socket : fd) + 1;

    /* Initialize the timeout structure */
    timeout.tv_sec  = 10;
    timeout.tv_usec = 0;

    /* Do the select */
    n = select(max_fd, &input, NULL, NULL, &timeout);

    /* See if there was an error */
    if (n < 0)
      perror("select failed");
    else if (n == 0)
      /* We have input */
      if (FD_ISSET(fd, input))
      if (FD_ISSET(socket, input))

You'll notice that we first check the return value of the select system call. Values of 0 and -1 yield the appropriate warning and error messages. Values greater than 0 mean that we have data pending on one or more file descriptors.

To determine which file descriptor(s) have pending input, we use the FD_ISSET macro to test the input set for each file descriptor. If the file descriptor flag is set then the condition exists (input pending in this case) and we need to do something.

Using SELECT with the X Intrinsics Library

The X Intrinsics library provides an interface to the select system call via the XtAppAddInput(3x) and XtAppRemoveInput(3x) functions:

int XtAppAddInput(XtAppContext context, int fd, int mask,
                  XtInputProc proc, XtPointer data);
void XtAppRemoveInput(XtAppContext context, int input);

The select system call is used internally to implement timeouts, work procedures, and check for input from the X server. These functions can be used with any Xt-based toolkit including Xaw, Lesstif, and Motif.

The proc argument to XtAppAddInput specifies the function to call when the selected condition (e.g. input available) exists on the file descriptor. In the previous example you could specify the process_fd or process_socket functions.

Because Xt limits your access to the select system call, you'll need to implement timeouts through another mechanism, probably via XtAppAddTimeout(3x).

Appendix A, Pinouts

This appendix provides pinout information for many of the common serial ports you will find.

RS-232 Pinouts

RS-232 comes in three flavors (A, B, C) and uses a 25-pin D-Sub connector:

Figure 2 - RS-232 Connector

Table 12 - RS-232 Signals
Pin Description Pin Description
1 Earth Ground 14 Secondary TXD
2 TXD - Transmitted Data 15 Transmit Clock
3 RXD - Received Data 16 Secondary RXD
4 RTS - Request To Send 17 Receiver Clock
5 CTS - Clear To Send 18 Unassigned
6 DSR - Data Set Ready 19 Secondary RTS
7 GND - Logic Ground 20 DTR - Data Terminal Ready
8 DCD - Data Carrier Detect 21 Signal Quality Detect
9 Reserved 22 Ring Detect
10 Reserved 23 Data Rate Select
11 Unassigned 24 Transmit Clock
12 Secondary DCD 25 Unassigned
13 Secondary CTS

RS-422 Pinouts

RS-422 also uses a 25-pin D-Sub connector, but with differential signals:

Figure 3 - RS-422 Connector

Table 13 - RS-422 Signals
Pin Description Pin Description
1 Earth Ground 14 TXD+
2 TXD- - Transmitted Data 15 Transmit Clock-
3 RXD- - Received Data 16 RXD+
4 RTS- - Request To Send 17 Receiver Clock-
5 CTS- - Clear To Send 18 Unassigned
6 DSR - Data Set Ready 19 RTS+
7 GND - Logic Ground 20 DTR- - Data Terminal Ready
8 DCD- - Data Carrier Detect 21 Signal Quality Detect
9 Reserved 22 Unassigned
10 Reserved 23 DTR+
11 Unassigned 24 Transmit Clock+
12 DCD+ 25 Receiver Clock+
13 CTS+

RS-574 (IBM PC/AT) Pinouts

The RS-574 interface is used exclusively by PC manufacturers and uses a 9-pin male D-Sub connector:

Figure 4 - RS-574 Connector

Table 14 - RS-574 (IBM PC/AT) Signals
Pin Description Pin Description
1 DCD - Data Carrier Detect 6 Data Set Ready
2 RXD - Received Data 7 RTS - Request To Send
3 TXD - Transmitted Data 8 CTS - Clear To Send
4 DTR - Data Terminal Ready 9 Ring Detect
5 GND - Logic Ground

SGI Pinouts

Older SGI equipment uses a 9-pin female D-Sub connector. Unlike RS-574, the SGI pinouts nearly match those of RS-232:

Figure 5 - SGI 9-Pin Connector

Table 15 - SGI 9-Pin DSUB Signals
Pin Description Pin Description
1 Earth Ground 6 DSR - Data Set Ready
2 TXD - Transmitted Data 7 GND - Logic Ground
3 RXD - Received Data 8 DCD - Data Carrier Detect
4 RTS - Request To Send 9 DTR - Data Terminal Ready
5 CTS - Clear To Send

The SGI Indigo, Indigo2, and Indy workstations use the Apple 8-pin MiniDIN connector for their serial ports:

Figure 6 - SGI 8-Pin Connector

Table 16 - SGI 8-Pin MiniDIN Signals
Pin Description Pin Description
1 DTR - Data Terminal Ready 5 RXD - Received Data
2 CTS - Clear To Send 6 RTS - Request To Send
3 TXD - Transmitted Data 7 DCD - Data Carrier Detect
4 GND - Logic Ground 8 GND - Logic Ground

Appendix B, ASCII Control Codes

This chapter lists the ASCII control codes and their names.

Control Codes

The following ASCII characters are used for control purposes:
Table 17 - ASCII Control Codes
Name Binary Octal Decimal Hexadecimal
NUL 00000000 000 0 00
SOH 00000001 001 1 01
STX 00000010 002 2 02
ETX 00000011 003 3 03
EOT 00000100 004 4 04
ENQ 00000101 005 5 05
ACK 00000110 006 6 06
BEL 00000111 007 7 07
BS 00001000 010 8 08
HT 00001001 011 9 09
NL 00001010 012 10 0A
VT 00001011 013 11 0B
NP, FF 00001100 014 12 0C
Name Binary Octal Decimal Hexadecimal
CR 00001101 015 13 0D
SO 00001110 016 14 0E
SI 00001111 017 15 0F
DLE 00010000 020 16 10
XON, DC1 00010001 021 17 11
DC2 00010010 022 18 12
XOFF, DC3 00010011 023 19 13
DC4 00010100 024 20 14
NAK 00010101 025 21 15
SYN 00010110 026 22 16
ETB 00010111 027 23 17
CAN 00011000 030 24 18
EM 00011001 031 25 19
SUB 00011010 032 26 1A
ESC 00011011 033 27 1B
FS 00011100 034 28 1C
GS 00011101 035 29 1D
RS 00011110 036 30 1E
US 00011111 037 31 1F

Appendix C, GNU Free Documentation License

Version 1.2, November 2002

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59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
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This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.


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Appendix D, Change History

This appendix lists the changes made to the document with each revision.

Edition 5, Revision 3

The following changes were made for the 3rd revision: