22.2. Introduction

22.2.1. Terminology


Bits per Second - the rate at which data is transmitted


Data Terminal Equipment - for example, your computer


Data Communications Equipment - your modem


EIA standard for hardware serial communications

When talking about communications data rates, this section does not use the term "baud". Baud refers to the number of electrical state transitions that may be made in a period of time, while "bps" (bits per second) is the correct term to use (at least it does not seem to bother the curmudgeons quite as much).

22.2.2. Cables and Ports

To connect a modem or terminal to your FreeBSD system, you will need a serial port on your computer and the proper cable to connect to your serial device. If you are already familiar with your hardware and the cable it requires, you can safely skip this section. Cables

There are several different kinds of serial cables. The two most common types for our purposes are null-modem cables and standard ("straight") RS-232 cables. The documentation for your hardware should describe the type of cable required. Null-modem Cables

A null-modem cable passes some signals, such as "Signal Ground", straight through, but switches other signals. For example, the "Transmitted Data" pin on one end goes to the "Received Data" pin on the other end.

You can also construct your own null-modem cable for use with terminals (e.g., for quality purposes). This table shows the RS-232C signals and the pin numbers on a DB-25 connector. Note that the standard also calls for a straight-through pin 1 to pin 1 Protective Ground line, but it is often omitted. Some terminals work OK using only pins 2, 3 and 7, while others require different configurations than the examples shown below.

Tabela 22.1. DB-25 to DB-25 Null-Modem Cable
SignalPin # Pin #Signal
SG7connects to7SG
TD2connects to3RD
RD3connects to2TD
RTS4connects to5CTS
CTS5connects to4RTS
DTR20connects to6DSR
DTR20connects to8DCD
DSR6connects to20DTR
DCD8connects to20DTR

Here are two other schemes more common nowadays.

Tabela 22.2. DB-9 to DB-9 Null-Modem Cable
SignalPin # Pin #Signal
RD2connects to3TD
TD3connects to2RD
DTR4connects to6DSR
DTR4connects to1DCD
SG5connects to5SG
DSR6connects to4DTR
DCD1connects to4DTR
RTS7connects to8CTS
CTS8connects to7RTS

Tabela 22.3. DB-9 to DB-25 Null-Modem Cable
SignalPin # Pin #Signal
RD2connects to2TD
TD3connects to3RD
DTR4connects to6DSR
DTR4connects to8DCD
SG5connects to7SG
DSR6connects to20DTR
DCD1connects to20DTR
RTS7connects to5CTS
CTS8connects to4RTS


When one pin at one end connects to a pair of pins at the other end, it is usually implemented with one short wire between the pair of pins in their connector and a long wire to the other single pin.

The above designs seems to be the most popular. In another variation (explained in the book RS-232 Made Easy) SG connects to SG, TD connects to RD, RTS and CTS connect to DCD, DTR connects to DSR, and vice-versa. Standard RS-232C Cables

A standard serial cable passes all of the RS-232C signals straight through. That is, the "Transmitted Data" pin on one end of the cable goes to the "Transmitted Data" pin on the other end. This is the type of cable to use to connect a modem to your FreeBSD system, and is also appropriate for some terminals. Ports

Serial ports are the devices through which data is transferred between the FreeBSD host computer and the terminal. This section describes the kinds of ports that exist and how they are addressed in FreeBSD. Kinds of Ports

Several kinds of serial ports exist. Before you purchase or construct a cable, you need to make sure it will fit the ports on your terminal and on the FreeBSD system.

Most terminals will have DB-25 ports. Personal computers, including PCs running FreeBSD, will have DB-25 or DB-9 ports. If you have a multiport serial card for your PC, you may have RJ-12 or RJ-45 ports.

See the documentation that accompanied the hardware for specifications on the kind of port in use. A visual inspection of the port often works too. Port Names

In FreeBSD, you access each serial port through an entry in the /dev directory. There are two different kinds of entries:

  • Call-in ports are named /dev/ttydN where N is the port number, starting from zero. Generally, you use the call-in port for terminals. Call-in ports require that the serial line assert the data carrier detect (DCD) signal to work correctly.

  • Call-out ports are named /dev/cuadN. You usually do not use the call-out port for terminals, just for modems. You may use the call-out port if the serial cable or the terminal does not support the carrier detect signal.


    Call-out ports are named /dev/cuaaN in FreeBSD 5.X and older.

If you have connected a terminal to the first serial port (COM1 in MS-DOS(R)), then you will use /dev/ttyd0 to refer to the terminal. If the terminal is on the second serial port (also known as COM2), use /dev/ttyd1, and so forth.

22.2.3. Kernel Configuration

FreeBSD supports four serial ports by default. In the MS-DOS(R) world, these are known as COM1, COM2, COM3, and COM4. FreeBSD currently supports "dumb" multiport serial interface cards, such as the BocaBoard 1008 and 2016, as well as more intelligent multi-port cards such as those made by Digiboard and Stallion Technologies. However, the default kernel only looks for the standard COM ports.

To see if your kernel recognizes any of your serial ports, watch for messages while the kernel is booting, or use the /sbin/dmesg command to replay the kernel's boot messages. In particular, look for messages that start with the characters sio.


To view just the messages that have the word sio, use the command:

# /sbin/dmesg | grep 'sio'

For example, on a system with four serial ports, these are the serial-port specific kernel boot messages:

sio0 at 0x3f8-0x3ff irq 4 on isa
sio0: type 16550A
sio1 at 0x2f8-0x2ff irq 3 on isa
sio1: type 16550A
sio2 at 0x3e8-0x3ef irq 5 on isa
sio2: type 16550A
sio3 at 0x2e8-0x2ef irq 9 on isa
sio3: type 16550A

If your kernel does not recognize all of your serial ports, you will probably need to configure your kernel in the /boot/device.hints file. You can also comment-out or completely remove lines for devices you do not have.

Please refer to the sio(4) manual page for more information on serial ports and multiport boards configuration. Be careful if you are using a configuration file that was previously used for a different version of FreeBSD because the device flags and the syntax have changed between versions.


port IO_COM1 is a substitution for port 0x3f8, IO_COM2 is 0x2f8, IO_COM3 is 0x3e8, and IO_COM4 is 0x2e8, which are fairly common port addresses for their respective serial ports; interrupts 4, 3, 5, and 9 are fairly common interrupt request lines. Also note that regular serial ports cannot share interrupts on ISA-bus PCs (multiport boards have on-board electronics that allow all the 16550A's on the board to share one or two interrupt request lines).

22.2.4. Device Special Files

Most devices in the kernel are accessed through "device special files", which are located in the /dev directory. The sio devices are accessed through the /dev/ttydN (dial-in) and /dev/cuadN (call-out) devices. FreeBSD also provides initialization devices (/dev/ttydN.init and /dev/cuadN.init on FreeBSD 6.X, /dev/ttyidN and /dev/cuaiaN on FreeBSD 5.X) and locking devices (/dev/ttydN.lock and /dev/cuadN.lock on FreeBSD 6.X, /dev/ttyldN and /dev/cualaN on FreeBSD 5.X). The initialization devices are used to initialize communications port parameters each time a port is opened, such as crtscts for modems which use RTS/CTS signaling for flow control. The locking devices are used to lock flags on ports to prevent users or programs changing certain parameters; see the manual pages termios(4), sio(4), and stty(1) for information on the terminal settings, locking and initializing devices, and setting terminal options, respectively.

22.2.5. Serial Port Configuration

The ttydN (or cuadN) device is the regular device you will want to open for your applications. When a process opens the device, it will have a default set of terminal I/O settings. You can see these settings with the command

# stty -a -f /dev/ttyd1

When you change the settings to this device, the settings are in effect until the device is closed. When it is reopened, it goes back to the default set. To make changes to the default set, you can open and adjust the settings of the "initial state" device. For example, to turn on CLOCAL mode, 8 bit communication, and XON/XOFF flow control by default for ttyd5, type:

# stty -f /dev/ttyd5.init clocal cs8 ixon ixoff

System-wide initialization of the serial devices is controlled in /etc/rc.d/serial. This file affects the default settings of serial devices.

To prevent certain settings from being changed by an application, make adjustments to the "lock state" device. For example, to lock the speed of ttyd5 to 57600 bps, type:

# stty -f /dev/ttyd5.lock 57600

Now, an application that opens ttyd5 and tries to change the speed of the port will be stuck with 57600 bps.

Naturally, you should make the initial state and lock state devices writable only by the root account.

All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.