22.2. Introduction

22.2.1. Terminology

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.

22.2.3. Kernel Configuration

FreeBSD supports four serial ports by default. In the MS-DOS 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.

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.

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.