27.2. Serial Terminology and Hardware

The following terms are often used in serial communications:


Bits per Second (bps) is the rate at which data is transmitted.


Data Terminal Equipment (DTE) is one of two endpoints in a serial communication. An example would be a computer.


Data Communications Equipment (DTE) is the other endpoint in a serial communication. Typically, it is a modem or serial terminal.


The original standard which defined hardware serial communications. It has since been renamed to TIA-232.

When referring to communication data rates, this section does not use the term baud. Baud refers to the number of electrical state transitions made in a period of time, while bps is the correct term to use.

To connect a serial terminal to a FreeBSD system, a serial port on the computer and the proper cable to connect to the serial device are needed. Users who are already familiar with serial hardware and cabling can safely skip this section.

27.2.1. Serial Cables and Ports

There are several different kinds of serial cables. The two most common types are null-modem cables and standard RS-232 cables. The documentation for the hardware should describe the type of cable required.

These two types of cables differ in how the wires are connected to the connector. Each wire represents a signal, with the defined signals summarized in Table 27.1, “RS-232C Signal Names”. A standard serial cable passes all of the RS-232C signals straight through. For example, 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 used to connect a modem to the FreeBSD system, and is also appropriate for some terminals.

A null-modem cable switches the Transmitted Data pin of the connector on one end with the Received Data pin on the other end. The connector can be either a DB-25 or a DB-9.

A null-modem cable can be constructed using the pin connections summarized in Table 27.2, “DB-25 to DB-25 Null-Modem Cable”, Table 27.3, “DB-9 to DB-9 Null-Modem Cable”, and Table 27.4, “DB-9 to DB-25 Null-Modem Cable”. While the standard calls for a straight-through pin 1 to pin 1 Protective Ground line, it is often omitted. Some terminals work using only pins 2, 3, and 7, while others require different configurations. When in doubt, refer to the documentation for the hardware.

Table 27.1. RS-232C Signal Names
RDReceived Data
TDTransmitted Data
DTRData Terminal Ready
DSRData Set Ready
DCDData Carrier Detect
SGSignal Ground
RTSRequest to Send
CTSClear to Send

Table 27.2. 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

Table 27.3. 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

Table 27.4. 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.

Serial ports are the devices through which data is transferred between the FreeBSD host computer and the terminal. Several kinds of serial ports exist. Before purchasing or constructing a cable, make sure it will fit the ports on the terminal and on the FreeBSD system.

Most terminals have DB-25 ports. Personal computers may have DB-25 or DB-9 ports. A multiport serial card may have RJ-12 or RJ-45/ ports. See the documentation that accompanied the hardware for specifications on the kind of port or visually verify the type of port.

In FreeBSD, each serial port is accessed through an entry in /dev. There are two different kinds of entries:

  • Call-in ports are named /dev/ttyuN where N is the port number, starting from zero. If a terminal is connected to the first serial port (COM1), use /dev/ttyu0 to refer to the terminal. If the terminal is on the second serial port (COM2), use /dev/ttyu1, and so forth. Generally, the call-in port is used for terminals. Call-in ports require that the serial line assert the Data Carrier Detect signal to work correctly.

  • Call-out ports are named /dev/cuauN on FreeBSD versions 8.X and higher and /dev/cuadN on FreeBSD versions 7.X and lower. Call-out ports are usually not used for terminals, but are used for modems. The call-out port can be used if the serial cable or the terminal does not support the Data Carrier Detect signal.

FreeBSD also provides initialization devices (/dev/ttyuN.init and /dev/cuauN.init or /dev/cuadN.init) and locking devices (/dev/ttyuN.lock and /dev/cuauN.lock or /dev/cuadN.lock). 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. Refer to termios(4), sio(4), and stty(1) for information on terminal settings, locking and initializing devices, and setting terminal options, respectively.

27.2.2. Serial Port Configuration

By default, FreeBSD supports four serial ports which are commonly known as COM1, COM2, COM3, and COM4. FreeBSD also supports dumb multi-port serial interface cards, such as the BocaBoard 1008 and 2016, as well as more intelligent multi-port cards such as those made by Digiboard. However, the default kernel only looks for the standard COM ports.

To see if the system recognizes the serial ports, look for system boot messages that start with uart:

# grep uart /var/run/dmesg.boot

If the system does not recognize all of the needed serial ports, additional entries can be added to /boot/device.hints. This file already contains hint.uart.0.* entries for COM1 and hint.uart.1.* entries for COM2. When adding a port entry for COM3 use 0x3E8, and for COM4 use 0x2E8. Common IRQ addresses are 5 for COM3 and 9 for COM4.

To determine the default set of terminal I/O settings used by the port, specify its device name. This example determines the settings for the call-in port on COM2:

# stty -a -f /dev/ttyu1

System-wide initialization of serial devices is controlled by /etc/rc.d/serial. This file affects the default settings of serial devices. To change the settings for a device, use stty. By default, the changed settings are in effect until the device is closed and when the device is reopened, it goes back to the default set. To permanently change the default set, open and adjust the settings of the initialization device. For example, to turn on CLOCAL mode, 8 bit communication, and XON/XOFF flow control for ttyu5, type:

# stty -f /dev/ttyu5.init clocal cs8 ixon ixoff

To prevent certain settings from being changed by an application, make adjustments to the locking device. For example, to lock the speed of ttyu5 to 57600 bps, type:

# stty -f /dev/ttyu5.lock 57600

Now, any application that opens ttyu5 and tries to change the speed of the port will be stuck with 57600 bps.

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

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