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.
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 26.1, “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 26.2, “DB-25 to DB-25 Null-Modem Cable”, Table 26.3, “DB-9 to DB-9 Null-Modem Cable”, and Table 26.4, “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.
| Acronyms | Names |
|---|---|
| RD | Received Data |
| TD | Transmitted Data |
| DTR | Data Terminal Ready |
| DSR | Data Set Ready |
| DCD | Data Carrier Detect |
| SG | Signal Ground |
| RTS | Request to Send |
| CTS | Clear to Send |
| Signal | Pin # | Pin # | Signal | |
|---|---|---|---|---|
| SG | 7 | connects to | 7 | SG |
| TD | 2 | connects to | 3 | RD |
| RD | 3 | connects to | 2 | TD |
| RTS | 4 | connects to | 5 | CTS |
| CTS | 5 | connects to | 4 | RTS |
| DTR | 20 | connects to | 6 | DSR |
| DTR | 20 | connects to | 8 | DCD |
| DSR | 6 | connects to | 20 | DTR |
| DCD | 8 | connects to | 20 | DTR |
| Signal | Pin # | Pin # | Signal | |
|---|---|---|---|---|
| RD | 2 | connects to | 3 | TD |
| TD | 3 | connects to | 2 | RD |
| DTR | 4 | connects to | 6 | DSR |
| DTR | 4 | connects to | 1 | DCD |
| SG | 5 | connects to | 5 | SG |
| DSR | 6 | connects to | 4 | DTR |
| DCD | 1 | connects to | 4 | DTR |
| RTS | 7 | connects to | 8 | CTS |
| CTS | 8 | connects to | 7 | RTS |
| Signal | Pin # | Pin # | Signal | |
|---|---|---|---|---|
| RD | 2 | connects to | 2 | TD |
| TD | 3 | connects to | 3 | RD |
| DTR | 4 | connects to | 6 | DSR |
| DTR | 4 | connects to | 8 | DCD |
| SG | 5 | connects to | 7 | SG |
| DSR | 6 | connects to | 20 | DTR |
| DCD | 1 | connects to | 20 | DTR |
| RTS | 7 | connects to | 5 | CTS |
| CTS | 8 | connects to | 4 | RTS |
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/ttyu
where NN 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/cuau
on FreeBSD versions 8.X and higher and
N/dev/cuad
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.N
FreeBSD also provides initialization devices
(/dev/ttyu
and
N.init/dev/cuau
or
N.init/dev/cuad)
and locking devices
(N.init/dev/ttyu
and
N.lock/dev/cuau
or
N.lock/dev/cuad).
The initialization devices are used to initialize
communications port parameters each time a port is opened,
such as N.lockcrtscts 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.
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/
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>.