The /dev/tty{1-15}[a-pA-P]
device files provide access to the standard and optional serial ports
of the computer using a clist-based serial driver
(sio).
Each file corresponds to one of the serial
ports (with or without modem control).
Files are named according to the following conventions:
ISA serial boards are assigned numbers
1 through 4, 1 for the
first COM expansion slot (COM1),
2 for the second (COM2), and so forth.
PCI serial boards are assigned
numbers 5 through 15 and
the numbers are determined by the order in which
the boards are discovered.
Final lowercase letters, a-p, indicate no modem control.
Final uppercase letters, A-P, indicate a line with modem control.
Each port is assigned both a lowercase and uppercase device node.
For example, with a four-port ISA expansion board
installed at COM1
and a single port ISA board installed at COM2,
you can access the following lines:
The device names in the following table refer to the serial ports,
with and without modem control.
The table describes boards installed at COM1
through COM4.
``Minor'' is the minor device number for the port (see
mknod(C)).
Serial lines
Board type
Non-modem control
Modem control
Minor
Name
Minor
Name
COM1
0
tty1a
128
tty1A
COM2
8
tty2a
136
tty2A
COM3
4
tty3a
132
tty3A
COM4
12
tty4a
140
tty4A
A dumb serial port board installed uses interrupt 4 if installed
as COM1, and uses interrupt 3 if installed as COM2.
Access
The files may only be accessed if the corresponding serial
interface card is installed and its I/O
address configured correctly. Also, for multiport expansion
cards, you must use the scoadmin serial
program to create more than the default number of files.
Unless you have added a serial board, only the port
described in your hardware documentation
(typically COM1 and COM2) may be used.
You should also ensure that any ports on the system board
are enabled and configured in the motherboard BIOS.
Serial communication
The serial ports can be used for a variety of serial
communication purposes such as connecting login terminals
to the computer, attaching printers, or forming a serial
network with other computers. Note that a serial port may
operate at most of the standard baud rates, and that the
ports (on most computers and terminals) have a
DTE (Data Terminal Equipment) configuration. The
serial ports on most modems are configured as DCE
(Data Circuit terminating Equipment). The following table
defines how each pin is used for 25-pin and 9-pin
connections (it is recommended that you check that the pin
outs listed here correspond to those on your machine before
attempting to make a connection):
25-pin
9-pin
Description
1
-
Shield
2
3
Transmit Data (TD or Tx; data out on DTE, data in on DCE)
3
2
Receive Data (RD or Rx; data in on DTE, data out on DCE)
4
7
Request To Send (RTS; out on DTE, in on DCE)
5
8
Clear To Send (CTS; in on DTE, out on DCE)
6
6
Data Set Ready (DSR; in on DTE, out on DCE)
7
5
Signal Ground (SG or Gnd)
8
1
Carrier Detect (CD or DCD; in on DTE, out on DCE)
20
4
Data Terminal Ready (DTR; out on DTE, in on DCE)
22
9
Ring Indicator (RI; in on DTE, out on DCE)
When physically connecting two pieces of equipment using a serial
cable, it is necessary to determine:
whether the serial ports are configured as DCE or DTE
what handshaking protocol is to be used: software (XON/XOFF flow
control) or hardware (unidirectional flow control using RTS/CTS or
CTS and DTR, or bidirectional flow control
using RTS/CTS)
what pins need to be asserted in each port to enable data to be sent or
received
Only TD, RD and Signal Ground need be
connected for a terminal (or direct) connection that uses
XON/XOFF flow control.
A port with modem control uses TD, RD, and
Signal Ground in the same way as a non-modem control port.
On a non-modem control port, RTS and DTR
are asserted and CD is not monitored.
With modem control, RTS and DTR
are asserted, and the state of CD is monitored.
No signal is transmitted on TD until
CD is asserted from another source (such as a modem).
It is generally recommended that the Shield is only connected at the
DTE end to prevent current (or ``hum'') loops
being set up. Elimination of noise, and protection against
electrical interference and lightning pulse overvoltage
should be referred to a communications expert.
Modem connections
The RS 232D standard has the
following 25-pin connections for linking DTE to DCE
(the 9-pin equivalents are given in parentheses):
DTE
DCE
1 (-)
- (-)
2 (3)
to
2 (3)
3 (2)
to
3 (2)
4 (7)
to
4 (7)
5 (8)
to
5 (8)
6 (6)
to
6 (6)
7 (5)
to
7 (5)
8 (1)
to
8 (1)
20 (4)
to
20 (4)
22 (9)
to
22 (9)
RI is only used by some types of modem.
Null-modem connections
For direct connection linking DTE to DTE
(computer to computer, DTE terminal to computer),
the 25-pin connections (9-pin equivalents given
in parentheses) are:
DTE
DTE
1 (-)
- (-)
2 (3)
to
3 (2)
3 (2)
to
2 (3)
4 (7)
to
5 (8)
5 (8)
to
4 (7)
6=8 (6=1)
to
20 (4)
7 (5)
to
7 (5)
20 (4)
to
6=8 (6=1)
The equals sign (=) denotes that the pins are to be
linked within a connector.
DTR from each serial port is connected to
CD and DSR on the other serial port.
This simulates the assertion of these lines by a port
configured as DCE.
Another way to achieve this is by connecting DTR,
CD and DSR at the same end. This is
often done when connecting DTE terminals to a
DTE serial port. Shown below is an example for
connecting a terminal which uses software flow control only
(no modem control):
Computer (DTE)
Terminal (DTE)
- (-)
1 (-)
2 (3)
to
3 (2)
3 (2)
to
2 (3)
6=8=20 (6=1=4)
- (-)
7 (5)
to
7 (5)
Many terminal manufacturers recommend that DTR, CD, and
DSR are not connected at the terminal end if a modem control
protocol is not being used.
Printer connections with RTS/CTS flow control
To connect an RTS/CTS unidirectional protocol printer (the
printer is assumed to be configured as DTE),
the 25-pin connections (9-pin equivalents given in parentheses) are:
Computer (DTE)
Printer (DTE)
- (-)
1 (-)
2 (3)
to
3 (2)
3 (2)
to
2 (3)
4 (7)
to
5 (8)
5 (8)
to
4 (7)
6=20 (6=4)
- (-)
7 (5)
to
7 (5)
If the printer is configured as DCE, the connections
become:
Computer (DTE)
Printer (DCE)
- (-)
1 (-)
2 (3)
to
2 (3)
3 (2)
to
3 (2)
4 (7)
to
4 (7)
5 (8)
to
5 (8)
6=20 (6=4)
- (-)
7 (5)
to
7 (5)
Printer connections with DTR handshaking
To connect a printer which uses DTR for handshaking
(the printer is assumed to be configured as DTE),
the 25-pin connections (9-pin equivalents given
in parentheses) are:
Computer (DTE)
Printer (DTE)
- (-)
1 (-)
2 (3)
to
3 (2)
3 (2)
to
2 (3)
5 (8)
to
20 (4)
6=20 (6=4)
- (-)
7 (5)
to
7 (5)
If the printer is configured as DCE, the connections
become:
Computer (DTE)
Printer (DCE)
- (-)
1 (-)
2 (3)
to
2 (3)
3 (2)
to
3 (2)
5 (8)
to
20 (4)
6=20 (6=4)
- (-)
7 (5)
to
7 (5)
Supported ioctls
See the
termio(M)
and
termios(M)
manual pages for a description of the
ioctl(S)
commands supported by the sio driver.
Diagnostics
The following error messages may be displayed on the console.
See
messages(M)
for general information about kernel error messages,
including a list of generic device driver errors.
WARNING: serial: Serial adapter (type=m, nports=n) not found
A serial adapter could not be found.
WARNING: serial: Garbage or loose cable on dev n, port shut down
Too many interrupts were received together. Check the connections on the
port.
The unit number is always one less than the board number
encoded in the device name. In this example, the board's
sole port will be accessed through tty2a.
Limitations
If you log in via a modem control serial line, hanging up
logs that line out and kills your background processes.
See
nohup(C)
and
csh(C).
You cannot use the same serial port with both modem and
non-modem control at the same time. For example, you
cannot use tty1a and tty1A
simultaneously.
Files
/dev/tty{1-4}[a-p]
non-modem control device nodes for ISA serial ports
/dev/tty{1-4}[A-P]
modem control nodes for ISA serial ports
/dev/tty{5-15}[a-p]
non-modem control device nodes for PCI serial ports