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SETSERIAL(8) SETSERIAL(8)
NAME
setserial - get/set Linux serial port information
SYNOPSIS
setserial [ -abqvVWz ] device [ parameter1 [ arg ] ] ...
setserial -g [ -abGv ] device1 ...
DESCRIPTION
setserial is a program designed to set and/or report the configuration information
associated with a serial port. This information includes what I/O port and IRQ a
particular serial port is using, and whether or not the break key should be inter-
preted as the Secure Attention Key, and so on.
During the normal bootup process, only COM ports 1-4 are initialized, using the
default I/O ports and IRQ values, as listed below. In order to initialize any
additional serial ports, or to change the COM 1-4 ports to a nonstadard configura-
tion, the setserial program should be used. Typically it is called from an
rc.serial script, which is usually run out of /etc/rc.local.
The device argument or arguments specifies the serial device which should be con-
figured or interrogated. It will usually have the following form: /dev/cua[0-3].
If no parameters are specified, setserial will print out the port type (i.e., 8250,
16450, 16550, 16550A, etc.), the hardware I/O port, the hardware IRQ line, its
"baud base," and some of its operational flags.
If the -g option is given, the arguments to setserial are interpreted as a list of
devices for which the characteristics of those devices should be printed.
Without the -g option, the first argument to setserial is interpreted as the device
to be modified or characteristics to be printed, and any additional arguments are
interpreted as parameters which should be assigned to that serial device.
For the most part, superuser privilege is required to set the configuration parame-
ters of a serial port. A few serial port parameters can be set by normal users,
however, and these will be noted as exceptions in this manual page.
OPTIONS
Setserial accepts the following options:
-a When reporting the configuration of a serial device, print all available
information.
-b When reporting the configuration of a serial device, print a summary of the
device’s configuration, which might be suitable for printing during the
bootup process, during the /etc/rc script.
-G Print out the configuration information of the serial port in a form which
can be fed back to setserial as command-line arguments.
-q Be quiet. Setserial will print fewer lines of output.
-v Be verbose. Setserial will print additional status output.
-V Display version and exit.
-W Do wild interrupt initialization and exit. This option is no longer rele-
vant in Linux kernels after version 2.1.
-z Zero out the serial flags before starting to set flags. This is related to
the automatic saving of serial flags using the -G flag.
PARAMETERS
The following parameters can be assigned to a serial port.
All argument values are assumed to be in decimal unless preceeded by "0x".
port port_number
The port option sets the I/O port, as described above.
irq irq_number
The irq option sets the hardware IRQ, as described above.
uart uart_type
This option is used to set the UART type. The permitted types are none,
8250, 16450, 16550, 16550A, 16650, 16650V2, 16654, 16750, 16850, 16950, and
16954. Using UART type none will disable the port.
Some internal modems are billed as having a "16550A UART with a 1k buffer".
This is a lie. They do not have really have a 16550A compatible UART;
instead what they have is a 16450 compatible UART with a 1k receive buffer
to prevent receiver overruns. This is important, because they do not have a
transmit FIFO. Hence, they are not compatible with a 16550A UART, and the
autoconfiguration process will correctly identify them as 16450’s. If you
attempt to override this using the uart parameter, you will see dropped
characters during file transmissions. These UART’s usually have other prob-
lems: the skip_test parameter also often must be specified.
autoconfig
When this parameter is given, setserial will ask the kernel to attempt to
automatically configure the serial port. The I/O port must be correctly
set; the kernel will attempt to determine the UART type, and if the auto_irq
parameter is set, Linux will attempt to automatically determine the IRQ.
The autoconfig parameter should be given after the port,auto_irq, and
skip_test parameters have been specified.
auto_irq
During autoconfiguration, try to determine the IRQ. This feature is not
guaranteed to always produce the correct result; some hardware configura-
tions will fool the Linux kernel. It is generally safer not to use the
auto_irq feature, but rather to specify the IRQ to be used explicitly, using
the irq parameter.
^auto_irq
During autoconfiguration, do not try to determine the IRQ.
skip_test
During autoconfiguration, skip the UART test. Some internal modems do not
have National Semiconductor compatible UART’s, but have cheap imitations
instead. Some of these cheasy imitations UART’s do not fully support the
loopback detection mode, which is used by the kernel to make sure there
really is a UART at a particular address before attempting to configure it.
So for certain internal modems you will need to specify this parameter so
Linux can initialize the UART correctly.
^skip_test
During autoconfiguration, do not skip the UART test.
baud_base baud_base
This option sets the base baud rate, which is the clock frequency divided by
16. Normally this value is 115200, which is also the fastest baud rate
which the UART can support.
spd_hi Use 57.6kb when the application requests 38.4kb. This parameter may be
specified by a non-privileged user.
spd_vhi
Use 115kb when the application requests 38.4kb. This parameter may be spec-
ified by a non-privileged user.
spd_shi
Use 230kb when the application requests 38.4kb. This parameter may be spec-
ified by a non-privileged user.
spd_warp
Use 460kb when the application requests 38.4kb. This parameter may be spec-
ified by a non-privileged user.
spd_cust
Use the custom divisor to set the speed when the application requests
38.4kb. In this case, the baud rate is the baud_base divided by the divi-
sor. This parameter may be specified by a non-privileged user.
spd_normal
Use 38.4kb when the application requests 38.4kb. This parameter may be
specified by a non-privileged user.
divisor divisor
This option sets the custom divison. This divisor will be used then the
spd_cust option is selected and the serial port is set to 38.4kb by the
application. This parameter may be specified by a non-privileged user.
sak Set the break key at the Secure Attention Key.
^sak disable the Secure Attention Key.
fourport
Configure the port as an AST Fourport card.
^fourport
Disable AST Fourport configuration.
close_delay delay
Specify the amount of time, in hundredths of a second, that DTR should
remain low on a serial line after the callout device is closed, before the
blocked dialin device raises DTR again. The default value of this option is
50, or a half-second delay.
closing_wait delay
Specify the amount of time, in hundredths of a second, that the kernel
should wait for data to be transmitted from the serial port while closing
the port. If "none" is specified, no delay will occur. If "infinite" is
specified the kernel will wait indefinitely for the buffered data to be
transmitted. The default setting is 3000 or 30 seconds of delay. This
default is generally appropriate for most devices. If too long a delay is
selected, then the serial port may hang for a long time if when a serial
port which is not connected, and has data pending, is closed. If too short
a delay is selected, then there is a risk that some of the transmitted data
is output at all. If the device is extremely slow, like a plotter, the
closing_wait may need to be larger.
session_lockout
Lock out callout port (/dev/cuaXX) accesses across different sessions. That
is, once a process has opened a port, do not allow a process with a differ-
ent session ID to open that port until the first process has closed it.
^session_lockout
Do not lock out callout port accesses across different sessions.
pgrp_lockout
Lock out callout port (/dev/cuaXX) accesses across different process groups.
That is, once a process has opened a port, do not allow a process in a dif-
ferent process group to open that port until the first process has closed
it.
^pgrp_lockout
Do not lock out callout port accesses across different process groups.
hup_notify
Notify a process blocked on opening a dial in line when a process has fin-
ished using a callout line (either by closing it or by the serial line being
hung up) by returning EAGAIN to the open.
The application of this parameter is for getty’s which are blocked on a
serial port’s dial in line. This allows the getty to reset the modem (which
may have had its configuration modified by the application using the callout
device) before blocking on the open again.
^hup_notify
Do not notify a process blocked on opening a dial in line when the callout
device is hung up.
split_termios
Treat the termios settings used by the callout device and the termios set-
tings used by the dialin devices as separate.
^split_termios
Use the same termios structure to store both the dialin and callout ports.
This is the default option.
callout_nohup
If this particular serial port is opened as a callout device, do not hangup
the tty when carrier detect is dropped.
^callout_nohup
Do not skip hanging up the tty when a serial port is opened as a callout
device. Of course, the HUPCL termios flag must be enabled if the hangup is
to occur.
low_latency
Minimize the receive latency of the serial device at the cost of greater CPU
utilization. (Normally there is an average of 5-10ms latency before charac-
ters are handed off to the line discpline to minimize overhead.) This is
off by default, but certain real-time applications may find this useful.
^low_latency
Optimize for efficient CPU processing of serial characters at the cost of
paying an average of 5-10ms of latency before the characters are processed.
This is the default.
CONSIDERATIONS OF CONFIGURING SERIAL PORTS
It is important to note that setserial merely tells the Linux kernel where it
should expect to find the I/O port and IRQ lines of a particular serial port. It
does *not* configure the hardware, the actual serial board, to use a particular I/O
port. In order to do that, you will need to physically program the serial board,
usually by setting some jumpers or by switching some DIP switches.
This section will provide some pointers in helping you decide how you would like to
configure your serial ports.
The "standard MS-DOS" port associations are given below:
/dev/ttys0 (COM1), port 0x3f8, irq 4
/dev/ttys1 (COM2), port 0x2f8, irq 3
/dev/ttys2 (COM3), port 0x3e8, irq 4
/dev/ttys3 (COM4), port 0x2e8, irq 3
Due to the limitations in the design of the AT/ISA bus architecture, normally an
IRQ line may not be shared between two or more serial ports. If you attempt to do
this, one or both serial ports will become unreliable if you try to use both simul-
taneously. This limitation can be overcome by special multi-port serial port
boards, which are designed to share multiple serial ports over a single IRQ line.
Multi-port serial cards supported by Linux include the AST FourPort, the Accent
Async board, the Usenet Serial II board, the Bocaboard BB-1004, BB-1008, and
BB-2016 boards, and the HUB-6 serial board.
The selection of an alternative IRQ line is difficult, since most of them are
already used. The following table lists the "standard MS-DOS" assignments of
available IRQ lines:
IRQ 3: COM2
IRQ 4: COM1
IRQ 5: LPT2
IRQ 7: LPT1
Most people find that IRQ 5 is a good choice, assuming that there is only one par-
allel port active in the computer. Another good choice is IRQ 2 (aka IRQ 9);
although this IRQ is sometimes used by network cards, and very rarely VGA cards
will be configured to use IRQ 2 as a vertical retrace interrupt. If your VGA card
is configured this way; try to disable it so you can reclaim that IRQ line for some
other card. It’s not necessary for Linux and most other Operating systems.
The only other available IRQ lines are 3, 4, and 7, and these are probably used by
the other serial and parallel ports. (If your serial card has a 16bit card edge
connector, and supports higher interrupt numbers, then IRQ 10, 11, 12, and 15 are
also available.)
On AT class machines, IRQ 2 is seen as IRQ 9, and Linux will interpret it in this
manner.
IRQ’s other than 2 (9), 3, 4, 5, 7, 10, 11, 12, and 15, should not be used, since
they are assigned to other hardware and cannot, in general, be changed. Here are
the "standard" assignments:
IRQ 0 Timer channel 0
IRQ 1 Keyboard
IRQ 2 Cascade for controller 2
IRQ 3 Serial port 2
IRQ 4 Serial port 1
IRQ 5 Parallel port 2 (Reserved in PS/2)
IRQ 6 Floppy diskette
IRQ 7 Parallel port 1
IRQ 8 Real-time clock
IRQ 9 Redirected to IRQ2
IRQ 10 Reserved
IRQ 11 Reserved
IRQ 12 Reserved (Auxillary device in PS/2)
IRQ 13 Math coprocessor
IRQ 14 Hard disk controller
IRQ 15 Reserved
MULTIPORT CONFIGURATION
Certain multiport serial boards which share multiple ports on a single IRQ use one
or more ports to indicate whether or not there are any pending ports which need to
be serviced. If your multiport board supports these ports, you should make use of
them to avoid potential lockups if the interrupt gets lost.
In order to set these ports specify set_multiport as a parameter, and follow it
with the multiport parameters. The multiport parameters take the form of specify-
ing the port that should be checked, a mask which indicate which bits in the
register are significant, and finally, a match parameter which specifies what the
significant bits in that register must match when there is no more pending work to
be done.
Up to four such port/mask/match combinations may be specified. The first such com-
binations should be specified by setting the parameters port1, mask1, and match1.
The second such combination should be specified with port2, mask2, and match2, and
so on. In order to disable this multiport checking, set port1 to be zero.
In order to view the current multiport settings, specify the parameter get_multi-
port on the command line.
Here are some multiport settings for some common serial boards:
AST FourPort port1 0x1BF mask1 0xf match1 0xf
Boca BB-1004/8 port1 0x107 mask1 0xff match1 0
Boca BB-2016 port1 0x107 mask1 0xff match1 0
port2 0x147 mask2 0xff match2 0
Hayes ESP Configuration
Setserial may also be used to configure ports on a Hayes ESP serial board.
The following parameters when configuring ESP ports:
rx_trigger
This is the trigger level (in bytes) of the receive FIFO. Larger values may
result in fewer interrupts and hence better performance; however, a value
too high could result in data loss. Valid values are 1 through 1023.
tx_trigger
This is the trigger level (in bytes) of the transmit FIFO. Larger values may
result in fewer interrupts and hence better performance; however, a value
too high could result in degraded transmit performance. Valid values are 1
through 1023.
flow_off
This is the level (in bytes) at which the ESP port will "flow off" the
remote transmitter (i.e. tell him to stop stop sending more bytes). Valid
values are 1 through 1023. This value should be greater than the receive
trigger level and the flow on level.
flow_on
This is the level (in bytes) at which the ESP port will "flow on" the remote
transmitter (i.e. tell him to resume sending bytes) after having flowed it
off. Valid values are 1 through 1023. This value should be less than the
flow off level, but greater than the receive trigger level.
rx_timeout
This is the amount of time that the ESP port will wait after receiving the
final character before signaling an interrupt. Valid values are 0 through
255. A value too high will increase latency, and a value too low will cause
unnecessary interrupts.
CAUTION
CAUTION: Configuring a serial port to use an incorrect I/O port can lock up your
machine.
FILES
/etc/rc.local /etc/rc.serial
SEE ALSO
tty(4), ttys(4), kernel/chr_drv/serial.c
AUTHOR
The original version of setserial was written by Rick Sladkey (jrs AT world.com),
and was modified by Michael K. Johnson (johnsonm AT stolaf.edu).
This version has since been rewritten from scratch by Theodore Ts’o (tytso AT mit.edu)
on 1/1/93. Any bugs or problems are solely his responsibility.
Setserial 2.17 January 2000 SETSERIAL(8)
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