SETSERIAL(8)SETSERIAL(8)NAMEsetserial ‐ get/set Linux serial port information
SYNOPSISsetserial [ ‐abqvVWz ] device [ parameter1 [ arg ] ] ...
setserial ‐g [ ‐abGv ] device1 ...
DESCRIPTIONsetserial 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 particu‐
lar serial port is using, and whether or not the break key
should be interpreted 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 non‐
stadard configuration, the setserial program should be
used. Typically it is called from an setserial script,
which is usually run out of /etc/init.d.
The device argument or arguments specifies the serial
device which should be configured or interrogated. It
will usually have the following form: /dev/ttyS[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 character‐
istics of those devices should be printed.
Without the ‐g option, the first argument to setserial is
interpreted as the device to be modified or characteris‐
tics 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 parameters of a serial port. A few
serial port parameters can be set by normal users, how‐
ever, and these will be noted as exceptions in this manual
page.
OPTIONS
Setserial accepts the following options:
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SETSERIAL(8)SETSERIAL(8)
‐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 configura‐
tion, 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 set‐
serial as command‐line arguments.
‐q Be quiet. Setserial will print fewer lines of out‐
put.
‐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 relevant 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 per‐
mitted 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
Setserial 2.17 January 2000 2
SETSERIAL(8)SETSERIAL(8)
with a 1k receive buffer to prevent receiver over‐
runs. This is important, because they do not have
a transmit FIFO. Hence, they are not compatible
with a 16550A UART, and the autoconfiguration pro‐
cess will correctly identify them as 16450’s. If
you attempt to override this using the uart parame‐
ter, you will see dropped characters during file
transmissions. These UART’s usually have other
problems: 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 configurations
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 inter‐
nal 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
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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 specified by a non‐privileged
user.
spd_shi
Use 230kb when the application requests 38.4kb.
This parameter may be specified by a non‐privileged
user.
spd_warp
Use 460kb when the application requests 38.4kb.
This parameter may be specified 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 divisor.
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 appli‐
cation. 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 sec‐
ond, that DTR should remain low on a serial line
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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‐sec‐
ond delay.
closing_wait delay
Specify the amount of time, in hundredths of a sec‐
ond, 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 transmit‐
ted. 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 dif‐
ferent session ID to open that port until the first
process has closed it.
^session_lockout
Do not lock out callout port accesses across dif‐
ferent 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 dif‐
ferent process groups.
hup_notify
Notify a process blocked on opening a dial in line
when a process has finished 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
Setserial 2.17 January 2000 5
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have had its configuration modified by the applica‐
tion 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 settings 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 call‐
out 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 char‐
acters are handed off to the line discpline to min‐
imize overhead.) This is off by default, but cer‐
tain 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 pro‐
cessed. 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.
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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 simultaneously. 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 parallel port active in the com‐
puter. 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 ver‐
tical 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 connec‐
tor, 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 hard‐
ware and cannot, in general, be changed. Here are the
"standard" assignments:
IRQ 0 Timer channel 0
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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 ser‐
viced. 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 specifying 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 speci‐
fied. The first such combinations should be specified by
setting the parameters port1, mask1, and match1. The sec‐
ond 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_multiport 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
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SETSERIAL(8)SETSERIAL(8)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 trans‐
mit FIFO. Larger values may result in fewer inter‐
rupts 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/serial.conf /etc/init.d/setserial
SEE ALSOtty(4), ttys(4), kernel/chr_drv/serial.c
Setserial 2.17 January 2000 9
SETSERIAL(8)SETSERIAL(8)AUTHOR
The original version of setserial was written by Rick
Sladkey (jrs@world.std.com), and was modified by Michael
K. Johnson (johnsonm@stolaf.edu).
This version has since been rewritten from scratch by
Theodore Ts’o (tytso@mit.edu) on 1/1/93. Any bugs or
problems are solely his responsibility.
Debian related problems with this system should be sent to
Gordon Russell (gor@debian.org).
Setserial 2.17 January 2000 10
