Manual browser: com(4)

COM(4) Kernel Interfaces Manual COM(4)


comserial communications interface for RS-232C


com0 at isa? port "IO_COM1" irq 4
com1 at isa? port "IO_COM2" irq 3
com* at acpi?
com* at cardbus?
com* at isapnp?
com* at mca? slot ?
com* at mhzc?
com* at ofisa?
com* at pcmcia?
com* at pcmcom?
com* at pnpbios? index ?
com* at puc? port ?
com* at xirc?
options COM_HAYESP
options PPS_SYNC
options RND_COM


com* at clockport?


com0 at mainbus? base 0x00210fe0
com1 at mainbus? base 0x00210be0

HP 9000/300 and 400 Series

com* at dio? scode ?
com* at frodo? offset ?

HP 9000/700 and 800 Series

com* at dino?
com* at gsc?
com* at ssio?

IBM PowerPC 4xx

com* at opb?


com* at ebus?
com* at obio0


com0 at intio0 addr 0xefff00 intr 240
com1 at intio0 addr 0xefff10 intr 241


The com driver provides support for NS8250-, NS16450-, and NS16550-based EIA RS-232C (CCITT V.28) communications interfaces. The NS8250 and NS16450 have single character buffers, and the NS16550 has a 16 character buffer.

Input and output for each line may set to one of following baud rates; 50, 75, 110, 134.5, 150, 300, 600, 1200, 1800, 2400, 4800, 9600, 19200, 38400, 57600, or 115200, or any other baud rate which is a factor of 115200.

The ttyXX devices are traditional dial-in devices; the dtyXX devices are used for dial-out. (See tty(4).)

options COM_HAYESP adds support for the Hayes ESP serial board.

options PPS_SYNC enables code to use the Data Carrier Detect (DCD) signal line for attachment to an external precision clock source (e.g., GPS, CDMA) which generates a Pulse Per Second (PPS) signal. This is used by ntpd(8) to discipline the system clock, and more accurately count/measure time. See options(4) for more discussion.

With options RND_COM enabled, the com driver can be used to collect entropy for the rnd(4) entropy pool. The entropy is generated from interrupt randomness.

Arm32 specific

If “flags 1” is specified, the com driver will not set the MCR_IENABLE bit on the UART. This is mainly for use on AST multiport boards, where the MCR_IENABLE bit is used to control whether or not the devices use a shared interrupt.




com%d: %d silo overflows
The input “silo” has overflowed and incoming data has been lost.
com%d: weird interrupt: iir=%x
The device has generated an unexpected interrupt with the code listed.


The com driver was originally derived from the HP9000/300 dca driver.


Data loss is possible on busy systems with unbuffered UARTs at high speed.

The name of this driver and the constants which define the locations of the various serial ports are holdovers from DOS.

April 29, 2012 NetBSD 7.0