asy - asynchronous serial port driver


#include <fcntl.h>

#include <sys/termios.h>

open("/dev/term/n", mode);

open("/dev/tty/n", mode);

open("/dev/cua/n", mode);


The asy module is a loadable STREAMS driver that provides basic support
for Intel-8250, National Semiconductor-16450, 16550, and some 16650 and
16750 and equivalent UARTs connected via the ISA-bus, in addition to
basic asynchronous communication support. The asy module supports those
termio(4I) device control functions specified by flags in the c_cflag
word of the termios structure, and by the IGNBRK, IGNPAR, PARMRK, INPCK,
IXON, IXANY, or IXOFF flags in the c_iflag word of the termios structure.
All other termio(4I) functions must be performed by STREAMS modules
pushed atop the driver. When a device is opened, the ldterm(4M) and
ttcompat(4M) STREAMS modules are automatically pushed on top of the
stream, providing the standard termio(4I) interface.

The character-special devices /dev/term/a, /dev/term/b, /dev/term/c and
/dev/term/d are used to access the four standard serial ports (COM1,
COM2, COM3 and COM4 at I/O addresses 3f8, 2f8, 3e8 and 2e8 respectively).
Serial ports on non-standard ISA-bus I/O addresses are accessed via the
character-special devices /dev/term/0, /dev/term/1, etc. Device names
are typically used to provide a logical access point for a dial-in line
that is used with a modem.

To allow a single tty line to be connected to a modem and used for
incoming and outgoing calls, a special feature is available that is
controlled by the minor device number. By accessing character-special
devices with names of the form /dev/cua/n, it is possible to open a port
without the Carrier Detect signal being asserted, either through hardware
or an equivalent software mechanism. These devices are commonly known as
dial-out lines.

Note -

This module is affected by the setting of certain eeprom variables,
ttya-ignore-cd and ttya-rts-dtr-off (and similarly for ttyb-, ttyc-,
and ttyd- parameters). For information on these parameters, see the
eeprom(8) man page.

Note -

For serial ports on the standard COM1 to COM4 I/O addresses above, the
default setting for ttya-ignore-cd and ttya-rts-dtr-off is true. If any
of these ports are connected to a modem, these settings should be
changed to false. For serial ports on non-standard I/O addresses, the
default setting for ttya-ignore-cd and ttya-rts-dtr-off is false.


Once a /dev/cua/n line is opened, the corresponding tty line cannot be
opened until the /dev/cua/n line is closed. A blocking open will wait
until the /dev/cua/n line is closed (which will drop Data Terminal Ready,
after which Carrier Detect will usually drop as well) and carrier is
detected again. A non-blocking open will return an error. If the
/dev/ttydn line has been opened successfully (usually only when carrier
is recognized on the modem), the corresponding /dev/cua/n line cannot be
opened. This allows a modem to be attached to /dev/term/[n] (renamed from
/dev/tty[n]) and used for dial-in (by enabling the line for login in
/etc/inittab) or dial-out (by tip(1) or uucp(1C)) as /dev/cua/n when no
one is logged in on the line.


The standard set of termio ioctl() calls are supported by asy.

Breaks can be generated by the TCSBRK, TIOCSBRK, and TIOCCBRK ioctl()

The input and output line speeds may be set to any speed that is
supported by termio. The speeds cannot be set independently; for example,
when the output speed is set, the input speed is automatically set to the
same speed.

When the asy module is used to service the serial console port, it
supports a BREAK condition that allows the system to enter the debugger
or the monitor. The BREAK condition is generated by hardware and it is
usually enabled by default.

A BREAK condition originating from erroneous electrical signals cannot be
distinguished from one deliberately sent by remote DCE. The Alternate
Break sequence can be used as a remedy against this. Due to a risk of
incorrect sequence interpretation, SLIP and certain other binary
protocols should not be run over the serial console port when Alternate
Break sequence is in effect. Although PPP is a binary protocol, it is
able to avoid these sequences using the ACCM feature in RFC 1662. For
Solaris PPP 4.0, you do this by adding the following line to the
/etc/ppp/options file (or other configuration files used for the
connection; see pppd(8) for details):

asyncmap 0x00002000

By default, the Alternate Break sequence is a three character sequence:
carriage return, tilde and control-B (CR ~ CTRL-B), but may be changed by
the driver. For more information on breaking (entering the debugger or
monitor), see kbd(1) and kb(4M).


An open() will fail under the following conditions:

The unit being opened does not exist.

The dial-out device is being opened while the dial-in device is
already open, or the dial-in device is being opened with a no-
delay open and the dial-out device is already open.

The unit has been marked as exclusive-use by another process
with a TIOCEXCL ioctl() call.

The open was interrupted by the delivery of a signal.


dial-in tty lines

dial-out tty lines

64-bit kernel module for 64-bit x86 platform

asy configuration file


See attributes(7) for descriptions of the following attributes:

|Architecture | x86 |


kbd(1), tip(1), uucp(1C), ioctl(2), open(2), termios(3C), termio(4I),
kb(4M), ldterm(4M), ttcompat(4M), attributes(7), eeprom(8), pppd(8)


asyn: silo overflow.

The hardware overrun occurred before the input character could be

asyn: ring buffer overflow.

The driver's character input ring buffer overflowed before it could
be serviced.

October 9, 2004 ASY(4D)