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asy(4d)
asy(4D) Device Drivers & /dev files asy(4D)
NAME
asy - asynchronous serial port driver
SYNOPSIS
#include <fcntl.h>
#include <sys/termios.h>
open("/dev/term/n", mode);
open("/dev/tty/n", mode);
open("/dev/cua/n", mode);
DESCRIPTION
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, IN‐
PCK, 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 re‐
spectively). Serial ports on non-standard ISA-bus I/O addresses are ac‐
cessed 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 in‐
coming and outgoing calls, a special feature is available that is con‐
trolled by the minor device number. By accessing character-special de‐
vices with names of the form /dev/cua/n, it is possible to open a port
without the Carrier Detect signal being asserted, either through hard‐
ware or an equivalent software mechanism. These devices are commonly
known as dial-out lines.
Note -
This module is affected by the setting of the eeprom variables, tty-
ignore-cd and tty-rts-dtr-off, where tty represents the name of the
tty interface in question: ttya, ttyb, ttyc, or ttyd. For example,
ttya is affected by variables ttya-ignore-cd and ttya-rts-dtr-off.
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.
APPLICATION PROGRAMMING INTERFACE
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 car‐
rier 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)) as /dev/cua/n when
no one is logged in on the line.
IOCTLS
The standard set of termio ioctl() calls are supported by asy.
Breaks can be generated by the TCSBRK, TIOCSBRK, and TIOCCBRK ioctl()
calls.
The input and output line speeds may be set to any speed that is sup‐
ported 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 sup‐
ports 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 Alter‐
nate 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.
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).
ERRORS
An open() will fail under the following conditions:
ENXIO The unit being opened does not exist.
EBUSY 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.
EBUSY The unit has been marked as exclusive-use by another process
with a TIOCEXCL ioctl() call.
EINTR The open was interrupted by the delivery of a signal.
FILES
/dev/term/[a-d] dial-in tty lines
/dev/term/[012...]
/dev/cua/[a-d] dial-out tty lines
/dev/cua/[012...]
/kernel/drv/amd64/asy 64-bit kernel module for 64-bit x86 platform
/kernel/drv/asy.conf asy configuration file
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Architecturex86 _ Availabilitydriver/serial/asy
SEE ALSO
kbd(1), tip(1), ioctl(2), open(2), termios(3C), termio(4I), kb(4M),
ldterm(4M), ttcompat(4M), attributes(7), eeprom(8)
DIAGNOSTICS
asyn: silo overflow.
The hardware overrun occurred before the input character could be
serviced.
asyn: ring buffer overflow.
The driver's character input ring buffer overflowed before it could
be serviced.
Oracle Solaris 11.4 15 Mar 2024 asy(4D)