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man pages section 4: Device and Network Interfaces

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Updated: Thursday, June 13, 2019



su - asynchronous serial port driver


#include <fcntl.h>
#include <sys/termios.h>
open("/dev/tty[a-z]", _mode);
open("/dev/term[a-z]", _mode);
open("/dev/cua[a-z]", _mode);


The su module is a loadable STREAMS driver that provides basic support for standard UARTS that use Intel-8250, National Semiconductor-16450/16550 hardware and Southbridge 1535D (16550 compatible) Super I/O hardware. The module also provides keyboard and mouse I/O support for Sun machines using those same Intel, National Semiconductor and Southbridge chipsets. The su driver provides basic asynchronous communication support for serial ports. Both the serial devices and keyboard/mouse devices will have streams built with appropriate modules pushed atop the su driver by means of either the autopush(8) or dacf.conf(5) facilities, depending on the OS revision and architecture in use.

The su 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, or INPCK 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/ttya and /dev/ttyb are used to access the two standard serial ports. The su module supports up to ten serial ports, including the standard ports. The tty[a-z] devices have minor device numbers in the range 00-03, and may be assigned names of the form /dev/ttyd_n_, where _n_ denotes the line to be accessed. These 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.


Once a /dev/cua_n_ line is opened, the corresponding tty, or ttyd 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/ttyd_n_ 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 a device, (for example, /dev/ttyd0, which is renamed from /dev/tty00) 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/cua0 when no one is logged in on the line.


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

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

The input and output line speeds may be set to any of the following baud rates: 0, 50, 75, 110, 134, 150, 200, 300, 600, 1200, 1800, 2400, 4800, 9600, 19200, 38400, 57600 or 115200. 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 su 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.



dial-out tty lines


dial-in tty lines


binary compatibility package device names


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


See Also

uucp(1C), kbd(1), strconf(1), tip(1), ioctl(2), open(2), termios(3C), termio(4I), kb(4M), ldterm(4M), ttcompat(4M), dacf.conf(5), attributes(7), autopush(8), kstat(8), pppd(8)


The su driver keeps track of various warning and error conditions using kstat counters. The output of the kstat su command provides kstat counters. The counters and their meaning follow:

silo overflow

The internal chip FIFO received more data than it could handle. This indicates that the Solaris operating environment was not servicing data interrupts fast enough possibly due to a system with too many interrupts or a data line with a data rate that is too high.

ring buffer overflow

The su module was unable to store data it removed from the chips internal FIFO into a software buffer. The user process is not reading data fast enough, possibly due to an overloaded system. If possible, the application should enable flow control (either CTSRTS or XONXOFF) to allow the driver to backpressure the remote system when the local buffers fill up.