asy - asynchronous serial port driver
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.
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)) 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() calls.
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.
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:
The hardware overrun occurred before the input character could be serviced.
The driver's character input ring buffer overflowed before it could be serviced.