asy(7) INTERACTIVE UNIX System asy(7)
NAME
asy - asynchronous serial port
DESCRIPTION
The asy driver is a character device driver written for a
front-end module and a common interface module implementa-
tion. The asy driver supports both the system board serial
port and additional serial adapters. Up to 16 serial ports
are supported simultaneously. If an adapter for a port is
not installed, an attempt to open it will fail. The port
can be programmed for different speeds (50-38400 baud), for
character length, and for parity. Output speed is always
the same as input speed. The port behaves as described in
termio(7).
The asynchronous port is a character-at-a-time device for
both input and output. This characteristic both limits the
bandwidth which can be achieved over a line and increases
the interrupt loading on the central processor. In particu-
lar, file transfer programs such as uucp(1) may not function
well at speeds over 9600 baud.
If the port is opened with the modem control bit present in
the minor device number for the port (see below), modem con-
trol will be enabled. If enabled, the driver will wait in
open until Data Carrier Detect (DCD) is present. Once
opened, if DCD drops, the driver will return errors on any
subsequent reads or writes of the asynchronous port by the
user. If the port is opened as a controlling teletype, sig-
nal SIGHUP will be generated to the process which did the
open.
The baud rates of the serial adapter's programmable baud
rate generator do not correspond exactly with system baud
rates. In particular, setting B0 will cause a disconnect,
i.e., cause the Ready-to-Send (RTS) and Data-Term-Ready
(DTR) lines to be dropped, whereas setting the baud rate
back to any non-zero value will assert the RTS and DTR
lines. Setting EXTA will set 19200 baud, and setting EXTB
will set 38400 baud. It is not possible to directly set
2000, 3600, or 7200 baud.
Minor Device Numbers
The asynchronous port is a character device. The low-order
4 bits (bits 0-3) of the minor device number correspond to
the asynchronous port. Each port has three devices associ-
ated with it:
Device 1.
Completely ignores modem control. The device is
intended for direct (hardwired) connections, and it
allows a simple three-wire connection.
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asy(7) INTERACTIVE UNIX System asy(7)
Device 2.
Used for dial-in connections, with opens blocked
until DCD is present.
Device 3.
Used for dial-out connections, which allows open
calls to complete immediately, although the device
obeys modem control once a carrier has been esta-
blished.
Bit 4 indicates the dial-in line, and bit 5 indicates the
dial-out line. Thus, minor device 0 corresponds to the
directly-connected serial port 0 (or async adapter COM1)
with no modem control, while minor device 16 corresponds to
the same port for dial-in use with modem control enabled,
and minor device 32 is the dial-out device with modem con-
trol enabled. Similarly, minor device number 1 corresponds
to the directly connected serial port 1 (or async adapter
COM2), while minor device 17 corresponds to the same port
for dial-in use, and minor device 33 is the dial-out device.
All three devices interlock to allow use of a single port
for bidirectional communication without requiring any spe-
cial software. If the incoming line has been successfully
opened (due to a carrier being detected), then subsequent
attempts to open either the direct or dial-out lines will
fail and errno will be set to EBUSY. If either of the
direct or dial-out lines has been opened, then a process
attempting to open the dial-in line will not complete the
open regardless of the DCD state as long as the other device
remains open. This interlocking allows a standard
/etc/getty to run on the dial-in line while allowing uucp to
dial out on the dial-out line whenever the incoming line is
not in use.
FILES
/dev/tty0*
directly connected device
/dev/ttyd*
dial-in device
/dev/acu*
dial-out device
SEE ALSO
termio(7).
signal(2) in the INTERACTIVE SDS Guide and Programmer's
Reference Manual.
ADDED VALUE
This entry, supplied by INTERACTIVE Systems Corporation,
contains enhancements to UNIX System V.
Rev. Page 2