.TH ASYNC 3 
.SH NAME
async \- framing for a serial line to Datakit
.SH SYNOPSIS
.nf
.B
Fctlfd = open(".../ctl", ORDWR);
.B
Fwrite(ctlfd, "push async", 10);
.fi
.SH DESCRIPTION
.PP
This is not a device, but rather a
.I stream module
(see
.IR stream (3))
that can be pushed onto a stream.
This module provides the framing necessary to treat
a serial line as a Datakit trunk.
It is usually pushed onto a stream before the
.I dkmux
module.
The frame includes a CRC.
Any received frames with an incorrect CRC are
discarded.
.PP
The format of a message upstream of the module is:
.IP
channel # low byte
.br
channel # high byte
.br
control byte (0 means none)
.br
data bytes
.LP
The format of a frame is:
.IP
.B 0x7d	
.br
.B 0x7d
.br
channel # low byte
.br
channel # high byte
.br
crc low byte
.br
crc high byte
.br
.B 0x7d	
.br
.B 0x7d
.LP
All control bytes in the frame are preceded by a
.B 0x9d
byte.
All
.B 0x9d
and
.B 0x7d
bytes in the data are followed by a
.B 0x00
byte to distinguish them from framing or control
specifiers.
.SH "SEE ALSO"
.IR stream (3),
.IR cons (3),
.IR dk (3)