ACT apricot - Serialdatalinkcontrolandformat

KEYBOARD
A list of character codes and their meaning, and the
MicroScreen character display codes are contained in
tabular format at the end of the section.
SERIAL DATA LINK CONTROL AND FORMAT
The data link passes asynchronous serial date at V28
levels
between the Keyboard and the System Unit. The
format is NRZ, with the "0" start bit followed by eight data
bits, followed by "1" stop bit, no parity information is
transmitted. The line idle condition is a continuous string
of" Is". The data transmission rate is approximately 7800
bit/s
There is no control of data flow from the Keyboard to the
System Unit so to avoid the loss of data the System Unit has
to be capable of receiving and processing data at the
maximum data rate i.e. 780 cps.
Data flow from the System Unit is controlled in one of
two ways. The first method is when the Keyboard buffer is
full,
it will then transmit an X-off character to the System
Unit. When the System Unit receives the X-off character
from
the Keyboard rather than the mouse port it must not
physically send more than four further characters in order to
prevent overflow at the Keyboard. The System Unit will
receive the X-on character from the Keyboard when the
Keyboard buffer is empty, it can resume transmission. With
the exception of data from the mouse port, which would be
preceded
by a prefix code, the X -on and X -off characters will
not appear in any other context.
The second method is when data is being received from
the mouse port. In this case the data constitutes an implicit
X-off condition and no more than four characters can be
physically
sent to the Keyboard. At the end of data reception
from
the mouse port transmission to the Keyboard may
resume if an X-off character had not previously been
received from the Keyboard.
The program is construct~d so that data for the time and
date will not have key data interleaved but it is possible that
data from the mouse port may be transmitted. In order to