ACT apricot - Page 135

SERIAL INTERFACE
(b) Conversion of the input serial data from the serial
channels into parallel form, and checking the
incoming data for errors. In asynchronous modes, this
involves stripping the start, stop and parity bits from
the serial data and checking for parity and framing
errors.
When operating in synchronous modes, similar
automatic facilities are available within the SIO, dependent
on the selected mode.
In Bisync and Monosync, pre-programmed sync
characters and CRC characters are automatically inserted
during transmissions. The SIO strips sync characters from
received data, and also analyses the received data for errors,
utilising the received CRC characters.
In
SDLC and HDLC, the SIO performs the following
functions:
(a) Abort sequence generation and detection.
(b) Zero insertion and deletion.
(c) Flag insertion between messages.
(d) Address field recognition.
(e) I-field residue handling.
(f) CRC generation and detection.
The baud rates for the two serial channels are determined
by clock sources external to the SIO. In asynchronous
modes, the clocks can be divided internally within the SIO
under program control, prior to being used to set the baud
rates. This facility is not available in synchronous modes.
,
Each channel also provides inputs and outputs which are
specifically tailored to function as modem control lines for
co-ordinating serial data transfer operations. The outputs
are under direct control by the programmer. The inputs can
be monitored under software control by polling, or be
programmed to generate an interrupt to the CPU. These
control lines can also be redefined as general purpose input
and output control signals.