DESCRIPTION AND OPERATION
INIIT02 COMMUNICATION PROTOCOL
I-E96-607A
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recovery. The INFI 90 application layers in both the central and
satellite loop INIIT02 modules use the link control layer to
transport control and configuration information and exception
reports.
Framing
The INIIT02 module uses message framing to help it track data
bytes that pass through the asynchronous communication
link. The receiving INIIT02 module looks for a one-byte syn-
chronization character at the start of every transmission. Once
it recognizes the synchronization character, it looks at a byte
count field in each message to determine the framing of suc-
cessive messages.
Sequence Control
All messages have a sequence number. The transmitting and
receiving communication modules use the sequence number to
keep track of outstanding, lost and bad messages.
Line Control
Line control limits the transmission time of each INIIT02 mod-
ule when they are in the half duplex mode. There are two lim-
its: The window of outstanding sequenced messages and the
maximum transmission time set by executive block specifica-
tions S5 and S6. When the transmitting module reaches one of
the limits (or has no more data to send), it flags the last mes-
sage in the transmission. When the receiving module sees this
flag it can then start a transmission sequence of its own. If the
transfer modules are in the full duplex mode they do not use
line control; they can transmit independently of each other.
When external communication equipment (i.e., transceiver) is
used and the INIIT02 module is operating in half duplex mode,
the INIIT02 module requires a digital I/O module to provide a
switching signal to the communication equipment. A digital I/
O module is also required for systems that have redundant
external communication equipment (see Figure
Error Detection and Recovery
A 16-bit cyclic redundancy check (CRC) code assures the integ-
rity of each transmission. The transfer module uses an auto-
matic repeat request to recover from errors. The receiving
INIIT02 module notifies the sending INIIT02 module of the last
sequence number it receives. The sending module then knows
which message packets to resend.
