6100 BSC Programming Manual

Writing Applications that Use 6100 BSC
Bidding for the Line (or Accepting a Bid)
A station that wishes to transmit a series of blocks or messages
must first bid for the line, as described in Section 1. The line
must be in CONTROL state for bidding to occur.
Figure 3-3 illustrates a typical bidding transaction. To make a
bid, the application issues a WRITE or WRITEREAD request. The
WRITE request connects the link, if needed, then makes the bid
and accepts the acknowledgement; it can also send the first
message or block and receive its acknowledgement. The WRITEREAD
request is like the WRITE, except that if it sends a message or
block, it can receive either an acknowledgement or a
conversational reply. The ENQ character shown on the figure must
not be in the application buffer; BSC sends it automatically.
The buffer should contain only the first message or block.
To accept a bid, the application issues a READ request. The
request connects the link, if needed, then responds to the bid
with ACK 0 and receives the first transmission. (The first
transmission is normally a message or block, although it can
be a control sequence like TTD or EOT.)
If the receiving station isn't ready to accept a bid, the
application simply doesn't issue a READ request. The bid
times out and is retried until successful or until the retry
count is exhausted.
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