6100 BSC Programming Manual

Writing Applications that Use 6100 BSC
Sometimes both stations bid for the line at the same time.
If a secondary station makes a bid and receives another bid in
response, the WRITE or WRITEREAD request completes with error
166. An application that receives this error should yield the
line to the primary station--both to be consistent with a
protocol convention and to end an otherwise wasteful stalemate.
If a primary station makes a bid and receives another bid in
response, BSC retries the bid on behalf of the application. If
the retry count is exhausted, the request completes with an
error. (Notice that if both stations were primaries, the one
with the higher retry count would win the bid.) Figure 3-4
shows a case of contention and its resolution.
Normally, the right to bid for the line alternates between the
stations. After winning a bid, transmitting its data, and
finally relinquishing the line, a station waits for three seconds
before it can bid again. The purpose of this delay is to let the
other station bid. If the other station does not want to bid, it
can take one of two actions: it can wait three seconds, until
the first station is free to bid again, or it can make a CONTROL
request with the SEND EOT option. The CONTROL request sends an
EOT sequence and does not await a response. The receiving
station can then bid immediately, instead of waiting for three
seconds to elapse. (To accept a bid, the station that sent
the EOT must quickly make a READ request.)
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