6100 BSC Programming Manual

6100 BSC Concepts and Context
WAIT FOR ACKNOWLEDGEMENT. After transmitting a message, a line
bid, or another sequence, the sending station expects an
acknowledgement within a certain time. The time is 2.7 seconds
for a primary station, 3 seconds for a secondary station. If no
acknowledgement arrives or if the acknowledgement is garbled,
BSC sends an ENQ sequence to mean "Please repeat that reply."
BSC sends ENQ as many times as its retry count allows; then if
the retries have failed, it reports an error to the application.
If an acknowledgement arrives, but the acknowledgement is a NAK,
BSC repeats the transmission as often as the retry count allows.
If the retries fail, BSC reports an error to the application.
WAIT FOR NEXT BLOCK. After receiving a reply to a message block
or a line bid, a station must make its next transmission within a
prescribed interval. The other station dictates the interval;
its "receive timeout" determines how long it is willing to wait
for the next transmission. For example, if the remote station
has a five-second receive timeout, the local station has five
seconds to make its next transmission.
If a station cannot send a block within the required time
interval, it can transmit a TTD sequence to retain the line.
(TTD is one of the options in the CONTROL request.) If a station
simply does not make its next transmission on time, the receiving
station reports an error to its application.
Notice that the retry count does not apply in this case. BSC
does not repeat the read if a timeout occurs before a TTD
arrives.
Many devices use a 20-second timeout and disconnect the line if
such a timeout occurs. (The IBM General Information Manual
describes this feature.) To achieve this behavior with BSC,
use a 20-second read timeout for the station; if the timeout
occurs, disconnect the line with a CONTROL request.
CONTINUE TIMEOUT. A station that cannot transmit its next block
within the "read timeout" of the other station must transmit a
TTD sequence within two seconds of the last reply received.
The application CONTROL request can perform this function.
If the application fails to send a TTD sequence, a read timeout
occurs.
1-40