6100 BSC Programming Manual

6100 BSC Concepts and Context
Wait-Before-Transmit Positive Acknowledgement (WACK). A station
sends WACK as a reply if it is temporarily unable to receive.
WACK is an acceptable response to an ID sequence for circuit
assurance, to a line bid, or to a message block (including a
heading block). It must be issued within two seconds after
receiving the transmission.
Except in the context of circuit assurance, the application makes
a CONTROL request to send a WACK reply; it does not place a WACK
in its output buffer. During circuit assurance, the application
puts WACK in its buffer (as described earlier).
If an application receives a WACK in response to an ID sequence
or a line bid, it normally waits and repeats the same sequence at
a later time. If a WACK arrives in response to a message block,
BSC sends an ENQ sequence; the replying station sends WACK again
if it is still not ready, ACK 0 or ACK 1 if it is ready to
proceed. BSC keeps sending ENQ until the reply is ACK 0 or ACK
1, or until the retry count is exhausted. Then it reports an
error to the application.
WACK is a positive acknowledgement to the current message block.
Therefore, when a station that sent a WACK is again ready to
receive, the other station transmits its next block, not a repeat
of the previous one. Figure 1-16 shows the use of WACK in
the context of data transfer.
Reverse Interrupt (RVI). A station sends RVI as a reply instead
of ACK 0 or ACK 1 if it has urgent information it wishes to send
to the transmitting station. The RVI sequence can be a reply to
any message block, and has the form:
SYN SYN ... SYN RVI PAD
RVI is an option in the READ request; the application does not
supply the sequence in its output buffer.
The transmitting station reacts to RVI by quickly transmitting
the rest of its data. Normally it sends just as many blocks
as will empty its buffers, and relinquishes the line. The
station that sent the RVI then bids for the line to make its
transmission. Figure 1-17 shows the use of RVI.
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