6100 BSC Programming Manual

6100 BSC Concepts and Context
Enquiry (ENQ). The ENQ character has two special functions,
apart from the line bid and TTD sequences:
• If BSC receives a garbled reply to a message block, or if it
doesn't receive a reply at all within the expected time, it
uses the ENQ sequence to request another reply. (The expected
time is 2.7 seconds for a primary station, 3 seconds for a
secondary station.) BSC sends the sequence as often as the
retry count allows; this activity is transparent to the
application. Figure 1-13 illustrates this use of ENQ.
• Some devices insert an ENQ within a message heading or block
to tell the receiving station to disregard the block. After
receiving a NAK reply from the remote station, the device
repeats the aborted block. Figure 1-14 shows this use of ENQ.
NOTE
BSC doesn't use ENQ this way. Nor should your application
send that character, except in the context of ID exchange or
as data in transparent text.
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