6100 BSC Programming Manual

Writing Applications that Use 6100 BSC
error by giving the station more chances to succeed in each
operation.
• The FETCH STATISTICS request retrieves line quality
information and other data useful for evaluating problems.
• The ABORT request cancels an active request, unless that
request is FETCH STATISTICS or FETCH CONFIGURATION. (Review
the list of requests in Figure 3-1.) ABORT does not undo any
actions, but completes the request abruptly with an error
message.
NOTE
An application can cause error behavior by including in
its output buffer a control character that shouldn't be
there. For example, if a message contains an embedded
ETX character, that character will cause repeated errors
in the block check comparison. Once the retry count is
exhausted, the request completes with an error. (This
phenomenon doesn't occur, of course, in the case of
transparent text.) A missing control character will
also cause errors.
If the application keeps getting errors but nothing seems
to be wrong with the line, use CMI to trace output frames
and look for embedded or missing control characters. Trace
the frames as they leave the I/O process, not as they reach
the line; a trace of the line captures only the beginning and
end of the text.
Cancelling a Request
There are two requests you can use to cancel earlier requests:
• The ABORT request cancels an active request, except FETCH
STATISTICS or FETCH CONFIGURATION. It does not undo any
actions, but completes the request abruptly with the
status "request aborted."
• The HALT READ request cancels an active READ request if the
local station has not received a line bid. (If a bid has
arrived from the other station, HALT READ has no effect.)
Notice that HALT READ can cancel only an initial READ.
Do not use the GUARDIAN CANCEL call with 6100 BSC.
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