6100 BSC Programming Manual

Writing Applications that Use 6100 BSC
HALT READ Request and Response
The HALT READ request cancels an outstanding READ request if the
station is waiting for a line bid when you issue the HALT READ.
If a line bid has already arrived or if this is not an initial
READ, HALT READ has no effect.
This request lets a station keep a READ posted whenever it has no
data to send. If a bid arrives, the application accepts it and
receives data from the other station. If on the other hand the
application finds some data--for example, if a local transaction
completes--the application cancels its READ and makes a bid for
the line.
The application receives two completions: one from the HALT READ
request, and the other from the READ request. The HALT READ
request completes with a status code of 0, even if no READ
request was active or if a line bid had already arrived. To find
out what actually happened, the application processes the READ
response:
• A status code of 0 in the READ response means the READ was
able to complete, i.e., it wasn't cancelled. The new
line state is READ.
• A status code of 162 in the READ response normally means the
request was cancelled. The line state is CONTROL; you can
issue a line bid.
• Any other status code in the READ response indicates an
error condition that may require recovery.
HALT READ can be issued at any time, with the line in any state.
It is effective only under the conditions mentioned above.
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