6100 MPS-B Programming Manual

WRITING APPLICATIONS THAT USE 6100 MPS-B
Transferring Data
The POLL/SELECT LIST CONTROL request and STATION CONTROL request
are described later in this section.
TRANSFERRING DATA
Once the link is properly established, data can be exchanged on
the line. To poll stations on the list, the application makes a
call to WRITEREAD containing a READ request. Depending on the
modifier included with the request, all stations on the list may
be polled in entry order (sequential poll), or just one station
may be polled (specific poll). In addition to these types of
polling, there is provision for the ADM-2 SEND function.
6100 MPS-B begins polling upon receipt of an explicit READ
request issued by your application process through CP6100. Note
that only one READ request from your application can be handled
at any given time by CP6100 and 6100 MPS-B.
Stations on the poll/select list can be in an enabled state,
disabled state, or on a separate list of nonresponding stations.
When polling begins, only stations in the enabled state are
polled. The READ request is completed (polling terminates) if a
station responds with data. It is the responsibility of your
application process to determine which of the polled stations
responded. A READ request can also be completed by an explicit
Halt Poll request from your application.
If a polled station does not respond, a per-station retry count
is incremented by one. If this retry count does not exceed the
retry count defined for the line, the station is polled again
during the next poll cycle. If a station responds with an error,
the retry count is incremented by one, and the station is polled
again in the same poll cycle until it either returns a valid
response or the line retry count is reached. In both cases, when
the line retry count is reached, the station is placed on the
nonresponding list. Except for a Specific Poll READ request, a
station on the nonresponding list is never polled until it is
removed from the nonresponding list using an explicit request to
remove all stations from the list.
A station in the disabled state, or one on the nonresponding
list, can be polled by your application process using a Specific
Poll READ request. In these cases, the station is always polled.
If it is a station on the nonresponding list and returns a valid
response, it is removed from the nonresponding list. Note that a
Specific Poll READ request is completed (terminates) even when
the station has no data to send.
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