6100 ADCCP Programming Manual

Polling
ADCCP Link and Station Management
069225 Tandem Computers Incorporated 2–13
Figure 2-6. Standard Polling
Primary
Station
Secondary
Stations
RR-P (1)
RNR-F (1)
RR-P (2)
RR-F (2)
RR-P (1)
RR-F (1)
I (1)
RR-P (2)
RR-F (2)
I (1)
RR-P (1)
RNR-F (1)
019
The most complex polling occurs in cases where the line is in NRM and multipoint,
and the local station is the primary station. In these cases, the local station controls all
transmission on the link. Whenever requests for data (RECEIVETEXT) are pending
but there are no requests to send data (SENDTEXT), the ADCCP protocol module
polls the remote stations in round-robin fashion. When every remote station has had a
chance to transmit, the ADCCP protocol module waits for an interval of IDLETIMER,
then starts polling again. (A SENDTEXT request from an application can interrupt the
poll.) Once the ADCCP protocol module has sent the data to the specified station,
polling resumes where it left off. Although a poll may be posted in the idle time
between two frames, the poll is transmitted until the SENDTEXT frames have been
transmitted.
Several factors determine which stations are polled and in what order. If the local
station has a NOPOLL or ERRORSTOP condition, the corresponding remote station is
not polled. Likewise, if the local station is not in ITS, the remote station is not polled.
Remember that the local station can be in different states for different remote stations.
Also, an application can control station states to manage flow control. For example, by
setting LRNR, the application can call for RNR polling frames to be sent to particular
stations, while RR frames are sent to other stations.
To determine the polling order, the ADCCP protocol module refers to two lists. Both
lists result from specific application requests: