6100 ADCCP Programming Manual

Polling
ADCCP Link and Station Management
2–12 069225 Tandem Computers Incorporated
Polling One of the important tasks of the Level 2 Protocol is polling, in which a primary
station gives each secondary, in turn, the right to transmit. There are two types of
polling provided in the Tandem implementation of the ADCCP protocol:
Standard Polling
Alternate Polling
Standard Polling If the line operates in Asynchronous Response Mode (ARM) or Asynchronous
Balanced Mode (ABM), there is no need for either station to poll the other for data.
Either station may transmit at any time (although stations using two-way alternate
transmission cannot transmit simultaneously). If you want polling to occur in ARM or
ABM so each station knows the other is ready, specify the SYSGEN POLL parameter.
If the POLL parameter is set, the ADCCP protocol module sends RR frames to the
remote station when the line is idle. The line is idle when the local station has no data
to send (no SENDTEXT requests are pending) but would like to receive data. The
IDLETIMER parameter specifies the interval between polls.
In Normal Response Mode (NRM), polling occurs automatically when the line is idle if
the local station is the primary station on a point-to-point line. The POLL parameter
does not apply in this case, but IDLETIMER does, specifying the interval between poll
frames.
If the local station represents one or more secondary stations, the local station does not
do any polling; it only responds to them. If a secondary station is polled and has data
to send (it has data if a local application made a SENDTEXT request for that station), it
responds to the poll with data. If the secondary station is polled and does not have
data to send, it responds with an RR frame. (The responses described here are the
normal responses. A station with an LRNR, NOPOLL, or ERRORSTOP condition
behaves as described earlier in the subsection “The Information Transfer State (ITS).”)
If a secondary station is busy, it responds to the primary station with an RNR frame
with the final bit set, and the primary station polls the next station in the list. When
the secondary station is again polled and responds by sending an RR frame with the
final bit set, indicating it is no longer busy, the primary station holds outstanding
I-frames for the secondary station until all other remote stations have been polled.
Figure 2-6 shows the sequence of frames exchanged in standard polling. (The
numbers in the illustration indicate the station.)