Texas Instruments 990 User Manual page 214

Throughput
When the primary station has a message to send to the secondary,
the message can be sent in place of the poll. Alternatively, the
primary can send a poll to the secondary indicating that a
message is to be sent and asking if the secondary can receive the
message. Many protocol variations exist, and within each
protocol many possible implementations can be used to handle this
case.
Another method of polling is hub polling. This method is
generally used on data links that are connected by modems over
long distances. The poll cycle begins when the primary polls the
station that is physically the greatest distance away. That
station then polls its nearest neighbor, and so on, until finally
the last station issues a return poll to the primary. This
method of polling requires more complex software at each
secondary than the roll call method. It also requires that each
secondary know the address of its neighbor (the next station to
be polled) and have recovery ability in case that station does
not respond to the poll. This method is intended, in part, to
overcome the propagation delay incurred when transmitting over
great distances.
B.2 HDLC PACKAGE POLLING METHOD
The DXIO HDLC Communications Package provides a method of polling
that is designed to improve throughput to the secondary stations.
The method is best described as time delayed roll call with data
priority. With this method, polls to each secondary station
occur after measured delays unless a message is queued for that
station during the delay period. In that case, the delay is
preempted and the message is transmitted. The time delay between
polls is selected by the user and can range from a minimum of 0
milliseconds to a maximum of 8000 seconds (133.33 minutes) in
steps of 250 milliseconds. A zero-second delay implies that the
station should be polled at every poll opportunity. If all
secondary stations on one multipoint line have a delay of zero,
the polling reverts to pure roll call (that is, each station is
polled every time the poller accesses that station~s entry in the
polling list).
If the poller finds no station ready for polling during the
entire scan, it starts the next scan by polling the first station
in the list. It then continues through the list, omitting any
station whose time delay has not expired. On the next scan, it
starts by polling the second station in the list: on the third
scan, it starts with the third station: and so on. Using this
method, all secondary stations in the list are polled in an
orderly manner during periods of no scheduled polling activity.
B-2
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