Texas Instruments 990 User Manual page 215

Throughput
After a message is transmitted to a station, the poller uses the
following process to ensure that a sufficient number of attempts
is made to collect any response data that the secondary station
needs to transmit. The poller sends a series of ready-to-receive
polls in the following sequence:
1. Ten polls with no delay
2. Ten polls with a delay of at
least 250 milliseconds
between each poll
3. Ten polls with a delay of at least 500
mi lliseconds
between each poll
4. Ten polls with a delay of at least 750 mi lliseconds
between each poll
After the last poll in this cycle is sent, the poller reverts to
the normal delay between polls for this station. This polling
activity gives the secondary station approximately 15.5 seconds
to formulate a response message and send it back to the primary
station.
The usefulness of this method can be seen in the operatl0n of the
PM550 Programmable Controller. After it is loaded with a
functioning program, it can operate for long periods of time
without any direction from a primary station program. The time
between messages to this device should be known when the network
is defined; the delay between polls to this device should be
specified to limit the number of polls sent to it. Ideally, the
PM550 should never be polled if the time between message
transmissions to it is fairly short, that is, less than one or
two minutes. The device would normally be polled for status at
one- or two-minute intervals; but if a message is queued for it
during this interval, the delay between polls is preempted and
the message is transmitted immediately. After the message is
transmitted, the primary station issues the polls at the faster
rate (described above) and eventually resumes polling the device
at the selected time interval. Under operational circumstances
such as these, it would be wasteful to poll the secondary every
250 milliseconds or so. In fact, it may be wasteful to ever poll
the secondary if some primary station program will be sending it
messages fairly frequently.
In contrast to the preceding example, a TX5 secondary can be
configured with software that operates independently of any
primary station software. For these secondary stations a maximum
acceptable time delay between polls must be selected to ensure
that the secondary station's data is collected as required. A
time value from 500 milliseconds up to 5 or 10 seconds may be
acceptable for such secondary stations, depending on the
application involved.
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