Texas Instruments 990 User Manual page 218

Throughput
B.4.2 Polling Time for a Single Line
Assuming that all secondary stations are polled in roll call
fashion, the total time to send polls and collect responses at
9600 bps on one line that has 32 secondary stations attached is
32 times 10 milliseconds, or about 320 milliseconds. Note also
that with the option of selecting a time delay between polls, a
delay value of 250 milliseconds between polls is not a realistic
selection if that delay value is selected for every secondary on
a line with more than about 25 secondary stations. With 250-
millisecond delays, the time delay will expire before the 24th or
25th station is polled, or sooner if data is sent as part of a
poll or a response. Once this time delay has expired for most of
the secondary stations, the delay time is of no value and the
polling reverts to pure roll call.
For any line data rate below 9600 bps, the polling reverts to
roll call for a smaller number of secondary station. Assuming a
line data rate of 300 bps, the time to send one poll with
response is 48/300 + 48/300
=
320 milliseconds. If a 250-
millisecond delay is selected for every secondary stations, it
will expire for all secondary stations before the response to the
first poll is completed.
The sum of the polling operations at the data link level is shown
below. The TAT (T3 T2) is the TAT for the data link level
software and any line turnaround time required: it does not
include TAT for upper-level or task-level replies.
TO T1
2280616
where:
T2
I
I
T3 T4 T5
TO is the start of the FCCC poller program scan through
the polling list.
Tl is the start of the poll transmission over the line.
T2 is the end of the poll transmission.
T3 is start of the response transmission.
T4 is the end of the response transmission.
T5 is when the FCCC poller program processes the response
and goes back to the function at TO to poll the next
station in the list.
TI-TO is the FCCC table scan and processing time.
B-6
2270526-9701