Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 69)

Questions and Answers D–3
A. In rate measurement, you select a time period in BTW words
21-24. The module will count pulses on channel A for this time
period, and then convert the number to frequency. It will begin
its next time period in about 10msec, and then begin counting
pulses again.
In period/rate or continuous/rate modes, the pulses coming in
on the gate/reset will gate the internal 4MHz clock using the
hardware scaler selected in words 21-24. If a scaler of 1 is
selected, you are measuring the number of 4MHz pulses that
occurred while the gate/reset was active, and the frequency
returned will only be accurate if the incoming pulses occur
with a 50% duty cycle. With all other scaler values, you are
measuring the number of 4MHz pulses that occur in the scaler
during a number of gate/reset periods that equal the the scalar
value. For example, with a scaler value of 8, the 4MHz clock
will be measured for four periods. The frequency value, 4MHz
count ND bit and outputs will then be updated and remain
constant as the counter is idle for the next four periods. Then
the 4MHz clock will be measured for the next four periods and
then be idle for four periods. Thus, the frequency is updated
every eight periods. For more information, refer to appendix E.
Q. How often is the BTR data updated at the module?
A. The data available to be read by the processor is updated every
1.5–2.9msec binary and every 3.0–5.8msec BCD. Note that
based on module configuration, some values may not be
updated at that rate. For example, a frequency value in rate
measurement mode will be updated at the time base selected in
the scaler word. The module can always be read by the
processor, but if read at a faster rate than determined by the
time base, you will read old data.
Q. How often can I do a BTW?
A. This varies with the length sent and whether or not any data has
changed. The worst case would be a 64 word BTW that
changes the module configuration. This would take about
5.5ms in binary (11.1ms in BCD) to process. You would not be
able to do another BTW for that length of time. If the BTW
data has not changed any module configuration on that
particular scan, the rate at which you can do a BTW will vary
(depending on module configuration) from about 1.5ms to
2.9ms. The best case configuration would be no channels in
period/rate or continuous rate; worst case configuration would
be all 4 channels in period/rate or continuous rate.