Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 70)

Questions and Answers D–4
Q. How do my output on-off values work?
A. The first value is always the on value, and the second value is
always the off value. For example, with a rollover value of
2000, if I specify an on value of 1999 and an off value of 0, the
output will only be on when the count equals 1999. If I specify
an on value of 0 and an off value of 1999, the output will only
be off at a count of 1999.
Q. How do my outputs work if I tie them to an input used in
frequency mode.
A. If an output is tied to an input used in period rate mode, the
output will be triggered by the counts, not by the frequency.
The module provides better resolution by tieing the output to
the 4MHz clock count value instead of the frequency. For
example, in period rate mode with a scaler value of 1, feeding
the gate input a constant frequency of 285Hz will return a
frequency of 284-285, but the counts returned will be
7017-7019. The output is tied to the count value of 7017-7019.
If an output is tied to an input used in rate measurement mode,
the output is tied directly to the frequency. For example, with a
time base of 500msec the count returned will be 142-143, and
the frequency will be 284-286Hz. The output will be tied to the
frequency 284-286Hz.
Q. If I change the on-off value of an output, how long before it
takes effect?
A. If the output is tied to an input used in any of the count modes,
the change will take effect at the end of the BTW processing
time. The BTW processing time is based on the number of
words sent (5.5msec binary and 11msec BCD for a 64 word
transfer) after the BTW done bit is set.
If the output is tied to an input in either frequency mode, the
output change will not take effect until the data is processed as
above.
Q. What are the counts returned in rate measurement mode?
A. These counts are the number of counts received on channel A
during the selected sample time period. The counts are divided
by the specified time base and converted to frequency. For
example, with a time base of 500msec and a fixed frequency of
285Hz on channel A, a count value of 142-143 will be
returned, resulting in a frequency of 284-285.
Q. What are the counts returned in period rate mode?
A. These counts are the number of 4MHz internal clock pulses
that occurred during the selected number of scaler gate pulses.
Each pulse would be equal to the gate terminal being high for
0.25
µ
sec.