Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 72)

Appendix
E
Period/Rate and
Continuous/Rate Examples
The totalizer is always active in period/rate and continuous rate
modes. To access the values, the BTR length must be changed to a
value between 20 and 26 in multiples of 2. A length of 20 will return
the total count for C0, a length of 22 will return the total count for
C1 and C0, and so on. A BTR length of 0 will still return 18 words.
The reset bits will now reset the total count in words 19-26. The
count will continue to accumulate until 999,999 and then rollover to
zero. The presets and rollover in the BTW are inactive
When using the reset, you can always be off by (1–scaler). For
example, with a scaler of 8 after issuing a reset, you may have only 1
pulse occur, but your count may immediately go to 8, making you off
by 7 counts. Also, if you dynamically change the scaler, your count
can be off by the larger of the new or old scaler, and changing the
scaler will reset the count to 0.
The total count value will be updated every scaler number of pulses
at the gate/reset pin.
The maximum allowable input frequency for the total count to be
accurate varies with module configuration and the scaler by about
340–520Hz times the scaler.
For example, the best case is with only 1 channel operating in
period/rate or continuous/rate modes.
scaler = 1 Frequency = 520Hz
scaler = 2 Frequency = 1040Hz
scaler = 4 Frequency = 2080Hz
scaler = 128 Frequency = 66.5KHz
For example, the worst case is with all 4 channels operating in
period/rate or continuous/rate modes.
scaler = 1 Frequency = 340Hz
scaler = 2 Frequency = 630Hz
scaler = 4 Frequency = 1260Hz
scaler = 128 Frequency = 43.5KHz
The above is only true if you are not doing BTWs, the BTW length
is less than 3, or the data in the BTW does not change for any
counter.
General