Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 73)

Period/Rate and Continuous/Rate Examples E–2
When doing BTWs whose length is greater than 3 or if the BTW
data changes, the potential to miss pulses is limited to (number of
pulses in 6ms – scaler) every time there is a transition of BTW data
if the number pulses that occur in 6ms is greater than scaler. If the
number of pulses that occur in 6ms is less than the scaler, the count
will remain accurate.
There is no minimum limit to the input pulse width as long as no
more than the scaler number of pulses appear in (1/above allowable
input frequency) seconds.
It is important to note that even if the above frequency is exceeded
the 4MHz count and frequency will still be accurate. Only the total
counts returned will be unreliable.
Changes made in Revision B
The following are changes made to the 1771-VHSC module in
revision B.
Period/Rate mode mode has been modified such that if the pulses at
the gate/reset stop, the frequency (BTR words 11–18) will go to zero
and the 4MHz count (BTR words 3–10) will go to 999,999.
Continuous/rate mode has been added. This mode operates identical
to period/rate mode with the exception of the outputs.
If the frequency is very near (scaler x 2Hz), the most significant
digits of the 4MHz count can be greater than 999. This value will be
accurate.
In period/rate and continuous/rate modes, the smallest frequency
returned equals (2 x scaler)Hz. Frequencies less than this will return
0Hz.
The frequency will go to 0 and the 4MHz count will go to 999,999
250–260ms after the last pulse. The new data bit will also be set.
Words 19 through 26 have added to the BTW.