Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 21)

1–13 Overview of the Very High Speed Counter Module
The value representing the sampled number of pulses is returned in
BTR words 3 thru 10, and the value indicating the incoming
frequency is returned in words 11-18. The total count equals the
number of pulses received during the sample period. The operation
of rate measurement mode is shown below in 1.10.
Figure 1.10
Operation of the Rate Measurement Mode
1771ĆVHSC
Module
From user's encoder/pulse generator
Gate/Reset terminal
(not used)
UserĆselectable sample time
From user's encoder/pulse generator
Channel A
Channel B
(not used)
10685ĆI
Example:
In 1.10, three counts have been accumulated during the user-selected
time period. If you had selected 50 milliseconds as the sample
period, the frequency returned to the programmable controller
processor in words 11-12 would be:
Frequency = Counts/Sample period = 3 counts/50 milliseconds = 60
Hz
You would read 60 Hz as the frequency in the Block Transfer Read
file (words 11 and 12). Words 3 and 4 would contain the value 3.
Since the default configuration for the VHSC module is the Counter
mode, the user must select the rate measurement mode through the
block transfer write initialization file. This is done by setting the
appropriate bits in words 3 and 4 of the block transfer write
initialization file (chapter 4). If frequency exceeds 500KHz, the
number 999,999 is returned.