Rockwell Automation 1794-VHSC 1794 FLEX I/O Very High Speed Counter Module User Manual - Manual (Page 19)

Rockwell Automation Publication 1794-UM010D-EN-E - July 2013 11
Overview of the Very High Speed Counter Module
Period/Rate and Continuous/Rate Output Operation with Scaler of 1
Rate Measurement Mode Use the rate measurement mode to count incoming pulses for a user-specified
time interval. At the end of the interval, the module returns a value representing
the sampled number of pulses and a value indicating the incoming frequency.
When the count and frequency are updated, any associated outputs are checked
against their associated presets.
The value representing the total number of pulses is returned in input file words 4
(LSW) and 5 (MSW) for channel 0 and 6 (LSW) and 7 (MSW) for channel 1,
and the value indicating the incoming frequency is returned in words 0 (LSW)
and 1 (MSW) and 2 (LSW) and 3 (MSW). The total count equals the running
sum of the number of pulses received during the sample period. The operation of
rate measurement mode is shown in Figure
Operation of the Rate Measurement
Mode on page 12 .
Incoming pulse train at Z
(gate/reset terminal)
5 MHz internal clock
A (not used)
B (not used)
Input A
Input B
Z
Encoder/Pulse generator
1794-VHSC
Input Z
(Gate/Reset)
scaler
5 MHz clock
1
Accumulated count
Outputs updated continuously
Frequency
updated here
10 20
Assumes symmetrical pulse, 50% duty cycle, so period = sample time on X 2 (on and off)
Frequency = 1/period
If count = 25, scaler = 1, and clock period = (1/5 MHz)
Frequency = 1 / [(25/1) X (1/5 MHz) X 2] = 100 KHz
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