Rockwell Automation 1734-VHSC24 Very High-Speed Counter Modules User Manual - Manual (Page 17)

Publication 1734-UM003B-EN-P - August 2005
About the Modules 1-7
Continuous/Rate Mode
The Continuous/Rate Mode returns an incoming frequency and a total
accumulated count to POINTBus, by gating an internal 5 MHz internal
clock with an external signal.
Similar to the Period/Rate mode except outputs in this mode are
updated continuously. This mode determines the frequency and total
number of input pulses by counting the number of internal 5 MHz
clock pulses over a user-specified number of input signal pulses. Each
output is turned on as soon as the turn-on count is reached, and
turned off as soon as the turn-off count is reached. As the internal 5
MHz clock is counted, the outputs dynamically track the 5 MHz count.
EXAMPLE
Example of Continuous/Rate Mode
As the frequency of the incoming pulse train at the Z Gate/Reset
terminal increases, the number of sampled pulses from the 5 MHz
clock decreases. Since accuracy is related to the number of pulses
received over the sample period, the accuracy decreases with
increasing frequencies at the Gate/Reset terminal. (Refer to the
“Operation of Scalar” information and table in the Period/Rate Mode.)
A ( Not Used )
B ( Not Used )
Encoder / Pulse Generator
1734-VHSC
Input A
Input B
Z Input ( Pulse )
5 MHz Internal
(Gate / Reset )
Input Z
Z
Accumulated Count
1
Scalar
5 MHz Clk
Sampling Clock
Assumes symmetrical pulse, 50% duty cycle, so Period = Sample Time On X 2 {On & Off}
10 20
Frequency = 1 / Period If Count = 20, Scalar = 1, and Clock Period = ( 1 / 5 MHz )
Frequency = 1 / [ ( 20 / 1 ) X ( 1 / 5 MHz ) X 2 ] = 125 kHz
Outputs Updated
Continuously
Frequency
Updated Here