Allen-Bradley POINT I/O 1734-VHSC24, User Manual, Page: 15 / 64

Publication 1734-UM003A-EN-P - August 2000
About the Very High Speed Counter Modules 1-7
Continuous/Rate Mode
The Continuous/Rate Mode will return an incoming frequency and a
total accumulated count to PointBus, by gating an internal 5MHz
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 5MHz
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
5MHz clock is counted, the outputs dynamically track the 5MHz
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 5MHz
clock decreases. Since accuracy is related to the number of pulses
received over the sample period, the accuracy will decrease 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