Allen-Bradley POINT I/O 1734-VHSC24, User Manual, Page: 16 / 64

Publication 1734-UM003A-EN-P - August 2000
1-8 About the Very High Speed Counter Modules
Rate Measurement Mode
The Rate Measurement mode determines the frequency and total
number of input pulses over a user-specified sample period. 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.
Frequency is calculated by dividing the accumulated count by the user
selected time period, and is returned in the read data. Allowable time
periods are 10ms to 3s in 10ms increments, with a default value of 1s.
Note that a “0” time period is equivalent to the 1s default.
EXAMPLE
Example of Rate Measurement Mode
Pulse Width Modulation (PWM) Mode
The Pulse Width Modulation mode uses the counter to generate a
continuous rolling sequence of numbers. The real-time PWM value
written to the module is converted to a window edge so that a
variable duty cycle signal can be generated. The counter will reset to
zero based upon the PWM Period programmed into the module. Any
output tied to Window 0 will transmit the PWM signal.
A Input
B ( Not Used )
Encoder/Pulse Generator
1734-VHSC
Input A
Input B
A Input ( Pulse )
Internal Sampling Gate
(Gate / Reset )
Input Z
Z ( Not Used )
User Selectable Sample Period,
If Sample Period is 50ms, and Count = 3, then Frequency = 3 /50ms = 60Hz
Accumulated Count
1
Time Base
2 3
Frequency Calculated,
Outputs Updated Here
10ms to 2s in 10ms increments.