Allen-Bradley POINT I/O 1734-VHSC24, User Manual, Page: 14 / 64

Publication 1734-UM003A-EN-P - August 2000
1-6 About the Very High Speed Counter Modules
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
following Scaling table.
Relationship Between Sampled Pulses and Input Frequency
Scaling the input frequency through the use of a scalar can lessen the
decrease in accuracy. A scalar value of 1 will only return an accurate
input frequency if incoming input pulses have a 50% duty cycle.
Operation of Scalar
In the Period/Rate and Continuous/Rate modes, the scalar lets the
incoming pulse train at the Z Gate/Reset pin be divided by a user
defined number. There is one scalar value for each counter.
Acceptable values for the scalar are 1, 2, 4, 8, 16, 32, 64, and 128. The
default value for each scalar is 1. Note that a “0” scalar is equivalent
to a “1”.
The product of the Sample Period times the scalar should be less than
6.71 seconds in order to avoid a zero frequency detect indication.
(5 MHz sample time = 200ns; 16,777,216 counts x 200ns x 2 half cycles
of Z = 6.71 seconds)
Input Frequency at Z Gate/
Reset Terminal Sample Pulses for 1/2 Cycle
of Z Gate/Reset Pulse
2.5Hz 1M
5Hz 500k
10Hz 250k
20Hz 125k
50Hz 50k
100Hz 25k
200Hz 12.5k
500Hz 5k
1kHz 2.5k
2 Hz 1.25k
5kHz 500
10kHz 250
20kHz 125
50kHz 50
100kHz 25