Operation Of Scalar - Allen-Bradley 1734-IJ User Manual

1-6 About the Modules
Publication 1734-UM006B-EN-P - August 2005
Relationship Between Sampled Pulses and Input Frequency
Input Frequency at Z
Gate/Reset Terminal
2.5Hz
5Hz
10Hz
20Hz
50Hz
100Hz
200Hz
500Hz
1kHz
2 Hz
5kHz
10kHz
20kHz
50kHz
100kHz
Scaling the input frequency through the use of a scalar can lessen the
decrease in accuracy. A scalar value of 1 returns an accurate input
frequency only if incoming input pulses have a 50% duty cycle.

Operation of Scalar

In the Period/Rate mode, 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)
Sample Pulses for 1/2 Cycle
of Z Gate/Reset Pulse
1M
500k
250k
125k
50k
25k
12.5k
5k
2.5k
1.25k
500
250
125
50
25