Rate Measurement Mode 1756-HSC, 1794-VHSC

Rate Measurement mode counts incoming pulses on channel A depending on the time interval you specify. At the end of the interval, the 1756-HSC and the 1794-VHSC modules return 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.

The total count equals the number of pulses received during the sample period. Rate Measurement mode operation is shown in the image.

In the figure above, three counts have been accumulated during the time period selected. If you specified 50ms as the sample period, the frequency returned to the programmable controller processor is:

Frequency = Counts/Sample period = 3 counts/50ms = 60Hz.

If the frequency exceeds 500KHz, the value 999,999 is returned to the controller.

Sample Period

You can set the sample period used in the frequency calculation in the Rate Measurement mode. Allowable values are 10ms to 2 seconds in 10ms increments. The default value is 1 second.

A value of 0 in the ladder logic equals 1 second.

Connection to Channel Inputs

The only user connection used in the Rate Measurement mode is to input A of the module. The Input B and Input Z terminals are not used in this mode.