Frequency mode – Rockwell Automation 1756-HSC ControlLogix High Speed Counter Module User Manual
Page 30
Rockwell Automation Publication 1756-UM007C-EN-P - November 2011
30 Frequency Modes
The difference between the Period Rate and Continuous Rate modes is the
outputs are dynamic (On/Off) throughout the sample period for Continuous
Rate while Period Rate outputs are updated only at the end of the sample
period. Your desired output behavior should determine whether one uses
Period Rate or Continuous Rate modes.
See
for details.
Frequency Mode
In Frequency mode, the module counts incoming pulses on channel A for a
user-specified time interval that is configured in the Scaler tag. At the end of
the interval, the module returns a value representing the sampled number of
pulses in the Present Value tag, a value indicating the incoming frequency in
the Stored Value tag and a value indicating the total number of pulses that have
occurred in the Totalizer tag.
When the count and frequency are updated at the end of the sample period,
any associated outputs are checked against their associated presets. The output
On/Off values are related to the value in the Stored Value tag.
As you increase the Scaler (see
Sample Period for Frequency Mode
), the accuracy
of the frequency and the time between samples will increase. In general, if you
are measuring a higher frequency, the Scaler can be small. If you are measuring
a lower frequency, the Scaler likely will be larger.
Preset and rollover tag settings are active in this Frequency mode. User-defined
preset and rollover commands provide control of the starting and ending
points of incoming pulses, thus affecting the values in the Totalizer tag.
See
in Chapter 2 for preset and rollover tag details.
Where Frequency Values are Stored in Tags
Mode Description
Present Value Tag
Stored Value Tag
Totalizer Tag
Frequency
No. of input pulses occurring
in Sample Period
Frequency
Accumulated pulse count
Period Rate Frequency
No. of 4 MHz pulses occurring
in Sample Period
Continuous Rate Frequency
EXAMPLE
Frequency = No. of pulses per sample period/Scaler Time.
For example, if the frequency = 30 Hz, and the Scaler = 100 ms, then
the Present Value tag returned = 3, and the Stored Value tag = 30.