Output operation – Rockwell Automation 1756-HSC ControlLogix High Speed Counter Module User Manual
Page 35
Rockwell Automation Publication 1756-UM007C-EN-P - November 2011
Frequency Modes
35
Output Operation
The Period Rate and Continuous Rate frequency operational modes differ in
the operation of their respective on-board outputs. Both modes use count
values that you enter in the ‘Output Turns On’ and ‘Output Turns Off ’ fields
on the Output Configuration tab. These user-defined presets turn an output
On and Off. These On and Off count values are compared to the internal
4 MHz counts returned in the Present Value tag.
The Period Rate output On/Off presets are checked only once per sample
period. Therefore, outputs are only checked against their On/Off values and
updated once per scaler number of incoming pulses.
The Continuous Rate output On/Off presets are checked continuously during
the sample period. Therefore, outputs are dynamically checked against their
On/Off values and can be updated multiple times per scaler number of
incoming pulses.
For example, assume that the module was programmed to turn On an output
with a count value = 20,000 and Off at a count value = 80,001. Also assume
that the incoming frequency resulted in the 4 MHz clock count in the
Present Value tag = 40,000 with a scaler of ‘1’.
In Period Rate mode, the output would always be On because at the end of
every sample period the Stored Value, Present Value, and Totalizer tags would
be updated and the outputs compared against their On/Off values. The
number of 4 MHz counts in the Present Value tag would be 40,000, which is
between 20,000 and 80,001, therefore, the output would be On.
In Continuous Rate mode, the output state would change from Off to On to
Off during the incoming external pulse. In this mode, the output presets are
checked continuously against the 4 MHz count on the module. Initially, the
4 MHz count is zero and begins incrementing on the leading edge of the
incoming pulse. The count continues to increment, whereupon it reaches
20,000 counts and the output turns On. The internal 4 MHz count continues
incrementing until 40,000 counts, whereupon the pulse goes low and resets the
4 MHz count to zero, and the cycle repeats.
In both Period Rate and Continuous Rate, the Present Value, Stored Value,
and Totalizer tags are updated at the end of the sample period.
See
for square wave examples in Period Rate and Continuous Rate
modes.