Rockwell Automation 1756-LSC8XIB8I ControlLogix Low-speed Counter Module - Manual (Page 13)

Rockwell Automation Publication 1756-UM536A-EN-P - April 2012 13
Module Operation Chapter 2
On/Off Windows
Each counter has two configurable On/Off windows that compare the
accumulated count or frequency of incoming pulses to user-defined On/Off
values. When the count or frequency values are within the user-defined window
parameters, the module sets the corresponding bit in the Counter[
x].InWindow0
or Counter[
x].InWindow1 input tag.
The module produces data to the system on the rising and falling edge of each
On/Off window. A rising edge occurs when a count or frequency value enters the
window, and a falling edge occurs when a count or frequency value exits a
window.
You define each On/Off window by using these parameters:
• Comparison method—Defines whether the On/Off window uses
accumulated count, instantaneous frequency, or average frequency. You
define the comparison method for a window on the Counter
Configuration tab of the Module Properties dialog box.
• On and Off values—Defines the count or frequency value that results in
an On/Off status for the window. On and Off values represent counts or
frequency depending on the window’s comparison method. You define
these values in a set of output tags for each window:
– Counter[x].Window0On and Counter[x].Window0Off
– Counter[x].Window1On and Counter[x].Window1Off
IMPORTANT Keep in mind the following when using frequency as a window comparison
method:
• When configured to compare frequency values, window On/Off values are
still DINT (32-bit signed integers) while the returned frequency values are
REAL (32-bit IEEE float). As a result, the frequency triggers for On/Off
windows can only be defined in 1 Hz increments.
• Fluctuations in high frequency values across window parameters could
cause the window to transition on each pulse if the input frequency is at a
window parameter and you are using instantaneous frequency as the
comparison method. In this case, the module will produce a COS message
on the backplane with each input. This high traffic could result in system
communication issues.
• For example, if you set a window Off value at 18 kHz, and the input is at
18 kHz, the instantaneous frequency calculation could result in frequency
fluctuations for each pulse between 17998.0 Hz and 18002.0 Hz. This
fluctuation would cause a COS message to be sent every 55 μs. If this
situation occurs for all eight counters, the module can generate a large
amount of backplane traffic possibly resulting in system communication
issues.