Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 86)

86 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Chapter 5 Fast Module Features
Pulse Capture
The 1756-IB16IF fast input module can be used to detect or latch short duration
pulses. The module can detect incoming pulses with a duration as short as 10 μs if
the frequency is under 4 kHz (period of 250 μs).
When the module detects a short duration pulse at an input point, it sets the
corresponding bit for the Pt[
x].NewDataOffOn or Pt[x].NewDataOnOff input
tag. This bit remains latched until acknowledged. As a result, you can use this bit
to detect a transition that is too fast to be detected by the program scan. You can
also determine how rapid the transition was by configuring the module to latch
timestamps for the point, as described in
Per Point Timestamping and Change of
State on page 87 .
To acknowledge the last captured pulse and reset the pulse latch, you set the rising
edge of the corresponding bit in these output tags:
• Pt[
x].NewDataOffOnAck—Acknowledges that the input point has
transitioned to an On state and resets the pulse latch.
• Pt[
x].NewDataOnOffAck—Acknowledges that the input point has
transitioned to an Off state and resets the pulse latch.
You can change output tag values in program logic while normal module
operation continues or through the RSLogix 5000 tag editor. For more
information about module tags, refer to
Appendix B .
Once a pulse latch is reset for an input point, the next pulse at that point sets the
corresponding bit in the Pt[x].NewDataOffOn or Pt[x].NewDataOnOff input
tags.