Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 222)

222 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Appendix C Use Ladder Logic To Perform Run Time Services and Reconfiguration
Use Timestamped Inputs and Scheduled Outputs
for Standard and Diagnostic I/O Modules
This section demonstrates the use of timestamped inputs and scheduled outputs
for standard and diagnostic digital I/O modules. The Change of State timestamp
can be used to synchronize the output turning On or Off based on the time that
the input transitions. The program can be extended to include synchronizing
multiple output modules by sending the same timestamp to all output modules.
In the example below, the output follows the state of input 0, but it is delayed by
exactly 10 ms. The advantage of using CST over timers is that the
synchronization is performed at the I/O module, which eliminates any jitter due
to controller or communication delays.
Your control becomes much more deterministic even under changing loads. For
this synchronization to work properly, the 10 ms delay must be long enough to
account for any controller, backplane, and network delays. The input and output
modules must reside in the same rack as a Time Master (controller). Timestamp
units are microseconds.
The following illustrations show the ladder instructions the program uses. The
rungs perform these tasks:
• Rungs 0 and 1 detect the transition from Program to Run mode. This is
used to turn On ‘init’, which causes the program to initialize its tags.
• Rung 2 only executes once and initializes the LastTimestamp.
LastTimestamp is used to detect a Change of State on the input point by
checking to see if the timestamp of the input data has changed.