Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 47)

Rockwell Automation Publication 1756-UM058G-EN-P - November 2012 47
Common Module Features Chapter 3
Use the System Clock to Timestamp Inputs and Schedule Outputs
This section describes how to use CST timestamps in standard and diagnostic
I/O modules and the CIP Sync timestamps in fast I/O modules.
Use Coordinated System Time with Standard and Diagnostic I/O Modules
Time masters generate a 64-bit coordinated system time (CST) for their
respective chassis. The CST is a chassis-specific time that is not synchronized
with, or in any way connected to, the time generated over the ControlNet
network to establish a network update time (NUT). For more information about
NUT, refer to
Use RSNetWorx and RSLogix 5000 Software on page 20 .
You can configure your digital input modules to access the CST and timestamp
input data with a relative time reference of when that input data changes state.
The following table describes how you can use CST timestamps.
IMPORTANT Because only one CST value is returned to the controller when any input point
changes state, we recommend that you use timestamping on only one input
point per module.
Topic Description
Timestamping for a
sequence of events The CST can be used to establish a sequence of events occurring at a particular input module
point by timestamping the input data. To determine a sequence of events, you must do the
following:
• Set the input module’s communication format to CST Timestamped Input Data.
• Enable COS for the input point where a sequence will occur, and disable COS for all other
points on the module.
TIP If you decide to configure multiple input points for COS, your
module generates a unique CST each time any of those input
points change state, as long as the changes do not occur within
500 μs of each other.
If multiple input points configured for COS change state within
500 μs of each other, a single CST value is generated for all
making it appear that they changed at exactly the same time.
Timestamping in
conjunction with
scheduled outputs Timestamping can be used in conjunction with the scheduled outputs feature, so that after
input data changes state and a timestamp occurs, an output point will actuate at some
configured time in the future.
You can schedule outputs up to 16 seconds into the future. When you use timestamping of
inputs and scheduled outputs, you must do the following:
• Choose a communication or connection format for each input and output module that
allows timestamping. For more information, refer to Communication or Connection
Formats on page 131 .
• Have a time master in the same chassis as both I/O modules.
• Disable COS for all input points on the input module except the point being timestamped.
TIP For scheduled outputs to work most effectively, remember the
following items:
• The time to schedule outputs to transition in the future must
account for any controller, backplane, and network delays.
• The I/O modules must reside in the same rack as the time
master.