Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 28)

28 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Chapter 2 Digital I/O Operation in the ControlLogix System
Because the RPI and COS functions are asynchronous to the program scan, it is
possible for an input to change state during program scan execution. The point
must be buffered to prevent this from occurring. To buffer the point, you can
copy the input data from your input tags to another structure and use the data
from there.
Trigger Event Tasks
When configured, ControlLogix digital input modules can trigger an event task.
The event task lets you execute a section of logic immediately when an event, or
receipt of new data, occurs.
Your ControlLogix digital I/O module can trigger event tasks whenever module
input data changes state. Refer to these considerations when using a digital input
module to trigger an event task:
• Only one input module can trigger a specific event task.
• Input modules trigger the event task based on the module’s COS
configuration. The COS configuration defines which points prompt the
module to produce data if they turn On or Off. This production of data
triggers the event task.
• Typically, enable COS for only one point on the module. If you enable
COS for multiple points, a task overlap of the event task may occur.
For more information on event tasks, refer to the Logix5000 Controllers Tasks,
Programs, and Routines Programming Manual, publication
1756-PM005
.
Input Modules in a
Remote Chassis If an input module physically resides in a chassis other than where the
owner-controller resides, the role of the RPI and the module’s COS behavior
changes slightly with respect to getting data to the owner.
The RPI and COS behavior still define when the module will multicast data
within its own chassis, as described in the previous section. But, only the value of
the RPI determines when the owner-controller will receive it over the network.
TIP To minimize traffic and conserve bandwidth, use a larger RPI value if COS is
enabled and the module is in the same chassis as its owner-controller.