Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 32)

32 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Chapter 2 Digital I/O Operation in the ControlLogix System
Output Modules in a
Remote Chassis If an output module physically resides in a chassis other than that of the
owner-controller, the owner-controller normally sends data to the output module
at the RPI rate specified. Updates are not performed at the end of the controller’s
tasks.
In addition, the role of the RPI for a remote output module changes slightly with
respect to getting data from the owner-controller.
Remote Output Modules Connected via the ControlNet Network
When an RPI value is specified for an output module in a remote chassis
connected to the owner-controller by a scheduled ControlNet network, in
addition to instructing the owner-controller to multicast the output data within
its own chassis, the RPI also reserves a spot in the stream of data flowing across
the ControlNet network.
The timing of this reserved spot may or may not coincide with the exact value of
the RPI. But, the control system will guarantee that the output module will
receive data at least as often as the specified RPI, as shown in the illustration
below.
Figure 5 - Remote Output Modules on ControlNet Network
The reserved spot on the network and the output data sent by the controller are
asynchronous to each other. This means there are best and worst case scenarios as
to when the owner-controller will receive updated data from the module in a
remote chassis.
Best Case RPI Multicast Scenario
In the best case scenario, the owner-controller sends the output data just before
the reserved network slot is made available. In this case, the remote output
module receives the data almost immediately.
42675
ControlNet Network
Local Chassis Remote Chassis
Data is sent from the
owner-controller. Output data is sent at
least as often as RPI.