Rockwell Automation 1756-IF4FXOF2F ControlLogix High-speed Analog I/O Module - Manual (Page 139)

Rockwell Automation Publication 1756-UM005B-EN-P - January 2013 139
Module Operation in a Remote Chassis Appendix E
When an RPI value is specified for a module in a remote chassis, in addition to
instructing the 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 guarantees that the output module receives data
at least as often
as the specified RPI.
Figure 17 - Owner-controller Sends Output Data to Remote Chassis
The reserved spot on the network and when the controller sends the output data
are asynchronous to each other. This means there are best and worst case
scenarios as to when the module receives the output data from the controller in a
remote chassis.
Best Case RPI Scenario
In the best case scenario, the controller sends the output data just before the
reserved network slot is available. In this case, the remotely located output
module receives the data almost immediately.
41360 ControlNet Network
Output data at least as often as RPI.
Immediate backplane
transfers to module. Data sent from owner at
module’s RPI rate.
Owner-controller
ControlNet
Communication Module Owner-controller
ControlNet
Communication Module