Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 29)

Rockwell Automation Publication 1756-UM058G-EN-P - November 2012 29
Digital I/O Operation in the ControlLogix System Chapter 2
Remote Input Modules Connected via the ControlNet Network
When an RPI value is specified for an input module in a remote chassis
connected by a scheduled ControlNet network, in addition to instructing the
module to multicast 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 owner-controller will
receive data at least as often as the specified RPI.
As shown in the illustration below, the input data within the remote chassis is
multicast at the configured RPI. The ControlNet communication module sends
input data back to the owner-controller at least as often as the RPI.
Figure 3 - Remote Input Modules on ControlNet Network
The module’s RPI and reserved spot on the network 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 module performs an RPI multicast with updated
channel data just before the reserved network slot is made available. In this case,
the remotely-located owner receives the data almost immediately.
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ControlNet Network
Local Chassis Remote Chassis
Multicast Data