Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 30)

30 Publication 1756-UM009C-EN-P - December 2010
Chapter 2 Analog I/O Operation in the ControlLogix System
Output Modules in
a Remote Chassis If an output module resides in a remote chassis, the role of the RPI changes
slightly with respect to getting data from the owner-controller, depending on
what network type you are using to connect to the modules.
Remote Output Modules Connected Via the ControlNet Network
When remote analog output modules are connected to the owner-controller
via a scheduled ControlNet network, 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 will guarantee that the output module will
receive data at least as often as the specified RPI.
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 will receive the output data from the
controller in a networked 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.
ControlNet
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 Bridge module ControlNet Bridge module Output module
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