Rockwell Automation 1756-CFM ControlLogix Configurable Flowmeter Module - Manual (Page 24)

24 Rockwell Automation Publication 1756-UM010B-EN-P - December 2011
24 Configurable Flowmeter Operation in the ControlLogix System
Configurable Flowmeter
Modules in a Remote
Chassis
If an CFM module resides in a networked chassis, the role of the RPI changes
slightly with respect to getting data to the owner.
The RPI not only defines when the module multicasts data within its own
chassis (as described in the previous section), but also determines how often
the owner-controller will receive it over
the network.
When an RPI value is specified for an CFM module in a remote chassis, 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 guarantees that the owner controller
receives data at least as often as the specified RPI.
See Figure 2 for a better understanding of the data flow with a CFM module in
a remote chassis.
Figure 2 - CFM Module in Remote Chassis with RPI Reserving a Spot in Flow of
Data
ControlNet
CFM data at least as often as RPI
Data in remote chassis
at the RPI rates
Owner controller ControlNet Bridge module ControlNet Bridge module CFM module
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