Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 27)

Publication 1756-UM009C-EN-P - December 2010 27
Analog I/O Operation in the ControlLogix System Chapter 2
Input Modules
in a Remote Chassis If an input module physically resides in a remote chassis, the role of the RPI
and the module’s RTS behavior change slightly with respect to getting data to
the owner-controller, depending on what network type you are using to
connect to the modules.
Remote Input Modules Connected Via the ControlNet Network
When remote analog I/O modules are connected to the owner-controller via a
scheduled ControlNet network, the RPI and RTS intervals still define when
the module will multicast data within its own chassis (as described in the
previous section). However, only the value of the RPI determines how often
the owner-controller will receive it over the 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 guarantees that the owner-controller
receives 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 bridge module sends input
data back to the owner-controller at least as often as the RPI.
The ‘reserved’ spot on the network and the module’s RTS are asynchronous to
each other. This means there are best and worst Case scenarios as to when the
owner-controller will receive updated channel data from the module in a
networked chassis.
ControlNet network
Multicast data
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Local Chassis Remote Chassis