Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules - Manual (Page 44)

44 Rockwell Automation Publication 1756-UM533C-EN-P - February 2011
Chapter 3 ControlLogix Module Operation
Input Modules in a
Remote Chassis If an input module resides in a remote chassis, the role of the requested packet
interval and the module’s real time sample 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 ControlNet Network
When remote analog I/O modules are connected to the owner-controller via a
scheduled ControlNet network, the requested packet interval and real time
sample intervals still define when the module multicasts data within its own
chassis
. However, only the value of the requested packet interval determines how
often the owner-controller receives it over the network.
When an requested packet interval 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 requested
packet interval also reserves a spot in the stream of data flowing across the
ControlNet network.
The timing of this reserved spot does not coincide with the exact value of the
requested packet interval, but the control system guarantees that the owner-
controller receives data at least as often as the specified requested packet interval.
The reserved spot on the network and the module’s real time sample are
asynchronous to each other. This means there are best and worst case scenarios as
to when the owner-controller receives updated channel data from the module in a
networked chassis.
ControlNet Network
Input data at least as often as RPI
Input data in remote
chassis at RTS and RPI
Owner-controller ControlNet Bridge Module ControlNet Bridge Module Input Module
Input Module in Remote Chassis with Requested Packet Interval Reserving Spot in Flow of Data
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