Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules - Manual (Page 46)

46 Rockwell Automation Publication 1756-UM533C-EN-P - February 2011
Chapter 3 ControlLogix Module Operation
Output Module Operation The requested packet interval parameter governs exactly when an analog output
module receives data from the owner-controller and when the output module
echoes data. An owner-controller sends data to an analog output module at the
period specified in the requested packet interval
. Data is not sent to the
module at the end of the controller’s program scan.
When an analog output module receives new data from an owner-controller
(every requested packet interval), the module automatically multicasts or echoes
a data value that corresponds to the analog signal present at the output terminals
to the rest of the control system. This feature, called Output Data Echo, occurs
whether the output module is local or remote.
Depending on the value of the requested packet interval, with respect to the
length of the controller program scan, the output module can receive and echo
data multiple times during one program scan.
When the requested packet interval is less than the program scan length, the
controller effectively lets the module’s output channels change values multiple
times during a single program scan because the output module is not dependent
on reaching the end of the program to send data.
Output Modules in a
Local Chassis When specifying a requested packet interval value for an analog output module,
you instruct the controller when to broadcast the output data to the module. If
the module resides in the same chassis as the owner-controller, the module
receives the data almost immediately after the controller sends it.
TIP If the output module is not responding according to how it has been
programmed, it could be for one of the following reasons:
•
The commanded value falls outside the Configured Limits and is thus
being clamped.
•
The commanded value changed faster than the configured max Rate
Limit, and is being clamped.
•
The module is in Start-up Hold mode, following a connection break or
Run mode transition waiting for the control system to synchronize
with the prevailing setting to facilitate a bumpless startup.
Data sent from owner
at the RPI
Owner-controller Output Module
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