Rockwell Automation Publication 1756-UM058G-EN-P - November 2012 31
Digital I/O Operation in the ControlLogix System Chapter 2
Output Module Operation
An owner-controller sends output data to an output module when either one of
two things occur:
• At the end of every one of its tasks (local chassis only)
• At the rate specified in the module’s RPI
When an output module physically resides in a remote chassis with respect to the
owner-controller, the owner-controller sends data to the output module only at
the RPI rate specified for the module. Updates are not performed at the end of
the owner-controller’s tasks.
Whenever the module receives data from the controller, it immediately
multicasts the output commands it received to the rest of the system. The actual
output data is echoed by the output module as input data and multicast back out
onto the network. This is called output data echo.
Output Modules in a
Local Chassis
The owner-controller updates ControlLogix digital output modules in the local
chassis at the end of every task and at the RPI.
When you specify an RPI value for a digital output module, you instruct the
owner-controller when to broadcast the output data to the module. If the module
resides in the same chassis as the owner-controller, as shown in the illustration
below, the module receives the data almost immediately after the owner-
controller sends it. Backplane transfer times are small.
Figure 4 - Local Output Modules
Depending on the value of the RPI with respect to the length of the program
scan, the output module can receive and echo data multiple times during one
program scan.
In this Producer/Consumer model, the output module is the consumer of the
controller’s output data and the producer of the data echo.
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Data is sent at the end of
every task and at the RPI.
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