Rockwell Automation Logix5000 Controllers Tasks, Programs, and Routines Programming Manual - Manual (Page 58)

Chapter 2 Manage event tasks
Maintain the integrity of data
An event task with a consumed tag trigger provides a simple mechanism to pass
data to a controller and make sure that the controller does not use the data while
the data is changing.
Description
RPI occurs for the produced tag.
The produced tag transfers old data to the consuming controller.
The producer controller starts to update the values of the produced tag.
RPI occurs again for the produced tag.
The produced tag transfers a mix of old and new data to the consuming controller.
The producer controller finishes updating the values of the produced tag.
The producer controller runs an Immediate Output (IOT) instruction.
The produced tag immediately transfers all the new data to the consuming controller.
When the consumer controller receives all the data, it runs its event task.
Although the producing controller runs the IOT instruction immediately after it
loads new data, the event task is not triggered (in the consuming controller) until
the consuming controller has received all the new data. This verifies that the
controller operates on a complete packet of new data.
58 Rockwell Automation Publication 1756-PM005-EN-P - October 2014