Rockwell Automation Publication 5069-UM002A-EN-P - January 2019 195
Develop Control Applications Chapter 8
Connection Overview
Logix 5000 controllers provide connection resources when communications
are established between two devices.
Connections are used when the system contains the following conditions:
• I/O modules, communication modules, and adapters are present in the
I/O configuration of the user project.
• Produced or Consumed tags are configured in the user project.
• Connected Messages are executed in the user application.
• External devices, programming terminals, or HMI terminals
communicate with the controller.
Controller Communication
Interaction with
Control Data
The CompactLogix 5480 controller runs the communications task separately
from the application code. The controller runs communications
asynchronously to the application.
Therefore, it is important to make sure communications that are delivered to
the controller are complete before the application executes on the newly
delivered data. This applies to data that is coming into the controller and data
that is going out from the controller.
For example, if an HMI device writes a large block of recipe data to the
controller, the application code can start to execute on that data before the data
is written. This action results in half of the current recipe and half of the last
recipe in the application space.
Traditionally, programmers have used the following to control the effects of
asynchronous communications:
• UID/UIE pairs
• Periodic tasks
• Moving data with CPS instructions.
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