Hello,
I was wondering if someone could help me understand how Ignition updates tags from a ControlLogix PLC, and maybe point me to the documentation about it as I can't seem to find it.
Does it poll every tag in the tag provider or is it report by exception? I was under the impression it polls every tag. Clarification on this would be very helpful, thanks in advance!
There's a few systems in between your OPC tag in its provider and the PLC.
All of Ignition's drivers are plugins for Ignition's OPC UA server. There's also an OPC connection set up to that server in every installation by default. The Logix drivers know how to browse Logix PLCs, which means you can use the OPC browser to drag and drop a tag from the OPC browser to the tag browser.
Once you've done this, you've created an OPC tag. There is a corresponding Monitored Item created on the OPC UA server. In this case, the Monitored Item is for a tag that belongs the some CLX device instance you've configured. That device now starts polling the PLC for that tag.
Between the OPC server and the PLC it's polled. From the OPC server to the OPC client (and then into the tag system) it's report by exception.
This is basically true for any OPC server you are connected to, the only thing that might vary is how the underlying data is collected from the PLC, and that usually just depends on the protocol.
Thanks Kevin, it's very clear now! Where can I find this in the documentation?
I don't think there's anything in the documentation that tries to lay it out like this.