I was looking for some clarification on how the Historian evaluates OnChange events when accessing a PLC via Ignition's UA Client to a PLC-based UA Server. In my specific scenario, I am using a Siemens S7-1500 UA Server.
I have tag groups defined within ignition. I see each tag group as a unique connection in my OPC Client subscriptions. In addition to the "Tag Group Rate" set by the tag group, I also see the Publishing Interval (which I assume is determined by the OPC Server on the PLC based on the Server settings).
What I am looking for clarification on is when an Ignition tag is set to Historize based upon OnChange of the data. Reading this page from the manual, I would expect that a tag could potentially historize OnChange data at a faster rate than defined by the Tag Group Rate (subject to limits of the Publishing Interval of the UA Server).
However, I do not see any additional subscriptions to the UA Server in the OPC Client diagnostics page unique to Historical data. If the Historical OnChange is supposed to ignore the rate of the associated tag group, yet the only connections to the UA Server are determined by the Tag Groups that have been defined, how is the Historian subscribing to the data to evaluate the OnChange event? Does it use the Publishing Interval of the UA Server and ignore the Tag Group rate?
Example image from the OPC Client subscriptions page of my test gateway (rates are not realistic / not optimized).
