A piece of the user manual below (from this site) says that values are received asynchronously as they change.
This portion of the same page says ‘Rate’ controls the rate at which tags will be executed.
For an OPC tag that appears to get its value asynchronously (assuming a subscribed tag group group) and write its value based on the Publishing Interval property, what happens in an execution? Boiled down, what does the Rate parameter control on a subscribed OPC tag?
The requested sampling interval for each MonitoredItem in an OPC UA Subscription.
That's not what the Publishing Interval is. Publishing Interval is a property of an OPC UA subscription - basically how often, in the server, that any changes for any MonitoredItems belonging to a Subscription are gathered and sent to the client.
OPC UA subscriptions report changes as a batched report by exception, not per-item.
If I understand correctly (there’s a high probability that’s not the case) there’s sort of three “places” a value for a particular PLC tag exists: Ignition Tag, Ignition OPC-UA Server, and in the PLC itself.
Is that right or have I misunderstood the whole system?
Nope, that's pretty much correct.
Is this a property of Ignition Tag <--> OPC-UA or of OPC-UA <--> PLC?
It’s a request to the OPC UA server to sample the underlying value at that rate. With most traditional servers and drivers that translates to polling the value in the PLC at that rate.
So from the first post, Subscribed vs. Polled controls how Ignition tags get values from the OPC-UA server. The rate property tries to control how frequently the OPC-UA server polls values from an end device (at least in the common case). Is this right?
Yep. In Polled mode Ignition is just issuing read requests at that rate instead of creating a subscription and requesting the items be sampled at that rate.
It all makes a lot more sense now. Thanks for taking the time to walk me through this. Your (both IA the company and you personally) support really is top notch.
a couple of other clarifications I'd like to follow up with. would a subscribed tag only write to the database, if the value had changed? versus a polled tag would write every time the rate ticks by? we are working through server sizing and trying to analyze the existing tag base that are getting logged to historian.
No, a tag change event is still only fired when the value or quality changes.
gotcha, which that makes sense. Are there any scripts you have/know of to analyze the existing percentage change on the tags with history enabled?
