My evidence suggests it gets refreshed every 5 minutes but how do I prove that? I am using the tag
[System]Client/Network/Hostnamein an expression and every 5 minutes that expression is firing off. The only thing I can think of is the expression thinks it is being updated (even though the value is always the same).
Anyone care to chime in?
It’s on a five minute poll, yes. The system tags don’t do any actual logic to push out changes, I’m guessing, so every value refresh fires a new event which triggers your expression downstream.
Interesting. Based on my previous understanding of expressions I would have thought this was impossible since the value never changed. It was quite the mystery for a while.
The tag values are fully qualified, so if you are not looking at the just the value, an update with the same value will fire a tag change event because the timestamp changed.
Aren’t regular tags fully qualified?
The expression doesn't "know" what was delivered, only that a new value was. In most contexts in Ignition (e.g. Vision), for optimization and other reasons, "matching" events aren't sent through. It would be possible to update the system tags to do the same thing, but it's probably not worth the effort (and potential risk; see: Hyrum's Law).
That is a GOOD law… I would just insist that this particular feature of system tags be documented somewhere for the next person.
I think this odd behavior is unique to Client system tags… it doesn’t sound quite right to me for Gateway system tags.