I have a tag change script that logs tag values to a database. It's set up to run on each tag's tagChange script.
Of course, this is fine for sensor values that are constantly changing, but some tags stay the same value for long periods of time and only change occasionally. How can I set up what would effectively be a "maximum sample time" so that I get a database entry at least every, say 3 seconds?
I did this for a production system where we wanted to log machine counter values on run / stop transitions but also for shift change (and the shift patterns were flexible). We scheduled it for every 15 minutes.
But why every 3 s? It seems excessive for a value that isn't changing. Why not an hour, for example?
Just for pretty charts.
Unless there's a way to get realtime/historic charts to hold/show the last value up until the current time that I'm not aware of.
For instance, if a tag's value is 16 and hasn't changed for 20 minutes and the realtime trend is only showing the last 5 minutes, it shows nothing.
Saying it out loud now, that doesn't sound right, but that's how I remember it working...
Let's see your query. There must be an SQL way of retrieving the last records before the start time UNION the records in the period of interest UNION the most recent record after the end time.
That's actually logged as a custom property of the tag. The only query that I have is an update query to add the row.
The pens in the chart are DB pens using the where clause.
That's actually logged as a custom property of the tag.
What is?
This sounds like a Vision chart problem. Can you add the appropriate tag?
These are the custom props on my tag.
Sorry, I might've misread your statement. I was not thinking SQL-ly
The value and time from the last database entry for the tag are kept as custom properties on the tag (tag.lastVal and tag.lastTime, respectively).
What do you mean by "add the appropriate tag"?
"add the appropriate tag?"
Forum tag under the thread title. I've done it.
Ok, I see. Is this specific to Vision though? I don't use Perspective enough to know how its chart works.
Not specific to Vision. Wherever Perspective uses time series that do not come from the historian, this same issue is present. If there's no data for a tag in a charted span, there's no trace.
For historian pens in Analog deadband style, historian queries inject the most recent sampled value as a virtual row, even if not yet recorded. (See the historian's docs for Analog deadband style.)
For historian pens in discrete mode, most of my clients specific a relatively short (one minute-ish) "max time between samples" to ensure there are chart traces for tight zooms.