queryTagHistory returning 0 for tag value

Ignition
1

I have a tag whose value was last updated about five months ago and has remained unchanged since then. In a script, I’m trying to retrieve the tag’s value from 30 minutes ago using system.tag.queryTagHistory.

However, the query is returning 0 instead of the expected value, and I’m not sure why.

Here’s the script I’m using:

startTime= system.date.now()
endTime = system.date.addMinutes(startTime, 1)

dataSet = system.tag.queryTagHistory(
    paths=[my_tag_path],
    startDate=startTime,
    endDate=endTime,
    returnSize=1,
    aggregationMode="LastValue",
    returnFormat="Wide",
    includeBoundingValues=True
)

print dataSet.getValueAt(0, 1)

Given that the tag hasn’t changed in months, I expected the last known value (from well before the query window) to be returned. Am I misunderstanding how includeBoundingValues, returnSize, or LastValue behave in this scenario?

Any guidance would be appreciated.

2

You are misunderstanding.

  • The documentation of Last Value aggregation says zero will be returned for intervals that do not have a value.

  • includeBoundingValues specifically applies to Seeded Values, which are defined to be values outside the begin/end timestamps, but near them. Note the other requirements, too.

I don't think any interpretation would place "three months ago" as "near" a begin timestamp of now().

Requesting a one-minute interval starting now() pretty much guarantees there won't be a stored value to return.

3

Hmm that makes sense. Thanks for the clarification.

So in this case, includeBoundingValues won’t help because the last stored value is far outside the query window, and LastValue correctly returns 0 since there are no samples within that interval. I also tried other aggregation modes like Maximum and Average, but those behave the same way, which I suppose is due to the same logic (right?).

Given that, what is the correct way to retrieve the effective value of a tag at a specific point in time (e.g., what the value was 30 minutes ago) when the tag hasn’t changed in a long time? In other words, how should I query “the most recent value at or before a timestamp,” regardless of how far back it was recorded?

I think I’m still missing the intended pattern for this use case.

4

There really isn't such a thing.

You should change your time span to be -6 months to now() with return size = 1.

Better, reconfigure your tags to have a max time between samples that isn't infinite. For best results when charting, the max time between samples should be smaller than the smallest normal chart timespan.

1 Like
5

Interesting. This may be it. I hadn’t considered the max time between samples. I’ll look into updating the tag history settings.

Thanks for the help, I appreciate it.

6

And also set the type to Discrete so you don't get screwed by the Analog compression algorithm which will still not store a value for a non-changing value even at the max time between samples.

2 Likes