Bizarre digital tag error?

I have been running 7.8.3 and noticed a strange error. I was monitoring a couple tags - binary or digital tags (1 or 0 tags), with the easy chart. And it seems like, on occasion if the value changes from 0 to 1 or vis-versa, it will re-write the history of that tag from the previous 24 hours? I’m not even sure how that’s possible.

So, if for example I’m monitoring a motor’s run status, I’m looking at the easy chart and the motor has been on for the last 24 hours I can see a straight line at 1, then the motor turns off, and the easy chart will show the motor turning off (the 1 turning to a 0) not now, but 24 hours ago and staying off for that period of time, when I know it was running. So, the line that was at 1 has disappeared, and is now at 0. I’m also tracking the actual speed of the motor and that is recorded normally, dropping to zero only when the motor is shut-off.

Has anyone else observed this? I thought it was a problem with the way I’ve setup “on-change” historian collection, but recently switched my motor tags to “on-time” and the same thing seems to be happening.


Digital historical tags can appear to act odd if you don’t understand the historian. With default settings, a historical tag will be evaluated based on the historical scan class, but it will only be logged if the value changes. So, in your case, the tag changed once in 24 hours - so you only have 2 data points for the chart to analyze, which can give whacky results. If extrapolation is enabled it gets even weirder - as your digital will look like an analog. A work around that I use is to use the max time between records option to force the tag to log on an interval regardless of if it changes. For a 10 sec scan class, I force it to log every 6 executions to ensure status charts and easy charts look right…