Alert Notification when return from Null

I’m currently testing a project using ver. 7.2 and I haven’t tried this on the previous release but…

I currently have a tag that is set to alert when the value = 1,
the value is currently at 1 and is in an alert mode,
communications are sketchy and the value of the tag goes null every few hours,
I get an updated alert every time the value transfers from null to 1 as if the alert is new. :scratch:

This is great only for the fact that I get a lovely email every time my value goes to null and returns to 1.

Is this a 7.2 issue or has it been documented in the past?


Is it just a standard OPC tag? Do you see the quality go bad, or does the value simply to to NULL?

The alert evaluation only occurs if the value’s quality is good. If the communication is going down, the quality should be bad- it might be possible that that quality is being reported as “good” too early when the comm comes back up and the value is null. This would indeed cause the alert to think that it was cleared, but the problem here would be with the quality system, and probably not how alerts are evaluated. So, the trick is to track down what’s happening with the value. If it happens fairly frequently, you can probably watch the tag in the designer. Setting the tag up to store history and putting it on a fairly quick historical scan class (1 sec or so) could help- we could examine the raw history to see how it’s changing. Otherwise, perhaps a few more details about the tag should help- what opc server is being used, what driver, etc.

The behavior would almost definitely be the same in 7.1.


It is happening when the value returns from null.

I’m using the OPC-UA Server and an Allen-Bradley SLC Ethernet driver.

Ok, thanks. I’ll create a ticket to look into whether the quality is being changed to good prematurely, before the value is delivered.


Just revisiting this topic.

I am still having this problem. It occurs when the quality overlay is “Unknown”. It forces “null” into the value and updates the SQL database with “null” which causes all my alarms to trigger.

Shouldn’t any quality overlay prevent a null value from firing?

FYI - when the quality overlay is any of the red overlays, it does hold the last value.