Intermittent Alarms at HMI Start Up?

[Twitch]

Hi, I have a project that exhibits some strange behavior. With the PLC up and running normally I now fire up the HMI and it will sometimes immediately trigger an alarm. The alarm message is configured to display the failing measured numeric and the test limit values being used, and so it is obvious that there is bogus data in those tag values, but they are stable and correct as I watch them in RS Studio. Once the HMI is active it behaves properly. It seems that there are transient values being used as the HMI boots up?

Through the history of this HMI development I've seen the problem move to a different alarm, disappear, reappear on a different alarm, and now disappear again as new HMI builds are generated.

Can anyone explain what might be happening or how to prevent it?

Tim

 

[Ron Beaufort]

first GUESS ...

the values that the HMI is supposed to be monitoring are final calculations (math) performed on "raw" data from the field ... in other words, raw data from the field is being mathematically "scaled" before being "picked up" by the HMI ...

so ...

what MIGHT be happening is that the HMI is picking up the data value "partway" through the mathematical computations - and so you MIGHT be seeing an intermediate value ...

big idea: the HMI can "pickup" or "monitor" a particular data value at ANY point during the processor's scan cycle - even during the middle of a mathematical calculation/instruction ...

take a look at the following link and see if that line of reasoning (GUESSING) fits your situation ...

http://www.plctalk.net/qanda/showthread.php?p=376167&postcount=61

good luck with it ...

 

[Bit_Bucket_07]

I don't know which HMI you are using, but some initiate tags with default values upon start-up, unless the tags are configured to be memory retentive, in which case those tags will be initialized to their last recorded value upon shutdown of the HMI. Some time elapses between starting the HMI graphics interface and the database being updated with values read from the PLC.

 

[Twitch]

Hi guys, thank you for helping!

I am using Factory Talk View ME v8.1 with RS Studio 5000 v24 on a 1769-L33ER controller.

I'm not sure that the issue is due to tags being snagged in the middle of a scan cycle because the value reported in the trip is not possible in any part of the scaling calculation, which I can easily see by watching RS Studio as the HMI boots. They are just bogus numbers. The math is correct because the alarms report out properly if tested after the HMI is stable.

These are all controller-base tags. Can they be somehow be defaulted in my HMI? How do you configure them to be memory-retentive? Any other ideas on how to prevent it?