Advise tags & scan classes - bit of clarification please

Discussion created by caffreys_col on Jul 15, 2013
Latest reply on Jul 24, 2013 by drabinowitz

Hi All,


I need a bit of clarification on how advise tags work with scan classes. I've read the manuals but I think I'm just confusing myself!!!


I have an OPC server and the PI OPC interface. I have 6 scan classes ranging from 1 second to 1 minute. All my tags are configured as advise tags. I have a tag created (Tag 1) with a tag span of 100, and an exception value of 1.


If I assign Tag 1 to a 5 second scan class, how often will that tag update (in the OPC interface)? Does the OPC interface update whenever there is a change in the value of Tag 1 that is greater than 1, or does it only pick up the change at the next 5 second scan?


To try and clarify:

  • 00:00 (time in seconds) - 1st scan of 5 second scan class. Tag 1 = 10
  • 02:00 (2 second after initial scan has "fired") Tag 1 changes value by more than the exception limit: Tag 1 = 12


Does the OPC server flag the change in value and therefore allows the interface pick up the change in value now (after 2 seconds), or does the interface pick up the new value at the next scan interval?? e.g....

  • 05:00 (5 seconds after initial scan), second scan is initiated, and change in Tag 1 value is picked up?


The reason for this clarification is that we have had some plant issues and we want to make sure that we aren't missing data between scans (it would be more noticeable at 30 second and 1 minute scan intervals, but for this example I chose 5 seconds).


What would happen if Tag 1 fell by more than exception (eg from 10 to 7), but then rose again to greater than exception (then from 7 to 15) within the scan interval??Which value would be sent to the PI server??


Any help much appreciated!!