It was my understanding that if the Group OPC Data Mode is set to “Read” then any OPC items would be refreshed before performing the transaction group operation (i.e. write tag values to a DB). Based on some testing I have done this is not the case. Can someone explain?
Example Setting A:
Trigger Tag (scanned every 1 second)
Data Tags (set to scan every 5 seconds)
Transaction Group (evaluating every 2 seconds, I assume this mean it’s only looking for an active trigger every 2 seconds)
OPC (set to “Read”)
In this case, the data tags are not refreshed prior to inserting into the DB. Old values are written.
Example Setting B:
Trigger Tag (scanned every 1 second)
Data Tags (set to scan every 1 second)
Transaction Group (evaluating every 2 seconds, I assume this mean it’s only looking for an active trigger every 2 seconds)
OPC (set to “Read”)
In this case, the correct data tag values are inserted to the DB.
Am I misunderstanding the OPC setting? My goal was to not scan the data tags at a very fast rate until they are actually needed, but it seems like they still need to scan at the same rate or faster than the trigger.
