I have noticed many tags that are recording two values for the same Timestamp. Our interface is an OPC DA classic interface (version 126.96.36.199).
This particular tag is in a 5 second advise scan class (00:00:05,00:00:02).
You can see above that the times are identical - down to the microsecond.
The tag settings are above.
We need to keep compression enabled but retain the ability to reasonably reconstruct this data in another system (which breaks when it sees duplicated timestamps) which is why the exception Max Time is set fairly low - i.e. keep the snapshot fresh-ish so the compression algorithm can store a recent value, which helps with interpolating the data later.
Ideally, we need both values stored, BUT with at least 1-microsecond difference between the two. This is important because with interpolating between the points, we want see the sharp increase (from 31 to 47) at this time.
What can be done to prevent these duplicated time stamps?
In the above case, the value 31 at ~00:19:31 is recorded. The exception time (15 seconds) seems to trigger an update of the snapshot ~00:21:16 (5x 15s intervals) right when a new value is pulled from the OPC server.
I imagine a short exception max time that is a multiple of the scan time can be the cause here - but we need to have this so as to ensure a value a recent value is in the snapshot for the compression to work in a way that we can reconstruct the data accurately.