Rockwell Automation 1756-HIST2G Historian ME 2.1 - Manual (Page 114)

F ACTORY T ALK H ISTORIAN ME M ODULE U SER ’ S G UIDE
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Exception reporting works by comparing the new event to the old event as follows:

If the time new event timestamp and old event timestamp is greater than or equal
to the ExcMax, the new event is sent to the Snapshot.

For digital points, if the new value differs from the old value, the new event is
sent to the Snapshot regardless of excmin time.

For numeric points, if the status changes from good to bad, or bad to good, the
new event is sent to the Snapshot.

For numeric points, if the time between the old event and the new event is greater
than or equal to excmin and the absolute value of the difference between the new
value and the old value is greater than excdev, the value is sent to the Snapshot.

If the new event was sent to the Snapshot, the old event is replaced by the new
event.
The last step is a test to see if the PrevEvent should also be sent the Snapshot. If the
PrevEvent was not equivalent to the original OldEvent, the PrevEvent is sent to the
Snapshot. The only time the PrevEvent is not sent to the Snapshot is when two
consecutive exception reports send the new event to the Snapshot. The PrevEvent is
used to accurately indicate what really happened to the value; without it, a step
change would look like a ramp change. Basically, if a measurement holds steady for
hours, then makes a step change, just sending the new value to the Snapshot results in
interpolating between the old value and the new value. By also sending the
PrevEvent, the step change is stored.
E XCEPTION D EVIATION
The Exception Deviation (ExcDev) attribute specifies in engineering units how much
a value may differ from the previous value before it is considered to be a significant
value. The exception deviation should be less than the compression deviation by at
least a factor of 2.
NewEvent The value/status/timestamp of event to test for exception.
Parameter Description