Exception deviation

Exception reporting is used to define the precision of a data stream, and the amount of deviation that constitutes a significant change. Most interface programs can execute an exception-reporting algorithm to determine when to send a point value to the PI Snapshot subsystem. An exception is an event that occurs either:

• After a specified minimum duration of time since the previous event, while exceeding a specified deviation in value from that event.

• After a specified maximum duration of time since the previous event.

This means that when activated, exception reporting filters events and stores only periodic values, including duplicates, unless an event represents a significant change in the short-term trend of values. An exception event, both timestamp and value, is sent with the previous event to the Snapshot.

• Exception Deviation

  The deviation in value required to store an event, either as a number of engineering units, or as a percentage of the point's Span value. The exception deviation should be less than the compression deviation by at least a factor of 2, and is ignored for digital, string and BLOB data type points.

• Min Time

  The minimum time that must elapse after an event before an exception value can be stored.

• Max Time

  The maximum time that can elapse after an event before automatically storing the next event as an exception value.

Set the minimum and maximum time values to 0 to turn off exception reporting.