Delta retrieval - how it works

The following illustration shows how values are returned for delta retrieval:

Data is retrieved in delta mode with a start time of T1 and an end time of T2. Each dot in the graphic represents an actual data point stored on the historian. From these points, the following are returned:

• P2, because there is no actual data point at T1

• P5, P8, P9, P10, and P11, because they represent changed values during the time period

For delta retrieval for replicated summary tags on a tier-2 historian, if a point with doubtful quality is returned as the result of a value selection from an input summary point with a contained gap, the same point can be returned again with good quality if the same value is selected again from the next input summary point that has good quality.