Maximum retrieval - how it works

The following illustration shows how the maximum value is selected for an analog tag.

This example has a start time of TC0 and an end time of TC2. The resolution has been set in such a way that the historian returns data for two complete cycles starting at TC0 and TC1, a "phantom" cycle starting at TCP, and an incomplete cycle starting at TC2. The phantom cycle has the same duration as the first cycle in the query period, extending back in time from the query start time.

For the queried tag, a total of 18 points are found throughout the cycles, represented by the markers P1 through P18. Of these points, 17 represent normal analog values. The point P13 represents a NULL due to an I/O Server disconnect, which causes a gap in the data between P13 and P14.

The maximum value for the "phantom" cycle starting at TCP is returned as the initial value at TC0. Point P18 is not considered at all because it is outside of the query time frame. All other points are considered, but only the points indicated by green markers on the graph are returned (P12, P13, and P15).

In total, four points are returned:

• P6 as the maximum value of the "phantom" cycle and the initial point

• P12 as the maximum value in the first cycle

• P13 as the first and only exception occurring in the first cycle

• P15 as the maximum value in the second cycle

No points are returned for the incomplete third cycle starting at the query end time, because the tag does not have a point exactly at that time.

If the maximum value of the first cycle is located exactly at the query start time, this value and the maximum value of the phantom cycle are returned.