Slope retrieval - how it works

The following illustration shows how the slope is calculated for an analog tag.

This example has a start time of TS and an end time of TE.

For the queried tag, a total of nine points are found, represented by the markers P1 through P9. Of these points, eight represent normal analog values. The point P5 represents a NULL due to an I/O Server disconnect, which causes a gap in the data between P5 and P6.

For every point in the time period, slope retrieval returns the slope of the line going through that point and the point immediately before it. For two points P1 and P2 occurring at times T1 and T2, the slope formula is as follows:

(P2 - P1) / (T2 - T1)

The difference between T1 and T2 is measured in seconds. Therefore, the returned value represents the change in Engineering Units per second.

In this example, point P2 is located at the query start time, and because there is a prior value (P1), the slope of the line through both points is calculated and returned at time TS. Similarly, slopes are calculated to be returned at times T3, T4, T7, and T8. The slope is also calculated for the line through P8 and P9, but that value is returned as point PTE at the query end time.

For point P6, there is no prior point with which to perform a slope calculation. Instead, the slope of the flat line going through the point (that is, the value 0) is calculated. At the time of P5, NULL is returned.

The quality detail and OPC quality returned with a slope point is always directly inherited from the point that also provides the time stamp. In this example, this means that point P2 provides the qualities for the slope point returned at the query start time, TS.