bounding values – OPC Unified Architecture

3.1.7 bounding values

values at the startTime and endTime needed for Aggregates to compute the result

Note 1

Note 1 to entry: If Raw data does not exist at the startTime and endTime a value shall be estimated. There are two ways to determine Bounding Values for an interval. One way (called Interpolated Bounding Values) uses the first non-Bad data points found before and after the timestamp to estimate the bound. The other (called Simple Bounding Values) uses the data points immediately before and after the boundary timestamps to estimate the bound even if these points are Bad. Subclauses 3.1.8 and 3.1.9 describe the two different approaches in more detail.

In all cases the TreatUncertainAsBad (see 4.2.1.2) flag is used to determine whether Uncertain values are Bad or non-Bad.

If a Raw value was not found and a non-Bad bounding value exists the Aggregate Bits (see 5.3.3) are set to ‘Interpolated’.

When calculating bounding values, the value portion of Raw data that has Bad status is set to null. This means the value portion is not used in any calculation and a null is returned if the raw value is returned. The status portion is determined by the rules specified by the bound or Aggregate.

The Interpolated Bounding Values approach (see 3.1.8) is the same as what is used in Classic OPC Historical Data Access (HDA) and is important for applications such as advanced process control where having useful values at all times is important. The Simple Bounding Values approach (see 3.1.9) is new in this standard and is important for applications which shall produce regulatory reports and cannot use estimated values in place of Bad data.