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5 Using the PHD OPC Server
5.4
PHD OPC Data Standards for Client Applications
Uniformance - PHD OPC Server User Guide
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Extrapolated values
You can use the PHD OPC Server to return extrapolated values. To use extrapolation, the
OPC items specified by the client must refer to PHD tags that are configured appropriately.
The PHD Server configuration allows you to use syntax to specify the dynamic use of
extrapolation when values are referenced - this syntax is specified in the source tag
specification of the tag configuration. The specific notation invokes a dynamic data reduction
(virtual calculation) by the PHD Server to perform the extrapolation.
For example, the following syntax specified in the source tag specification field of the PHD
tag SRC_A.A100PV.extrap invokes extrapolation of the current value from the PHD tag
SRC_A.A100.PV:
SRC_A.A100.PV{,,NOW,NOW,}
Since an extrapolated value is derived from actual data prior to the specific request time, it
has a confidence factor of less than 100%.
The PHD OPC Server translates the PHD value confidence to OPC data quality. For
extrapolated values, the OPC data quality is always Uncertain, and the sub-status is Non-
Specific.
For the definition of the mapping of PHD data confidence to OPC data quality, refer to
Interpreting data quality
in this guide.
Interpolated values
The PHD OPC Server calculates interpolative and time average aggregates as follows.
INTERPOLATIVE (snapshot)
For PHD tags configured with non-float data types (not Float or Double), or with Manual
Input enabled, or with Interpolation Method of Step, the PHD OPC Server uses stepped
interpolation; otherwise it uses linear interpolation.
TIMEAVERAGE
For the time-weighted average aggregate, the PHD OPC Server uses linear interpolation.
REFERENCE: For other aggregates, refer to the OPC HDA Specification (Custom Interface
Specification Version 1.2_Errata 1) Feb 9, 2004.
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