The nature of OPC UA Historical Accessrequires that a single timestamp reference be used to relate the multiple data points, and the Clientmay request which timestamp will be used as the reference. See OPC 10000-4for details on the TimestampsToReturnenumeration. An OPC UA Serversupporting Historical Access will treat the various timestamp settings as described below. A HistoryRead with invalid settings will be rejected with Bad_TimestampsToReturnInvalid (see OPC 10000-4).
For HistoricalDataNodes, the SourceTimestamp is used to determine which historical data values are to be returned.
The request is in terms of SourceTimestampbut the reply could be in SourceTimestamp, ServerTimestampor both timestamps. If the reply has the Servertimestamp the timestamps could fall outside of the range of the requested time.
SOURCE_0Return the SourceTimestamp.
SERVER_1Return the ServerTimestamp.
Any reference to timestamps in this context throughout this standard will represent either ServerTimestampor SourceTimestampas dictated by the type requested in the HistoryRead Service. Some Serversmay not support historizing both SourceTimestampand ServerTimestamp, but it is expected that all Serverswill support historizing SourceTimestamp(see OPC 10000-7for details on Server Profiles).