OPC UA Historical Access (HA) 

Available from 2019.6

Standard OPC UA HA enables the OPC UA clients to access the historical data of the PLCnext Technology DataLogger. You can call selected variables from a database using OPC UA HA. To use OPC UA HA for the data of GDS variables, the variables must be present in at least one DataLogger session (available for data sink types db and volatile). For further information on the DataLogger, please refer to the DataLogger section.

Note: Only scalars and strings are supported. If a variable is present in more than one DataLogger session, the session with the smallest sampling interval will automatically be used.

OPC UA server capabilities

The OPC UA server capabilities provide the OPC UA client with information on the server properties and possibilities. The OPC UA client can access the server capabilities in the information model of the OPC UA server. The HA-specific OPC UA server capabilities (HistoryServerCapabilities) provide information on access to the historical data. The AccessHistoryDataCapability node is TRUE. Any other nodes are FALSE. Additional information is available for the OPC UA client via a ServerCapabilityIdentifier. The HD (Historical Data) entry informs the OPC UA client about historical data being provided.

OPC UA AccessLevel

For GDS variables that are present in a DataLogger session, the HistoryRead bit in the AccessLevel or UserAccessLevel OPC UA attribute is TRUE. In this way, the historical data is available and can be read via the OPC UA server.

Retrieving historical data

The OPC UA server implements the ReadRaw function in accordance with OPC UA Specification 1.03 Part 11. The ReadRawModified service can be used to retrieve the historical data. Section 6.4.3 of the specification describes the service. The ReadModified function is not supported, as only raw data can be read. The isReadModified parameter must therefore be FALSE.

The following parameters of the ReadRawModified are supported in accordance with the OPC UA specification:

Parameter Description
ReadRawModifiedDetails Specifies the request of historical data
startTime Start time for the period of data retrieval
endTime End time for the period of data retrieval
numValuesPerNode Maximum number of values that is to be returned for each node within the period.
Value 0 = No maximum value specified
returnBounds TRUE = Limit values are to be returned
FALSE = Any other cases


Further information, application examples, and error codes can be found in the OPC UA specification.

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• Published/reviewed: 2022-09-14 • Revision 046 •