This subclause lists the Attributes of Variablesthat have particular importance for Data Access. They are specified in detail in OPC 10000-3. The following Attributes are particularly important for Data Access:

  • Value
  • DataType
  • AccessLevel
  • MinimumSamplingInterval

Valueis the most recent value of the Variablethat the Serverhas. Its data type is defined by the DataType Attribute. The AccessLevel Attributedefines the Server’sbasic ability to access current data and MinimumSamplingIntervaldefines how current the data is.

When a client requests the Value Attributefor reading or monitoring, the Serverwill always return a StatusCode(the quality and the Server’sability to access/provide the value) and, optionally, a ServerTimestampand/or a SourceTimestamp– based on the Client’srequest. See OPC 10000-4for details on StatusCodeand the meaning of the two timestamps. Specific status codes for Data Access are defined in 6.3.