This subclause lists the Attributes of Variables that 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

Value is the most recent value of the Variable that the Server has. Its data type is defined by the DataType Attribute. The AccessLevel Attribute defines the Server’s basic ability to access current data and MinimumSamplingInterval defines how current the data is.

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