The OPC Data Access Specification is the first in a series of specifications known as OPC Classic.

OPC Data Access is a set of standards that provides the specifications for the communication of real-time data from data acquisition devices such as PLC for display and human-machine interface (SCADA / HMI). The specific focus on continuous communication of data. The OPC Data Access specification is also known as OPC DA.
There are three attributes associated with OPC DA:

1. the value
2. the quality of the value
3. a timestamp

The OPC DA specification states that these three attributes must be returned to an OPC client to make a request. Therefore, if the data source is not able to provide a timestamp, for example, the OPC DA server needs to create a timestamp.

Classic OPC specifications are based on the Microsoft COM technology and define a C/C++. A wrapper is defined standard for access from Visual Basic, Delphi and other languages.