OPC Data Access is a group of standards that provides specifications for communicating real-time data from data acquisition devices such as PLCs to display and interface devices like Human-Machine Interfaces (HMI). The specifications focus on the continuous communication of data.
The OPC Data Access specification is also known as OPC DA. OPC DA deals only with real-time data and not historical data (for historical data you need to use OPC Historical Data Access, or OPC HDA). There are three attributes associated with OPC DA. These are; 1) a value, 2) the quality of the value, and 3) a timestamp. The OPC DA specification states that these three attributes have to be returned to an OPC client making a request. Therefore, if the data source is not capable of providing a timestamp, for example, the OPC DA server must create a timestamp.