Cogent

The OPC Data Access Specification is the first of a group of specifications known as the OPC Classic Specifications.[1]

OPC Data Access is a group of client–server 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), SCADA systems[2] and also ERP/MES systems.[3] 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) or events (for Alarms and Events you need to use OPC Alarms and Events, or OPC AE). There are three attributes associated with OPC DA data. These are

a value,
the quality of the value, and
a timestamp.