The OPC Data Access Specification is the first of a group of specifications known as the OPC Classic Specifications. 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, as well as ERP/MES systems.
The specifications focus on the continuous communication of data. OPC DA deals only with real-time data and not historical data or events. There are three attributes associated with OPC DA data: a value, the quality of the value, and a timestamp. The OPC DA specification states that these three attributes have to be returned to an OPC client making a request.
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