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 , or OPC AE). There are three attributes associated with OPC DA data. 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.

The OPC Classic specifications are based on the Microsoft COM technology[4] and define a C/C++ interface. A standard Automation wrapper interface is also defined for access from Visual Basic, Delphi and other automation-enabled languages.[5] Several vendors offer .NET toolkits to make the OPC interface accessible in .NET applications.

The newer OPC .NET (OPC Xi) specification is based on WCF (Windows Communication Foundation) and defines a .NET interface with the functionality of the OPC Classic specifications OPC DA, OPC HDA and OPC AE (Alarms&Events).[6]

The more recent OPC Unified Architecture allows the same functionality but offers platform independence and optionally complex information modelling capabilities.[7]

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