The main use case for OPC standards is the online data exchange between devices and HMI or SCADA systems using Data Access functionality. In this use case the device data is provided by an OPC server and is consumed by an OPC client integrated into the HMI or SCADA system. OPC DA provides functionality to browse through a hierarchical namespaces containing data items and to read, write and to monitor these items for data changes. The classic OPC standards are based on Microsoft COM/DCOM technology for the communication between software components from different vendors. Therefore classic OPC server and clients are restricted to Windows PC based automation systems.

OPC UA incorporates all features of classic OPC standards like OPC DA, A&E and HDA but defines platform independent communication mechanisms and generic, extensible and object-oriented modelling capabilities for the information a system wants to expose.

The OPC UA network communication part defines different mechanisms optimized for different use cases. The first version of OPC UA is defining an optimized binary TCP protocol for high performance intranet communication as well as a mapping to accepted internet standards like Web Services. The abstract communication model does not depend on a specific protocol mapping and allows adding new protocols in the future. Features like security, access control and reliability are directly built into the transport mechanisms. Based on the platform independence of the protocols, OPC UA servers and clients can be directly integrated into devices and controllers.

The OPC UA Information Modelprovides a standard way for Serversto expose Objectsto Clients. Objectsin OPC UA terms are composed of other Objects, Variablesand Methods. OPC UA also allows relationships to other Objectsto be expressed.

The set of Objectsand related information that an OPC UA Servermakes available to Clientsis referred to as its AddressSpace. The elements of the OPC UA ObjectModel are represented in the AddressSpaceas a set of Nodesdescribed by Attributesand interconnected by References. OPC UA defines eight classes of Nodesto represent AddressSpacecomponents. The classes are Object, Variable, Method, ObjectType, VariableType, DataType, ReferenceType andView. Each NodeClasshas a defined set of Attributes.

This specification makes use of three essential OPC UA NodeClasses: Objects, Methodsand Variables.

Objectsare used to represent components of a system. An Objectis associated to a corresponding ObjectTypethat provides definitions for that Object.

Methods are used to represent commands or services of a system.

Variablesare used to represent values. Two categories of Variablesare defined, Propertiesand DataVariables.

Propertiesare Server-defined characteristics of Objects, DataVariablesand other Nodes. Propertiesare not allowed to have Propertiesdefined for them. An example for Propertiesof Objectsis the Version Propertyof the CncInterfaceType.

DataVariablesrepresent the contents of an Object. DataVariablesmay have component DataVariables. This is typically used by Serversto expose individual elements of arrays and structures. This specification uses DataVariablesto represent parameters, state data, process and command data, for example ActSpeedof a Objectof CncSpindleType.