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OPC UA basics

Introduction to the core concepts of OPC Unified Architecture (nodes, NodeIds, address space, views, node managers, and services).

This content was machine translated and may contain errors. Read in German

Concept

OPC UA basics ​

OPC UA ​

OPC UA stands for OPC Unified Architecture. What sets OPC UA apart from its predecessor OPC is that it does not just transport machine data such as measured values and parameters. It also describes their semantics in a machine-readable form.

Today, OPC stands for Open Platform Communications. Originally, it stood for OLE for Process Control. Unified Architecture refers to the uniform, platform-independent architecture that distinguishes OPC UA from the classic OPC specifications.

Node ​

The node is the fundamental element of OPC UA. Almost every element can be traced back to a node. Nodes are related to each other.

The Wikipedia article on OPC Unified Architecture describes the term node well:

"The OPC information model is no longer just a hierarchy of folders, items, and properties. It is a so-called full-mesh network of nodes, which represents not only the payload data of a node but also metadata and diagnostic information."[1]

  • A node is similar to an object in object-oriented programming.
  • A node has attributes that you can read, for example through Data Access (DA) and Historical Data Access (HDA).
  • Nodes represent both the payload data and all other kinds of metadata.
  • The OPC address space modeled this way contains a type model that specifies all data types.

NodeId ​

NodeId: every node is uniquely identified by its NodeId.NodeID

The OPC specification requires that every node can be uniquely identified in the address space by an identifier, the NodeId. The NodeId consists of a GUID (Globally Unique Identifier), a numeric value, a byte array, or a string. The NodeId usually contains the namespace, but it does not have to.

Address space ​

Address Space: nodes and their relations are managed in an address space.NodeNodeNodeNodeNodeNodeNodeNodeNodeAddress Space

All nodes that are provided and processed in OPC UA reside in an address space. The address space is a kind of logical storage. The nodes it contains can logically reference one or more nodes in the same or in another address space.

View ​

View: an address space can be divided into one or more views.NodeNodeNodeNodeNodeNodeNodeNodeNodeAddress SpaceView

You can logically divide the address space shown above into one or more views.

Besides the default view, there are custom views that can contain one or more nodes.

Node manager ​

NodeManager: a node manager provides nodes and defines their relations.NodeNodeNodeManager

A node manager provides one or more nodes and defines their relationships with each other. The predefined system node managers are:

  • Core node manager (defines type nodes and system nodes, among others)
  • Diagnostics node manager (provides nodes for diagnostic purposes)
  • Master node manager (manages all node managers and delegates calls to the responsible node manager)

Service ​

Service: clients interact with the server nodes through services.NodeService

OPC UA defines a set of services through which the client interacts with the server. The server implements these services as methods. Among others, the services cover these tasks:

  • read and write attributes and values of nodes
  • manage references of nodes
  • browse nodes
  • read and write historical values
  • call methods
  • manage subscriptions

  1. Source: Wikipedia, OPC Unified Architecture (German article, quote translated) ↩︎