An OPC UA (Unified Architecture) node is the fundamental data object within an OPC UA address space. Every piece of information exposed by an OPC UA server — whether a sensor reading, a device parameter, a method, or a folder of related data — is represented as a node. Nodes are identified by a unique NodeId, carry a defined NodeClass that describes their type, and are linked to one another by references, forming a structured, browsable information model that client applications can discover and interact with at runtime.
What Is the OPC UA Address Space?
The address space is the complete collection of nodes exposed by an OPC UA server — a live directory of everything the server knows about, organized as a hierarchical, cross-linked graph.
Unlike older OPC Classic standards (DA, HDA, A&E), which exposed flat tag lists, the OPC UA address space is a fully navigable object model. A client can start at the root node and browse down through folders, objects, and variables to discover data without needing prior knowledge of the server’s layout.
The 8 OPC UA Node Classes
Every node belongs to exactly one NodeClass. The OPC UA specification, maintained by the OPC Foundation, defines eight:
| NodeClass | Purpose |
|---|---|
| Object | Represents a real-world entity (e.g., a motor, a PLC) |
| Variable | Holds a data value (e.g., temperature, speed) |
| Method | Defines a callable function on the server |
| ObjectType | A reusable template for Object nodes |
| VariableType | A reusable template for Variable nodes |
| DataType | Defines the data type of a variable (e.g., Float, String) |
| ReferenceType | Defines a relationship category between nodes |
| View | A filtered subset of the address space |
In day-to-day industrial use, Variable and Object are the two classes you will encounter most. Variables hold live process values; Objects group related variables into logical structures that mirror real equipment.
Core Node Attributes
Every node carries a standard set of attributes regardless of its class:
- NodeId — A server-wide unique identifier (namespace index + identifier value).
- NodeClass — One of the eight types above.
- BrowseName / DisplayName — Human-readable names used when browsing or displaying the node.
- Description — An optional plain-text explanation of the node’s purpose.
Variable nodes carry additional data-specific attributes:
- Value — The actual process value.
- DataType — The value’s type (e.g., Float, Int32, Boolean).
- AccessLevel — Whether the value is readable, writable, or both.
- StatusCode — Data quality: Good, Bad, or Uncertain.
- Timestamps — Source timestamp (device measurement time) and server timestamp (server record time).
How Nodes Connect: References
Nodes are linked by references — typed, directional relationships that express both hierarchy and meaning. A folder Organizes its children. An object HasComponent variables and methods. A node HasTypeDefinition pointing to its ObjectType template.
This reference model is what allows OPC UA to represent complex real-world structures — a machine with sub-assemblies, sensors, and controls — as a coherent, self-describing information model, rather than a flat list of tags.
Real-World Example
A boiler in an OPC UA server might be modeled as:
- Boiler_01 (Object)
- TemperatureSensor (Variable — Float, °C, live value: 312.4)
- PressureSensor (Variable — Float, bar, live value: 4.2)
- EmergencyShutdown (Method)
An OPC UA client connects, browses to Boiler_01, discovers its child nodes through references, and subscribes to TemperatureSensor for live updates. No pre-configured tag list required.
Why the Node Model Matters
The node-based address space is what makes OPC UA self-describing and genuinely interoperable. Clients discover structure at runtime, values travel with quality and timestamps built in, and the entire model is vendor-neutral under the IEC 62541 standard. For industrial automation and IIoT integration, nodes are the foundation that makes plug-and-play device connectivity possible.
To deepen your understanding of OPC UA concepts, the OPC Training Institute (OPCTI) offers hands-on training and the industry-recognised Certified OPC Professional (COP) designation for automation professionals.