IEC 62541-8:2025

Overview

IEC 62541-8:2025 - "OPC unified architecture - Part 8: Data access" defines the information model for OPC UA Data Access (DA). It specifies additional VariableTypes, Properties, NodeClass/Attribute descriptions and behavior needed to represent real-time automation data in an OPC UA address space. This fourth edition (2025) is a technical revision that adds a Quantity Model for engineering units and extends rules for the ValuePrecision property. Annex A provides normative guidance for mapping legacy OPC COM DA servers to the OPC UA DA model.

Key topics and technical requirements

• DataItem and VariableTypes: Detailed VariableType hierarchy for DataItemType, AnalogItem, DiscreteItem, ArrayItem (including XY/Y arrays, image and N-dimensional arrays), and related subtypes used to expose measured and calculated values.
• Attributes & Properties: Definitions and semantics for DA-specific Attributes (e.g., EUInformation, Range, ValuePrecision, PercentDeadband) and how they affect read/write and subscription behavior.
• Quantities and Units model: New reusable QuantityType/UnitType model that can be referenced from EngineeringUnit properties. Supports alternative units and conversions (including MathML examples and external syntax references like UCUM/QUDT).
• ValuePrecision rules: Expanded usage to additional data subtypes (Duration, Decimal) and clarified behavior for negative precision values.
• DataTypes: Structures such as Range, EUInformation, ComplexNumber/DoubleComplexNumber, AxisInformation, XVType for advanced numeric and multi-axis data.
• Service usage & status codes: Guidance for percent deadband, DA-specific status codes and limit bits to support efficient subscriptions and reliable operation.
• OPC COM DA mapping: Normative Annex A describing information model, data/error mapping and wrapper/proxy patterns for interoperability with legacy OPC COM DA systems.

Applications and users

IEC 62541-8:2025 is essential for:

• OPC UA server implementers (industrial device vendors, PLC/RTU suppliers) exposing process and telemetry data.
• OPC UA client developers (SCADA/HMI, historian, analytics, IIoT platforms) consuming DA data.
• System integrators and automation engineers designing interoperable control systems.
• Software architects and test labs validating DA behavior, unit conversions, and legacy OPC COM DA migration. Practical benefits include interoperable real-time data exchange, consistent engineering unit handling, deterministic precision semantics, and straightforward migration paths from OPC COM DA.

Related standards

• IEC 62541-3 - Address Space Model (complete NodeClasses & Attributes)
• IEC 62541-4 - Services (discovery, read/write, subscriptions)
• OPC COM DA - legacy mapping covered in Annex A
• UCUM/QUDT syntax references included for unit semantics

Keywords: IEC 62541-8:2025, OPC UA Data Access, OPC Unified Architecture, Quantity Model, ValuePrecision, EUInformation, OPC COM DA to UA mapping, engineering units, industrial interoperability.