SIST EN IEC 62541-8:2026

OPC unified architecture - Part 8: Data access (IEC 62541-8:2025)

This part of IEC 62541 is part of the overall OPC Unified Architecture (OPC UA) standard series and defines the information model associated with Data Access (DA). It particularly includes additional VariableTypes and complementary descriptions of the NodeClasses and Attributes needed for Data Access, additional Properties, and other information and behaviour.
The complete address space model, including all NodeClasses and Attributes is specified in IEC 62541-3. The services to detect and access data are specified in IEC 62541-4.
Annex A specifies how the information received from OPC COM Data Access (DA) Servers is mapped to the Data Access model.

OPC Unified Architecture - Teil 8: Zugriff auf Automatisierungsdaten (IEC 62541-8:2025)

Architecture unifiée OPC - Partie 8: Accès aux données (IEC 62541-8:2025)

IEC 62541-8:2025 définit le modèle d'information associé à l'Accès aux données (DA). Elle spécifie notamment des VariableTypes supplémentaires et fournit des descriptions complémentaires concernant les NodeClasses et Attributs nécessaires pour l'Accès aux données, ainsi que des Propriétés supplémentaires et d'autres paramètres relatifs aux informations et au comportement.
Le modèle d'espace d'adressage complet, comprenant toutes les NodeClasses et tous les Attributs, est spécifié dans l'IEC 62541‑3. Les services de détection et d'accès aux données sont spécifiés dans l'IEC 62541‑4.
L'Annexe A spécifie la méthode de mapping des informations reçues des Serveurs d'Accès aux données (DA, Data Access) OPC COM avec le modèle d'Accès aux données.
Cette quatrième édition annule et remplace la troisième édition parue en 2020. Cette édition constitue une révision technique.
Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) ajout d'un "Modèle de quantité" qui peut être référencé à partir des Propriétés EngineeringUnit. Le modèle définit les grandeurs et les unités attribuées. En outre, il fournit des unités alternatives et la conversion vers elles;
b) ajout de règles pour la Propriété ValuePrecision:
- peut également être utilisée pour d'autres sous-types tels que Duration et Decimal;
- ajout de règles lorsque ValuePrecision a des valeurs négatives.

Enotna arhitektura OPC - 8. del: Dostop do podatkov (IEC 62541-8:2025)

Ta del standarda IEC 62541 je del celotne serije standardov OPC Unified Architecture (OPC UA) in opredeljuje informacijski model, povezan z dostopom do podatkov (Data Access - DA). Posebej vključuje dodatne tipe spremenljivk (VariableTypes) in dopolnilne opise razredov vozlišč (NodeClasses) in atributov, potrebnih za dostop do podatkov, dodatne lastnosti in druge informacije ter vedenje.
Celoten model naslovnega prostora, vključno z vsemi razredi vozlišč in atributi, je določen v standardu IEC 62541-3. Storitve za zaznavanje in dostop do podatkov so določene v standardu IEC 62541-4.
Priloga A določa, kako se informacije, prejete od strežnikov OPC COM Data Access (DA), preslikajo v model dostopa do podatkov.

General Information

• Status: Published
• Public Enquiry End Date: 30-Mar-2024
• Publication Date: 12-Apr-2026
• ICS: 25.040.40 - Industrial process measurement and control
• ICS: 35.100.05 - Multilayer applications
• ICS: 35.240.50 - IT applications in industry
• Technical Committee: MOV - Measuring equipment for electromagnetic quantities
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 24-Feb-2026
• Due Date: 01-May-2026
• Completion Date: 13-Apr-2026
• Mandate: M/490 - Standardisation mandate to the European Standardisation Organisations (ESOs) to support European Smart Grid deployment
• Ref Project: EN IEC 62541-8:2026 - OPC unified architecture - Part 8: Data access

Relations

• Refers: SIST EN IEC 62541-19:2026 - OPC unified architecture - Part 19: Dictionary Reference (IEC 62541-19:2025)
• Effective Date: 03-Feb-2026
• Refers: SIST EN IEC 62541-1:2026 - OPC unified architecture - Part 1: Overview and concepts (IEC 62541-1:2025)
• Effective Date: 03-Feb-2026
• Refers: SIST EN IEC 62541-4:2026 - OPC unified architecture - Part 4: Services (IEC 62541-4:2025)
• Effective Date: 03-Feb-2026
• Refers: SIST EN IEC 62541-5:2026 - OPC unified architecture - Part 5: Information model (IEC 62541-5:2026)
• Effective Date: 03-Feb-2026
• Refers: SIST EN IEC 62541-3:2026 - OPC unified architecture - Part 3: Address space model (IEC 62541-3:2025)
• Effective Date: 03-Feb-2026
• Refers: SIST EN IEC 62541-4:2020 - OPC Unified Architecture - Part 4: Services (IEC 62541-4:2020)
• Effective Date: 03-Feb-2026
• Referred By: SIST EN 62769-3:2015 - Devices and integration in enterprise systems; Field Device Integration - Part 3: FDI Server (IEC 62769-3:2015)
• Effective Date: 20-Feb-2026
• Referred By: SIST EN IEC 62769-7:2021 - Field Device Integration (FDI) - Part 7: Communication devices (IEC 62769-7:2021)
• Effective Date: 20-Feb-2026
• Referred By: SIST EN IEC 62769-5:2021 - Field Device Integration (FDI) - Part 5: Information Model (IEC 62769-5:2021)
• Effective Date: 20-Feb-2026
• Referred By: SIST EN 62541-4:2012 - OPC unified architecture - Part 4: Services (IEC 62541-4:2011)
• Effective Date: 20-Feb-2026
• Referred By: SIST EN 62769-1:2015 - Devices and integration in enterprise systems; Field Device Integration - Part 1: Overview (IEC 62769-1:2015)
• Effective Date: 20-Feb-2026
• Referred By: SIST EN IEC 62541-18:2025 - OPC unified architecture - Part 18: Role-Based Security (IEC 62541-18:2025)
• Effective Date: 20-Feb-2026
• Referred By: SIST EN IEC 62541-7:2020 - OPC unified architecture - Part 7: Profiles (IEC 62541-7:2020)
• Effective Date: 20-Feb-2026
• Referred By: SIST EN 62541-4:2015 - OPC Unified Architecture - Part 4: Services (IEC 62541-4:2015)
• Effective Date: 20-Feb-2026
• Referred By: SIST EN 62541-13:2015 - OPC Unified Architecture -- Part 13: Aggregates (IEC 62541-13:2015)
• Effective Date: 20-Feb-2026

Overview

The oSIST prEN IEC 62541-8:2024 standard, titled OPC Unified Architecture - Part 8: Data Access, defines the critical information model associated with Data Access (DA) within the OPC Unified Architecture (UA) framework. Published by the CLC and aligned with IEC 62541 series, this fourth edition revises and extends previous versions to enhance interoperability and precision in accessing industrial automation data.

This standard specifies additional VariableTypes, comprehensive descriptions of NodeClasses and Attributes imperative for Data Access, and an enriched model supporting Units and Quantities. The overall address space model for NodeClasses and Attributes is detailed in IEC 62541-3, with corresponding services defined in IEC 62541-4. Annex A of this standard provides a detailed mapping guide for OPC COM Data Access Servers to the UA Data Access model.

Key enhancements in this edition include the integration of a Quantity Model linking Engineering Units with alternative units and conversion capabilities, and expanded rules for the ValuePrecision Property, including its application to Duration and Decimal subtypes with clarified guidelines for negative values.

Key Topics

• Data Access Information Model
  Defines the structure and behavior of Data Access nodes, encompassing VariableTypes such as DataItemType, AnalogItemType, DiscreteItemType, and various ArrayItemTypes, enabling consistent representation of real-time and historical automation data.

• Governance of NodeClasses and Attributes
  Detailed descriptions for nodes and attributes focused on Data Access improve clarity for developers and integrators, facilitating accurate data exchange and system interoperability.

• Quantities and Units Model
  Introduces a formalized Quantity Model referencing EngineeringUnit properties for standardized unit representation, alternative unit handling, and unit conversions-critical for multi-domain systems managing measurements in diverse units.

• ValuePrecision Property Rules
  Enhanced rules accommodate additional subtypes such as Duration and Decimal, clarifying behavior when ValuePrecision carries negative values to promote consistent precision management across data types.

• Mapping OPC COM DA to OPC UA
  Annex A provides normative instructions for transitioning OPC COM Data Access servers to the OPC UA Data Access model, including detailed data and error status code mappings and guidelines for subscription and data reading/writing processes.

• Extended DataTypes and Attributes
  Supports complex data types including ComplexNumberType, DoubleComplexNumberType, AxisInformation, and more to represent diverse types of process data accurately.

Applications

The oSIST prEN IEC 62541-8:2024 standard is essential for industrial automation systems requiring robust, standardized access to real-time data. Its practical applications include:

• Industrial Control Systems
  Facilitates precise, interoperable access to sensor and actuator data across manufacturing automation, enabling real-time monitoring and control.

• Process Instrumentation and Measurement
  Supports consistent data representation and unit conversions crucial for applications in process industries such as chemical plants, oil and gas, and utilities.

• Enterprise Integration
  Enhances data consistency for IT applications within industry, allowing secure and uniform data exchange between control level and enterprise systems.

• Legacy System Modernization
  Provides a structured approach to map existing OPC COM DA servers to OPC UA architecture, aiding digital transformation efforts.

• Complex Data Handling
  Enables support for multi-dimensional and complex numeric data such as images, arrays, and complex numbers, broadening use cases in advanced analytics and diagnostics.

Related Standards

• IEC 62541-3 (Address Space Model)
  Defines the overarching OPC UA address space, including all NodeClasses and Attributes, forming the foundation for encoding Data Access models.

• IEC 62541-4 (Services)
  Specifies services for browsing, reading, writing, and subscribing to data, which complement and extend the Data Access model.

• IEC 62541-1 (Overview and Concepts)
  Provides general information and the conceptual framework within which Data Access and other OPC UA parts operate.

• IEC 62541-5 (Information Model)
  Describes general information modeling principles supporting specialized models such as Data Access.

• Industry-specific OPC UA Companion Standards
  Various companion specifications may leverage or extend the Data Access model for vertical applications in automotive, energy, and manufacturing sectors.

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Keywords: OPC Unified Architecture, OPC UA Data Access, Industrial Automation, Data Access Model, Quantity Model, Engineering Unit, OPC COM Data Access, Industrial Process Control, IEC 62541-8, Data Interoperability, Unit Conversion, Industrial IT Applications.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2026
Enotna arhitektura OPC - 8. del: Dostop do podatkov (IEC 62541-8:2025)
OPC unified architecture - Part 8: Data access (IEC 62541-8:2025)
OPC Unified Architecture - Teil 8: Zugriff auf Automatisierungsdaten (IEC 62541-8:2025)
Architecture unifiée OPC - Partie 8: Accès aux données (IEC 62541-8:2025)
Ta slovenski standard je istoveten z: EN IEC 62541-8:2026
ICS:
25.040.40 Merjenje in krmiljenje Industrial process
industrijskih postopkov measurement and control
35.240.50 Uporabniške rešitve IT v IT applications in industry
industriji
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 62541-8

NORME EUROPÉENNE
EUROPÄISCHE NORM February 2026
ICS 25.040.40; 35.100.05 Supersedes EN IEC 62541-8:2020
English Version
OPC unified architecture - Part 8: Data access
(IEC 62541-8:2025)
Architecture unifiée OPC - Partie 8: Accès aux données OPC Unified Architecture - Teil 8: Zugriff auf
(IEC 62541-8:2025) Automatisierungsdaten
(IEC 62541-8:2025)
This European Standard was approved by CENELEC on 2026-01-27. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 62541-8:2026 E

European foreword
The text of document 65E/1055/CDV, future edition 4 of IEC 62541-8, prepared by SC 65E "Devices
and integration in enterprise systems" of IEC/TC 65 "Industrial-process measurement, control and
automation" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2027-02-28
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2029-02-28
document have to be withdrawn
This document supersedes EN IEC 62541-8:2020 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
This document has been prepared under a standardization request addressed to CENELEC by the
European Commission. The Standing Committee of the EFTA States subsequently approves these
requests for its Member States.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 62541-8:2025 was approved by CENELEC as a European
Standard without any modification.
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 62541-1 - OPC Unified Architecture - Part 1: EN IEC 62541-1 -
Overview and concepts
IEC 62541-3 - OPC Unified Architecture - Part 3: Address EN IEC 62541-3 -
Space Model
IEC 62541-4 - OPC unified architecture - Part 4: Services EN IEC 62541-4 -
IEC 62541-5 - OPC Unified architecture - Part 5: EN IEC 62541-5 -
Information Model
IEC 62541-19 - OPC unified architecture - Part 19: EN IEC 62541-19 -
Dictionary Reference
IEC 62541-8 ®
Edition 4.0 2025-12
INTERNATIONAL
STANDARD
OPC unified architecture -
Part 8: Data access
ICS 25.040.40; 35.100.05 ISBN 978-2-8327-0849-1

IEC 62541-8:2025-12(en)
IEC 62541-8:2025 © IEC 2025
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions and abbreviated terms . 7
3.1 Terms and definitions. 7
3.2 Abbreviated terms . 8
4 Concepts . 9
5 Model . 9
5.1 General . 9
5.2 SemanticsChanged . 10
5.3 Variable Types . 10
5.3.1 DataItemType . 10
5.3.2 AnalogItem VariableTypes . 12
5.3.3 DiscreteItemType . 15
5.3.4 ArrayItemType . 18
5.4 Address Space model . 25
5.5 Attributes of DataItems . 26
5.6 DataTypes . 27
5.6.1 Overview . 27
5.6.2 Range. 27
5.6.3 EUInformation . 27
5.6.4 ComplexNumberType . 30
5.6.5 DoubleComplexNumberType . 30
5.6.6 AxisInformation . 31
5.6.7 AxisScaleEnumeration . 32
5.6.8 XVType . 32
6 Quantities and Units model . 33
6.1 General . 33
6.2 Quantities entry point . 34
6.3 Syntax References . 34
6.3.1 General . 34
6.3.2 Using Dictionary References. 35
6.3.3 Syntax Reference Identifier . 36
6.4 ObjectTypes . 37
6.4.1 QuantityType ObjectType definition . 37
6.4.2 UnitType and subtypes . 39
6.4.3 SyntaxReferenceEntryType ObjectType definition . 43
6.5 References . 44
6.5.1 HasEngineeringUnitDetails . 44
6.5.2 HasQuantity . 45
6.6 DataTypes . 45
6.6.1 AnnotationDataType DataType definition . 45
6.6.2 LinearConversionDataType DataType definition . 46
6.6.3 ConversionLimitEnum . 47
6.6.4 QuantityDimension . 48
7 Data Access specific usage of Services . 50
IEC 62541-8:2025 © IEC 2025
7.1 General . 50
7.2 PercentDeadband . 50
7.3 Data Access status codes . 51
7.3.1 Overview . 51
7.3.2 Operation level result codes . 51
7.3.3 LimitBits . 52
Annex A (normative) OPC COM DA to UA mapping . 53
A.1 Introduction . 53
A.2 Security Considerations . 53
A.3 COM UA wrapper for OPC DA Server . 53
A.3.1 Information Model mapping . 53
A.3.2 Data and error mapping . 58
A.3.3 Read data . 60
A.3.4 Write Data . 61
A.3.5 Subscriptions . 62
A.4 COM UA proxy for DA Client . 62
A.4.1 Guidelines . 62
A.4.2 Information Model and Address Space mapping . 63
A.4.3 Data and error mapping . 67
A.4.4 Read data . 70
A.4.5 Write data . 71
A.4.6 Subscriptions . 72
Annex B (normative) UCUM Symbols . 73
B.1 Introduction – License . 73
B.2 Representation . 73
B.3 Tables of terminal symbols . 74
B.3.1 General . 74
B.3.2 Prefixes . 74
B.3.3 Base units . 75
B.3.4 Derived unit atoms . 76
B.3.5 Customary unit atoms . 79
B.3.6 Other legacy units . 83
Annex C (informative) Outline of syntax references . 88
C.1 UCUM syntax reference . 88
C.2 QUDT syntax reference. 88
C.3 UNECE syntax reference . 89
C.4 IEC CDD Syntax Reference . 90
C.5 LATEX_SIUNITX Syntax Reference . 91
Bibliography . 92

Figure 1 – OPC DataItems are linked to automation data . 9
Figure 2 – DataItem VariableType hierarchy . 10
Figure 3 – Graphical view of a YArrayItem . 20
Figure 4 – Representation of DataItems in the AddressSpace . 26
Figure 5 – Enhanced EUInformation example . 28
Figure 6 – Quantity model overview . 34
Figure 7 – References to external works . 36
IEC 62541-8:2025 © IEC 2025
Figure 8 – QuantityType . 37
Figure 9 – Units model . 39
Figure 10 – MathML example linear conversion . 43
Figure 11 – MathML example inverse linear conversion . 43
Figure A.1 – Sample OPC UA Information Model for OPC DA . 54
Figure A.2 – OPC COM DA to OPC UA data and error mapping . 58
Figure A.3 – Status Code mapping . 59
Figure A.4 – Sample OPC DA mapping of OPC UA Information Model and Address
Space . 64
Figure A.5 – OPC UA to OPC DA data & error mapping . 68
Figure A.6 – OPC UA Status Code to OPC DA quality mapping . 69

Table 1 – DataItemType definition . 11
Table 2 – BaseAnalogType definition . 12
Table 3 – AnalogItemType definition . 14
Table 4 – AnalogUnitType definition. 14
Table 5 – AnalogUnitRangeType definition . 15
Table 6 – DiscreteItemType definition . 15
Table 7 – TwoStateDiscreteType definition . 16
Table 8 – MultiStateDiscreteType definition . 16
Table 9 – MultiStateValueDiscreteType definition . 17
Table 10 – ArrayItemType definition. 18
Table 11 – YArrayItemType definition . 19
Table 12 – YArrayItem item description . 21
Table 13 – XYArrayItemType definition . 22
Table 14 – ImageItemType definition . 23
Table 15 – CubeItemType definition . 24
Table 16 – NDimensionArrayItemType definition . 25
Table 17 – Range DataType structure . 27
Table 18 – Range definition . 27
Table 19 – EUInformation DataType structure . 28
Table 20 – EUInformation definition . 29
Table 21 – Examples from the UNECE Recommendation . 29
Table 22 – ComplexNumberType DataType structure . 30
Table 23 – ComplexNumberType definition . 30
Table 24 – DoubleComplexNumberType DataType structure. 31
Table 25 – DoubleComplexNumberType definition . 31
Table 26 – AxisInformation DataType structure . 31
Table 27 – AxisInformation definition . 32
Table 28 – AxisScaleEnumeration values . 32
Table 29 – AxisScaleEnumeration definition . 32
Table 30 – XVType DataType structure . 33
Table 31 – XVType definition . 33
IEC 62541-8:2025 © IEC 2025
Table 32 – Quantities definition . 34
Table 33 – List of Syntax References . 35
Table 34 – Definition of NodeId for instances of the SyntaxReferenceEntryType . 35
Table 35 – List of Syntax Reference Identifiers . 36
Table 36 – QuantityType definition . 38
Table 37 – QuantityType Additional Subcomponents. 39
Table 38 – UnitType definition . 40
Table 39 – Non-exhaustive list of well-known systems of units . 40
Table 40 – ServerUnitType definition . 41
Table 41 – ServerUnitType Additional Subcomponents . 41
Table 42 – AlternativeUnitType definition . 42
Table 43 – SyntaxReferenceEntryType Definition . 44
Table 44 – HasEngineeringUnitDetails definition . 44
Table 45 – HasQuantity definition . 45
Table 46 – AnnotationDataType Structure . 45
Table 47 – AnnotationDataType examples . 46
Table 48 – AnnotationDataType definition . 46
Table 49 – LinearConversionDataType Structure . 47
Table 50 – LinearConversionDataType Definition . 47
Table 51 – ConversionLimitEnum Items . 47
Table 52 – ConversionLimitEnum Definition . 48
Table 53 – QuantityDimension DataType structure. 48
Table 54 – QuantityDimension definition . 49
Table 55 – QuantityDimension examples . 49
Table 56 – Operation level result codes for BAD data quality . 51
Table 57 – Operation level result codes for UNCERTAIN data quality . 52
Table 58 – Operation level result codes for GOOD data quality . 52
Table A.1 – OPC COM DA to OPC UA Properties mapping . 57
Table A.2 – DataTypes and mapping . 59
Table A.3 – Quality mapping . 60
Table A.4 – OPC DA Read error mapping . 61
Table A.5 – OPC DA Write error code mapping . 62
Table A.6 – DataTypes and Mapping . 69
Table A.7 – Quality mapping . 70
Table A.8 – OPC UA Read error mapping . 71
Table A.9 – OPC UA Write error code mapping . 71

IEC 62541-8:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPC unified architecture -
Part 8: Data access
FOREWORD
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IEC 62541-8 has been prepared by subcommittee 65E: Devices and integration in enterprise
systems, of IEC technical committee 65: Industrial-process measurement, control and
automation. It is an International Standard.
This fourth edition cancels and replaces the third edition published in 2020. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) addition of a "Quantity Model" which can be referenced from EngineeringUnit Properties.
The model defines quantities and assigned units. In addition it provides alternative units
and the conversion to them.
IEC 62541-8:2025 © IEC 2025
b) addition of rules for ValuePrecision Property:
• can also be used for other subtypes like Duration and Decimal.
• rules have been added when ValuePrecision has negative values.
The text of this International Standard is based on the following documents:
Draft Report on voting
65E/1055/CDV 65E/1108/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
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Throughout this document and the other parts of the IEC 62541 series, certain document
conventions are used:
Italics are used to denote a defined term or definition that appears in the "Terms and definitions"
clause in one of the parts of the IEC 62541 series.
Italics are also used to denote the name of a service input or output parameter or the name of
a structure or element of a structure that are usually defined in tables.
The italicized terms and names are, with a few exceptions, written in camel-case (the practice
of writing compound words or phrases in which the elements are joined without spaces, with
each element's initial letter capitalized within the compound). For example, the defined term is
AddressSpace instead of Address Space. This makes it easier to understand that there is a
single definition for AddressSpace, not separate definitions for Address and Space.
A list of all parts in the IEC 62541 series, published under the general title OPC Unified
Architecture, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
IEC 62541-8:2025 © IEC 2025
1 Scope
This part of IEC 62541 is part of the overall OPC Unified Architecture (OPC UA) standard series
and defines the information model associated with Data Access (DA). It particularly includes
additional VariableTypes and complementary descriptions of the NodeClasses and Attributes
needed for Data Access, additional Properties, and other information and behaviour.
The complete address space model, including all NodeClasses and Attributes is specified in
IEC 62541-3. The services to detect and access data are specified in IEC 62541-4.
Annex A specifies how the information received from OPC COM Data Access (DA) Servers is
mapped to the Data Access model.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 62541-1, OPC Unified Architecture - Part 1: Overview and Concepts
IEC 62541-3, OPC Unified Architecture - Part 3: Address Space Model
IEC 62541-4, OPC Unified Architecture - Part 4: Services
IEC 62541-5, OPC Unified Architecture - Part 5: Information Model
IEC 62541-19, OPC Unified Architecture - Part 19: Dictionary References
UN/CEFACT: UNECE Recommendation N°20, Codes for Units of Measure Used in International
Trade
https://www.unece.org/cefact/codesfortrade/codes_index.html
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 62541-1,
IEC 62541-3, and IEC 62541-4 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
IEC 62541-8:2025 © IEC 2025
3.1.1
DataItem
link to arbitrary, live automation data, that is, data that represents currently valid information
Note 1 to entry: Examples of such data are
• device data (such as temperature sensors),
• calculated data,
• status information (open/closed, moving),
• dynamically-changing system data (such as stock quotes),
• diagnostic data.
3.1.2
AnalogItem
DataItem that represents continuously-variable physical quantities (e.g., length, temperature),
in contrast to the digital representation of data in discrete items
Note 1 to entry: Typical examples are the values provided by temperature sensors or pressure sensors. OPC UA
defines specific VariableTypes to identify an AnalogItem. Properties describe the possible ranges of AnalogItems.
3.1.3
DiscreteItem
DataItem that represents data that can take on only a certain number of possible values (e.g.,
OPENING, OPEN, CLOSING, CLOSED)
Note 1 to entry: Specific VariableTypes are used to identify DiscreteItems with two states or with multiple states.
Properties specify the string values for these states.
3.1.4
ArrayItem
DataItem that represents continuously-variable physical quantities and where each individual
data point consists of multiple values represented by an array (e.g., the spectral response of a
digital filter)
Note 1 to entry: Typical examples are the data provided by analyser devices. Specific VariableTypes are used to
identify ArrayItem variants.
3.1.5
EngineeringUnits
units of measurement for AnalogItems that represent continuously-variable physical quantities
(e.g., length, mass, time, temperature)
Note 1 to entry: This standard defines Properties to inform about the unit used for the DataItem value and about
the highest and lowest value likely to be obtained in normal operation.
3.2 Abbreviated terms
DA Data Access
EU Engineering Unit
NaN Not a Number" defined in IEEE 754
UA Unified Architecture
IEC 62541-8:2025 © IEC 2025
4 Concepts
Data Access deals with the representation and use of automation data in Servers.
Automation data can be located inside the Server or on I/O cards directly connected to the
Server. It can also be located in sub-servers or on other devices such as controllers and
input/output modules, connected by serial links via field buses or other communication links.
OPC UA Data Access Servers provide one or more OPC UA Data Access Clients with
transparent access to their automation data.
The links to automation data instances are called DataItems. Which categories of automation
data are provided is completely vendor-specific. Figure 1 illustrates how the AddressSpace of
a Server can contain a broad range of different DataItems.

Figure 1 – OPC DataItems are linked to automation data
Clients can read or write DataItems or monitor them for value changes. The Services needed
for these operations are specified in IEC 62541-4. Changes are defined as a change in status
(quality) or a change in value that exceeds a client-defined range called a Deadband. To detect
the value change, the difference between the current value and the last reported value is
compared to the Deadband.
5 Model
5.1 General
The DataAccess model extends the variable model by defining VariableTypes. The
DataItemType is the base type. ArrayItemType, BaseAnalogType and DiscreteItemType are
specializations. See Figure 2. Each of these VariableTypes can be further extended to form
domain or server specific DataItems.
IEC 62541-8:2025 © IEC 2025
Figure 2 – DataItem VariableType hierarchy
5.2 SemanticsChanged
The StatusCode also contains an informational bit called SemanticsChanged.
Servers that implement Data Access shall set this Bit in notifications if certain Property values
defined in this standard change. The corresponding Properties are specified individually for
each VariableType.
Clients that use any of these Properties should re-read them before they process the data value.
5.3 Variable Types
5.3.1 DataItemType
This VariableType defines the general characteristics of a DataItem. All other DataItem Types
derive from it. The DataItemType derives from the BaseDataVariableType and therefore shares
the variable model as described in IEC 62541-3 and IEC 62541-5. It is formally defined in
Table 1.
IEC 62541-8:2025 © IEC 2025
Table 1 – DataItemType definition
Attribute Value
BrowseName DataItemType
IsAbstract False
ValueRank −2 (−2 = 'Any')
DataType BaseDataType
References NodeClass BrowseName DataType TypeDefinition ModellingRule
Subtype of the BaseDataVariableType defined in IEC 62541-5; i.e the Properties of that type are inherited.
HasSubtype VariableType BaseAnalogType Defined in 5.3.2.2
HasSubtype VariableType DiscreteItemType Defined in 5.3.3
HasSubtype VariableType ArrayItemType Defined in 5.3.4
HasProperty Variable Definition String PropertyType Optional
HasProperty Variable ValuePrecision Double PropertyType Optional
Conformance Units
Data Access DataItems
Definition is a vendor-specific, human readable string that specifies how the value of this
DataItem is calculated. Definition is non-localized and will often contain an equation that can
be parsed by certain clients.
Example: Definition: = "(TempA – 25) + TempB"
ValuePrecision specifies the maximum precision that the Server can maintain for the item based
on restrictions in the target environment.
ValuePrecision can be used for the following DataTypes:
• For Float, Double, and Decimal values it specifies the number of digits after the decimal
place when it is a positive number. When it is a negative number, it specifies the number of
insignificant digits to the left of the decimal place. For example, a ValuePrecision of −2
specifies that the precision of the Value is to the nearest 100. The ValuePrecision should
always be a whole number and it shall always be interpreted as a whole number by rounding
it to the nearest whole number.
• For DateTime values it shall always be a positive number which indicates the minimum time
difference in nanoseconds. For example, a ValuePrecision of 20 000 000 defines a precision
of 20 ms. The ValuePrecision should always be a whole number and it shall always be
interpreted as a whole number by rounding it to the nearest whole number.
• ValuePrecision can also be used for other subtypes of Double (like Duration) and other
Number subtypes that can be represented by a Double.
The ValuePrecision Property is an approximation that is intended to provide guidance to a
Client. A Server is expected to silently round any value with more precision that it supports.
This implies that a Client can encounter cases where the value read back from a Server differs
from the value that it wrote to the Server. This difference shall be no more than the difference
suggested by this Property.
The algorithm for rounding should follow the so-called "Banker's rounding" (aka Round half to
even), in which numbers which are equidistant from the two nearest integers are rounded to the
nearest even integer. Thus, 0.5 rounds down to 0; 1.5 rounds up to 2.
Other decimal fractions round as you would expect: 0.4 to 0, 0.6 to 1, 1.4 to 1, 1.6 to 2, etc.
Only x.5 numbers get the "special" treatment.
IEC 62541-8:2025 © IEC 2025
5.3.2 AnalogItem VariableTypes
5.3.2.1 General
The VariableTypes in 5.3.2.2 to 5.3.2.5 define the characteristics of AnalogItems. The types
have identical semantics and Properties but with diverging ModellingRules for individual
Properties.
The Properties are only described once – in 5.3.2.2. The descriptions apply to the Properties
for the other VariableTypes as well.
5.3.2.2 BaseAnalogType
This VariableType is the base type for analog items. All Properties are optional. Subtypes of
this base type will mandate some of the Properties. The BaseAnalogType derives from the
DataItemType. It is formally defined in Table 2.
Table 2 – BaseAnalogType definition
Attribute Value
BrowseName BaseAnalogType
IsAbstract False
ValueRank −2 (−2 = 'Any')
DataType Number
References NodeClass BrowseName DataType TypeDefinition ModellingRule
Subtype of the DataItemType defined in 5.3.1; i.e. the Properties of that type are inherited.
HasSubtype VariableType AnalogItemType Defined in 5.3.2.3
HasSubtype VariableType AnalogUnitType Defined in 5.3.2.4
HasProperty Variable InstrumentRange Range PropertyType Optional
HasProperty Variable EURange Range PropertyType Optional
HasProperty Variable EngineeringUnits EUInformation PropertyType Optional
Conformance Units
Data Access BaseAnalogType
The following paragra
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