prEN IEC 62541-8:2024

OPC unified architecture - Part 8: Data access

IEC 62541-8:2025 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. 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. 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.

OPC Unified Architecture - Teil 8: Zugriff auf Automatisierungsdaten

Architecture unifiée OPC - Partie 8: Accès aux données

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

General Information

• Status: Not Published
• Publication Date: 18-Aug-2025
• ICS: 25.040.40 - Industrial process measurement and control
• ICS: 35.100.05 - Multilayer applications
• Technical Committee: CLC/TC 65X - Industrial-process measurement, control and automation
• Current Stage: 4060 - Enquiry results established and sent to TC, SR, BTTF - Enquiry
• Start Date: 19-Apr-2024
• Completion Date: 19-Apr-2024
• Mandate: M/490 - Standardisation mandate to the European Standardisation Organisations (ESOs) to support European Smart Grid deployment
• Ref Project: oSIST prEN IEC 62541-8:2024 - OPC Unified Architecture - Part 8: Data Access

Relations

• Revises: EN IEC 62541-8:2020 - OPC Unified Architecture - Part 8: Data Access
• Effective Date: 31-Jan-2023

Overview

The 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 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.

---

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-marec-2024
Enotna arhitektura OPC - 8. del: Dostop do podatkov
OPC Unified Architecture - Part 8: Data Access
OPC Unified Architecture - Teil 8: Zugriff auf Automatisierungsdaten
Architecture unifiée OPC - Partie 8: Accès aux données
Ta slovenski standard je istoveten z: prEN IEC 62541-8:2024
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.

65E/1055/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 62541-8 ED4
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2024-01-26 2024-04-19
SUPERSEDES DOCUMENTS:
65E/980/RR
IEC SC 65E : DEVICES AND INTEGRATION IN ENTERPRISE SYSTEMS
SECRETARIAT: SECRETARY:
United States of America Mr Donald (Bob) Lattimer
OF INTEREST TO THE FOLLOWING COMMITTEES: PROPOSED HORIZONTAL STANDARD:

Other TC/SCs are requested to indicate their interest, if any, in
this CDV to the secretary.
FUNCTIONS CONCERNED:
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Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
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This document is still under study and subject to change. It should not be used for reference purposes.
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are aware and to provide supporting documentation.
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to be included should this proposal proceed. Recipients are reminded that the CDV stage is the final stage for submitting ISC
clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
OPC Unified Architecture - Part 8: Data Access

PROPOSED STABILITY DATE: 2026
NOTE FROM TC/SC OFFICERS:
electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without
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IEC CDV 62541-8 © IEC 2023
1 CONTENTS
3 FOREWORD . vii
4 1 Scope . 1
5 2 Normative references . 1
6 3 Terms, definitions and abbreviated terms . 1
7 3.1 Terms and definitions . 1
8 3.2 Abbreviated terms . 2
9 4 Concepts . 2
10 5 Model . 4
11 5.1 General . 4
12 5.2 SemanticsChanged . 4
13 5.3 Variable Types . 4
14 5.3.1 DataItemType . 4
15 5.3.2 AnalogItem VariableTypes . 5
16 5.3.3 DiscreteItemType . 8
17 5.3.4 ArrayItemType . 10
18 5.4 Address Space model . 15
19 5.5 Attributes of DataItems . 16
20 5.6 DataTypes . 17
21 5.6.1 Overview . 17
22 5.6.2 Range . 17
23 5.6.3 EUInformation . 17
24 5.6.4 ComplexNumberType . 19
25 5.6.5 DoubleComplexNumberType . 20
26 5.6.6 AxisInformation . 20
27 5.6.7 AxisScaleEnumeration . 21
28 5.6.8 XVType . 21
29 6 Quantities and Units model . 23
30 6.1 General . 23
31 6.2 Quantities entry point . 23
32 6.3 Syntax References . 23
33 6.3.1 General . 23
34 6.3.2 Using Dictionary References . 24
35 6.3.3 Syntax Reference Identifier . 25
36 6.4 ObjectTypes . 27
37 6.4.1 QuantityType ObjectType definition . 27
38 6.4.2 UnitType and subtypes . 28
39 6.4.3 SyntaxReferenceEntryType ObjectType definition . 31
40 6.5 References . 32
41 6.5.1 HasEngineeringUnitDetails. 32
42 6.5.2 HasQuantity . 32
43 6.6 DataTypes . 32
44 6.6.1 AnnotationDataType DataType definition . 32
45 6.6.2 LinearConversionDataType DataType definition . 33
46 6.6.3 ConversionLimitEnum . 34
47 6.6.4 QuantityDimension . 34
48 7 Data Access specific usage of Services . 37

iii IEC CDV 62541-8 © IEC 2023
49 7.1 General . 37
50 7.2 PercentDeadband. 37
51 7.3 Data Access status codes . 37
52 7.3.1 Overview . 37
53 7.3.2 Operation level result codes . 37
54 7.3.3 LimitBits . 39
55 Annex A (normative) OPC COM DA to UA mapping . 40
56 A.1 Introduction . 40
57 A.2 Security Considerations . 40
58 A.3 COM UA wrapper for OPC DA Server . 40
59 A.3.1 Information Model mapping . 40
60 A.3.2 Data and error mapping . 44
61 A.3.3 Read data . 47
62 A.3.4 Write Data . 48
63 A.3.5 Subscriptions . 48
64 A.4 COM UA proxy for DA Client . 49
65 A.4.1 Guidelines . 49
66 A.4.2 Information Model and Address Space mapping . 49
67 A.4.3 Data and error mapping . 53
68 A.4.4 Read data . 55
69 A.4.5 Write data . 56
70 A.4.6 Subscriptions . 56
71 Annex B (normative) UCUM Symbols . 58
72 B.1 Introduction - License . 58
73 B.2 Representation . 58
74 B.3 Tables of Terminal Symbols . 59
75 B.1.1 Prefixes . 59
76 B.1.2 Base Units . 60
77 B.1.3 Derived Unit Atoms . 60
78 B.1.4 Customary Unit Atoms . 62
79 B.1.5 Other Legacy Units . 65
80 Annex C (informative) Outline of Syntax References . 69
81 C.1 UCUM Syntax Reference . 69
82 C.2 QUDT Syntax Reference . 69
83 C.3 UNECE Syntax Reference . 70
84 C.4 IEC CDD Syntax Reference . 70
85 C.5 LATEX_SIUNITX Syntax Reference . 71
IEC CDV 62541-8 © IEC 2023 iv
87 FIGURES
89 Figure 1 – OPC DataItems are linked to automation data . 3
90 Figure 2 – DataItem VariableType hierarchy . 4
91 Figure 3 – Graphical view of a YArrayItem . 12
92 Figure 4 – Representation of DataItems in the AddressSpace . 16
93 Figure 5 – Enhanced EUInformation example . 18
94 Figure 6 – Quantity model overview . 23
95 Figure 7 – References to external works . 25
96 Figure 8 - QuantityType . 27
97 Figure 9 - Units model . 28
98 Figure A.1 – Sample OPC UA Information Model for OPC DA . 41
99 Figure A.2 – OPC COM DA to OPC UA data and error mapping . 45
100 Figure A.3 – Status Code mapping . 46
101 Figure A.4 – Sample OPC DA mapping of OPC UA Information Model and Address Space . 50
102 Figure A.5 – OPC UA to OPC DA data & error mapping . 53
103 Figure A.6 – OPC UA Status Code to OPC DA quality mapping . 55
v IEC CDV 62541-8 © IEC 2023
105 TABLES
107 Table 1 – DataItemType definition . 5
108 Table 2 – BaseAnalogType definition . 6
109 Table 3 – AnalogItemType definition . 7
110 Table 4 – AnalogUnitType definition . 7
111 Table 5 – AnalogUnitRangeType definition . 8
112 Table 6 – DiscreteItemType definition . 8
113 Table 7 – TwoStateDiscreteType definition . 8
114 Table 8 – MultiStateDiscreteType definition . 9
115 Table 9 – MultiStateValueDiscreteType definition . 10
116 Table 10 – ArrayItemType definition . 11
117 Table 11 – YArrayItemType definition . 11
118 Table 12 – YArrayItem item description . 12
119 Table 13 – XYArrayItemType definition . 13
120 Table 14 – ImageItemType definition . 14
121 Table 15 – CubeItemType definition . 14
122 Table 16 – NDimensionArrayItemType definition . 15
123 Table 17 – Range DataType structure . 17
124 Table 18 – Range definition . 17
125 Table 16 – EUInformation DataType structure . 18
126 Table 17 – EUInformation definition . 18
127 Table 18 – Examples from the UNECE Recommendation . 19
128 Table 19 – ComplexNumberType DataType structure . 19
129 Table 20 – ComplexNumberType definition . 20
130 Table 21 – DoubleComplexNumberType DataType structure . 20
131 Table 22 – DoubleComplexNumberType definition . 20
132 Table 23 – AxisInformation DataType structure . 20
133 Table 24 – AxisInformation definition . 21
134 Table 25 – AxisScaleEnumeration values . 21
135 Table 26 – AxisScaleEnumeration definition . 21
136 Table 27 – XVType DataType structure . 21
137 Table 28 – XVType definition . 22
138 Table 29 – Quantities definition . 23
139 Table 30 – List of Syntax References . 24
140 Table 31 – Definition of NodeId for instances of the SyntaxReferenceEntryType . 24
141 Table 32 – List of Syntax Reference Identifiers . 25
142 Table 33 – QuantityType definition . 27
143 Table 34 – QuantityType Additional Subcomponents . 28
144 Table 35 – UnitType definition . 29
145 Table 36 – Non-exhaustive list of well-known systems of units . 29
146 Table 37 – ServerUnitType definition . 29
147 Table 38 – ServerUnitType Additional Subcomponents . 30

IEC CDV 62541-8 © IEC 2023 vi
148 Table 39 – AlternativeUnitType definition . 30
149 Table 40 – SyntaxReferenceEntryType Definition . 31
150 Table 41 – HasEngineeringUnitDetails definition . 32
151 Table 42 – HasQuantity definition . 32
152 Table 43 – AnnotationDataType Structure . 33
153 Table 44 – AnnotationDataType examples . 33
154 Table 45 – AnnotationDataType definition . 33
155 Table 46 – LinearConversionDataType Structure . 33
156 Table 47 – LinearConversionDataType Definition . 34
157 Table 48 – ConversionLimitEnum Items . 34
158 Table 49 – ConversionLimitEnum Definition . 34
159 Table 50 – QuantityDimension DataType structure . 35
160 Table 51 – QuantityDimension definition . 35
161 Table 52 – QuantityDimension examples . 35
162 Table 53 – Operation level result codes for BAD data quality . 38
163 Table 54 – Operation level result codes for UNCERTAIN data quality . 38
164 Table 55 – Operation level result codes for GOOD data quality . 38
165 Table A.1 – OPC COM DA to OPC UA Properties mapping . 43
166 Table A.2 – DataTypes and mapping . 46
167 Table A.3 – Quality mapping . 47
168 Table A.4 – OPC DA Read error mapping. 48
169 Table A.5 – OPC DA Write error code mapping . 48
170 Table A.6 – DataTypes and Mapping . 54
171 Table A.7 – Quality mapping . 55
172 Table A.8 – OPC UA Read error mapping. 56
173 Table A.9 – OPC UA Write error code mapping . 56
vii IEC CDV 62541-8 © IEC 2023
176 INTERNATIONAL ELECTROTECHNICAL COMMISSION
177 ____________
179 OPC UNIFIED ARCHITECTURE –
181 Part 8: Data access
183 FOREWORD
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211 for the correct application of this publication.
212 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights.
213 IEC shall not be held responsible for identifying any or all such patent rights.
214 International Standard IEC 62541-8 has been prepared by subcommittee 65E: Devices and integration
215 in enterprise systems, of IEC technical committee 65: Industrial-process measurement, control and
216 automation.
217 This fourth edition cancels and replaces the third edition published in 2020. This edition constitutes a
218 technical revision.
219 This edition includes the following significant technical changes with respect to the previous edition:
220 a) Added a “Quantity Model” which can be referenced from EngineeringUnit Properties. The model
221 defines quantities and assigned units. In addition it provides alternative units and the conversion
222 to them.
223 b) Added additional rules for ValuePrecision Property:
224 • Can also be used for other subtypes like Duration and Decimal.
225 • Added rules when ValuePrecision has negative values.

IEC CDV 62541-8 © IEC 2023 viii
226 The text of this International Standard is based on the following documents:
CDV Report on voting
65E/XX/CDV 65E/XX/RVC
228 Full information on the voting for the approval of this International Standard can be found in the report
229 on voting indicated in the above table.
230 This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
231 Throughout this document and the other parts of the IEC 62541 series, certain document conventions
232 are used:
233 Italics are used to denote a defined term or definition that appears in the "Terms and definition" clause
234 in one of the parts of the IEC 62541 series.
235 Italics are also used to denote the name of a service input or output parameter or the name of a
236 structure or element of a structure that are usually defined in tables.
237 The italicized terms and names are, with a few exceptions, written in camel-case (the practice of
238 writing compound words or phrases in which the elements are joined without spaces, with each
239 element's initial letter capitalized within the compound). For example, the defined term is
240 AddressSpace instead of Address Space. This makes it easier to understand that there is a single
241 definition for AddressSpace, not separate definitions for Address and Space.
242 A list of all parts of the IEC 62541 series, published under the general title OPC Unified Architecture,
243 can be found on the IEC website.
244 The committee has decided that the contents of this document will remain unchanged until the stability
245 date indicated on the IEC website under "http://webstore.iec.ch" in the data related to the specific
246 document. At this date, the document will be
247 • reconfirmed,
248 • withdrawn,
249 • replaced by a revised edition, or
250 • amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.
1 IEC CDV 62541-8 © IEC 2023
253 OPC UNIFIED ARCHITECTURE –
255 Part 8: Data Access
257 1 Scope
258 This part of IEC 62541 is part of the overall OPC Unified Architecture (OPC UA) standard series
259 and defines the information model associated with Data Access (DA). It particularly includes
260 additional VariableTypes and complementary descriptions of the NodeClasses and Attributes
261 needed for Data Access, additional Properties, and other information and behaviour.
262 The complete address space model, including all NodeClasses and Attributes is specified in
263 IEC 62541-3. The services to detect and access data are specified in IEC 62541-4.
264 Annex A specifies the recommended way how the information received from OPC COM Data
265 Access (DA) Servers shall be mapped to the model in this document.
266 2 Normative references
267 The following referenced documents are indispensable for the application of this document. For
268 dated references, only the edition cited applies. For undated references, the latest edition of
269 the referenced document (including any amendments and errata) applies.
270 IEC 62541-1, OPC Unified Architecture - Part 1: Overview and Concepts
271 IEC 62541-3, OPC Unified Architecture - Part 3: Address Space Model
272 IEC 62541-4, OPC Unified Architecture - Part 4: Services
273 IEC 62541-5, OPC Unified Architecture - Part 5: Information Model
274 IEC 62541-19, OPC Unified Architecture - Part 19: Dictionary References
275 UN/CEFACT: UNECE Recommendation N° 20, Codes for Units of Measure Used in
276 International Trade
277 https://www.unece.org/cefact/codesfortrade/codes_index.html
278 3 Terms, definitions and abbreviated terms
279 3.1 Terms and definitions
280 For the purposes of this document, the terms and definitions given in IEC 62541-1, IEC 62541-
281 3, and IEC 62541-4 and the following apply.
282 3.1.1
283 DataItem
284 link to arbitrary, live automation data, that is, data that represents currently valid information
285 Note 1 to entry: Examples of such data are
286 • device data (such as temperature sensors),
287 • calculated data,
288 • status information (open/closed, moving),
289 • dynamically-changing system data (such as stock quotes),
290 • diagnostic data.
291 3.1.2
292 AnalogItem
293 DataItem that represents continuously-variable physical quantities (e.g., length, temperature),
294 in contrast to the digital representation of data in discrete items
295 Note 1 to entry: Typical examples are the values provided by temperature sensors or pressure sensors. OPC UA
296 defines specific VariableTypes to identify an AnalogItem. Properties describe the possible ranges of AnalogItems.

IEC CDV 62541-8 © IEC 2023 2
297 3.1.3
298 DiscreteItem
299 DataItem that represents data that may take on only a certain number of possible values (e.g.,
300 OPENING, OPEN, CLOSING, CLOSED)
301 Note 1 to entry: Specific VariableTypes are used to identify DiscreteItems with two states or with multiple states.
302 Properties specify the string values for these states.
303 3.1.4
304 ArrayItem
305 DataItem that represents continuously-variable physical quantities and where each individual
306 data point consists of multiple values represented by an array (e.g., the spectral response of a
307 digital filter)
308 Note 1 to entry: Typical examples are the data provided by analyser devices. Specific VariableTypes are used to
309 identify ArrayItem variants.
310 3.1.5
311 EngineeringUnits
312 units of measurement for AnalogItems that represent continuously-variable physical quantities
313 (e.g., length, mass, time, temperature)
314 Note 1 to entry: This standard defines Properties to inform about the unit used for the DataItem value and about
315 the highest and lowest value likely to be obtained in normal operation.
316 3.2 Abbreviated terms
317 DA Data Access
318 EU Engineering Unit
319 NaN „Not a Number“ defined in IEEE 754
320 UA Unified Architecture
321 4 Concepts
322 Data Access deals with the representation and use of automation data in Servers.
323 Automation data can be located inside the Server or on I/O cards directly connected to the
324 Server. It can also be located in sub-servers or on other devices such as controllers and
325 input/output modules, connected by serial links via field buses or other communication links.
326 OPC UA Data Access Servers provide one or more OPC UA Data Access Clients with
327 transparent access to their automation data.
328 The links to automation data instances are called DataItems. Which categories of automation
329 data are provided is completely vendor-specific. Figure 1 illustrates how the AddressSpace of
330 a Server might contain a broad range of different DataItems.

3 IEC CDV 62541-8 © IEC 2023
OPC UA Server
Root
AddressSpace
with data items
332 Figure 1 – OPC DataItems are linked to automation data
333 Clients may read or write DataItems, or monitor them for value changes. The Services needed
334 for these operations are specified in IEC 62541-4. Changes are defined as a change in status
335 (quality) or a change in value that exceeds a client-defined range called a Deadband. To detect
336 the value change, the difference between the current value and the last reported value is
337 compared to the Deadband.
IEC CDV 62541-8 © IEC 2023 4
338 5 Model
339 5.1 General
340 The DataAccess model extends the variable model by defining VariableTypes. The
341 DataItemType is the base type. ArrayItemType, BaseAnalogType and DiscreteItemType are
342 specializations. See Figure 2. Each of these VariableTypes can be further extended to form
343 domain or server specific DataItems.
BaseDataVariableType
Defined in
[UA Part 5]
Type
DataItemType
ArrayItemType BaseAnalogType DiscreteItemType
AnalogItemType
AnalogUnitType
TwoStateDiscreteType
AnalogUnitRange MultiState MultiStateValue
DiscreteType DiscreteType
Type
345 Figure 2 – DataItem VariableType hierarchy
346 5.2 SemanticsChanged
347 The StatusCode also contains an informational bit called SemanticsChanged.
348 Servers that implement Data Access shall set this Bit in notifications if certain Property values
349 defined in this standard change. The corresponding Properties are specified individually for
350 each VariableType.
351 Clients that use any of these Properties should re-read them before they process the data value.
352 5.3 Variable Types
353 5.3.1 DataItemType
354 This VariableType defines the general characteristics of a DataItem. All other DataItem Types
355 derive from it. The DataItemType derives from the BaseDataVariableType and therefore shares
356 the variable model as described in IEC 62541-3 and IEC 62541-5. It is formally defined in Table
357 1.
5 IEC CDV 62541-8 © IEC 2023
358 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
360 Definition is a vendor-specific, human readable string that specifies how the value of this
361 DataItem is calculated. Definition is non-localized and will often contain an equation that can
362 be parsed by certain clients.
363 Example: Definition::= “(TempA – 25) + TempB”
364 ValuePrecision specifies the maximum precision that the Server can maintain for the item based
365 on restrictions in the target environment.
366 ValuePrecision can be used for the following DataTypes:
367 • For Float, Double, and Decimal values it specifies the number of digits after the decimal
368 place when it is a positive number. When it is a negative number, it specifies the number
369 of insignificant digits to the left of the decimal place.
370 For example a ValuePrecision of -2 specifies that the precision of the Value is to the
371 nearest 100. The ValuePrecision should always be a whole number and it shall always
372 be interpreted as a whole number by rounding it to the nearest whole number.
373 • For DateTime values it shall always be a positive number which indicates the minimum
374 time difference in nanoseconds. For example, a ValuePrecision of 20 000 000 defines
375 a precision of 20 ms. The ValuePrecision should always be a whole number and it shall
376 always be interpreted as a whole number by rounding it to the nearest whole number.
377 • ValuePrecision can also be used for other subtypes of Double (like Duration) and other
378 Number subtypes that can be represented by a Double.
379 The ValuePrecision Property is an approximation that is intended to provide guidance to a
380 Client. A Server is expected to silently round any value with more precision that it supports.
381 This implies that a Client may encounter cases where the value read back from a Server differs
382 from the value that it wrote to the Server. This difference shall be no more than the difference
383 suggested by this Property.
384 The algorithm for rounding should follow the so-called “Banker’s rounding” (aka Round half to
385 even), in which numbers which are equidistant from the two nearest integers are rounded to the
386 nearest even integer. Thus, 0.5 rounds down to 0; 1.5 rounds up to 2.
387 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.
388 Only x.5 numbers get the "special" treatment.
390 5.3.2 AnalogItem VariableTypes
391 5.3.2.1 General
392 The VariableTypes in this subclause define the characteristics of AnalogItems. The types have
393 identical semantics and Properties but with diverging ModellingRules for individual Properties.
394 The Properties are only described once - in 5.3.2.2. The descriptions apply to the Properties
395 for the other VariableTypes as well.

IEC CDV 62541-8 © IEC 2023 6
396 5.3.2.2 BaseAnalogType
397 This VariableType is the base type for analog items. All Properties are optional. Subtypes of
398 this base type will mandate some of the Properties. The BaseAnalogType derives from the
399 DataItemType. It is formally defined in Table 2.
400 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
402 The following paragraphs describe the Properties of this VariableType. If the analog item’s
403 Value contains an array, the Properties shall apply to all elements in the array.
404 InstrumentRange defines the value range that can be returned by the instrument.
405 Example: InstrumentRange::= {-9999.9, 9999.9}
406 Although defined as optional, it is strongly recommended for Servers to support this Property.
407 Without an InstrumentRange being provided, Clients will commonly assume the full range
408 according to the DataType.
409 The InstrumentRange Property may also be used to restrict a Built-in DataType such as Byte
410 or Int16) to a smaller range of values.
411 Examples:
412 UInt4: InstrumentRange::= {0, 15}
413 Int6: InstrumentRange::= {-32, 31}
414 The Range DataType is specified in 5.6.2.
415 EURange defines the value range likely to be obtained in normal operation. It is intended for
416 such use as automatically scaling a bar graph display.
417 Sensor or instrument failure or deactivation can result in a returned item value which is actually
418 outside of this range. Client software must be prepared to deal with this possibility. Similarly a
419 Client may attempt to write a value that is outside of this range back to the server. The exact
420 behaviour (accept, reject, clamp, etc.) in this case is Server-dependent. However, in general
421 Servers shall be prepared to handle this.
422 Example: EURange::= {-200.0,1400.0}
423 See also 7.2 for a special monitoring filter (PercentDeadband) which is based on the
424 engineering unit range.
425 NOTE If EURange is not provided on an instance, the PercentDeadband filter cannot be used for that instance (see
426 clause 7.2).
427 EngineeringUnits specifies the units for the DataItem’s value (e.g., DEGC, hertz, seconds). The
428 EUInformation type is specified in 5.6.3. The NonHierarchical References HasQuantity (see
429 6.5.2) and HasEngineeringUnitDetail (see 6.5.1) can be used to expose further information for
430 the unit.
431 It is important to note that understanding the units of a measurement value is essential for a
432 uniform system. In an open system in particular where Servers from different cultures might be
433 used, it is essential to know what the units of measurement are. Based on such knowledge,

7 IEC CDV 62541-8 © IEC 2023
434 values can be converted if necessary before being used. Therefore, although defined as
435 optional, support of the EngineeringUnits Property i
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