IEC 62541 -3:2020 pdf download – OPC unified architecture – Part 3: Address Space Model.
ISO 8601 (all parts), Date and time – Representations for information interchange IETF RFC 5646, Tags for Identifying Languages http://tools.ietf.org/html/rfc5646 Unicode Standard Annex #1 5: Unicode Normalization Forms, http://www.unicode.org/reports/tr1 5/ W3C XML Schema Definition Language (XSD) Part 2: DataTypes http://www.w3.org/TR/xmlschema-2/ TAI: International Atomic Time http://www.bipm.org/en/bipm-services/timescales/tai.html 3 Terms, definitions, abbreviated terms and conventions 3.1 Terms and definitions For the purposes of this document, the terms and definitions given in IEC TR 62541 ‑1 and the following apply. ISO and IEC maintain terminological databases for use in standardization at the following addresses: • IEC Electropedia: available at http://www.electropedia.org/ • ISO Online browsing platform: available at http://www.iso.org/obp 3.1.1 DataType instance of a DataType Node that is used together with the ValueRank Attribute to define the data type of a Variable 3.1 .2 DataTypeId NodeId of a DataType Node
This document also defines ReferenceTypes, but ReferenceTypes can may also be specified by a Server or by a Client using the NodeManagement Services specified in IEC 62541 ‑4. Thus, the NodeClass tables contained in this document can may contain the base ReferenceType called References identifying that any ReferenceType may be used for the NodeClass, including system specific ReferenceTypes. The NodeClass tables only specify how the NodeClasses can be used as SourceNodes of References, not as TargetNodes. If a NodeClass table allows a ReferenceType for its NodeClass to be used as SourceNode, this is also true for subtypes of the ReferenceType. However, subtypes of the ReferenceType may restrict its SourceNodes. This document defines Properties, but Properties can be defined by other standards organizations or vendors and Nodes can have Properties that are not standardized. Properties defined in this document are defined by their name, which is mapped to the BrowseName having the NamespaceIndex 0, which represents the Namespace for OPC UA. The Use column (optional or mandatory) does not imply a specific ModellingRule for Properties. Different Server implementations will choose to use ModellingRules appropriate for them.
4.3.4 References References are used to relate Nodes to each other. They can be accessed using the browsing and querying Services defined in IEC 62541 ‑4. Like Attributes, they are defined as fundamental components of Nodes. Unlike Attributes, References are defined as instances of ReferenceType Nodes. ReferenceType Nodes are visible in the AddressSpace and are defined using the ReferenceType NodeClass (see 5.3). The Node that contains the Reference is referred to as the SourceNode and the Node that is referenced is referred to as the TargetNode. The combination of the SourceNode, the ReferenceType and the TargetNode is used in OPC UA Services to uniquely identify References. Thus, each Node can reference another Node with the same ReferenceType only once. Any subtypes of concrete ReferenceTypes are considered to be equal to the base concrete ReferenceTypes when identifying References (see 5.3 for subtypes of ReferenceTypes). Figure 4 illustrates this model of a Reference.
4.4.2 Properties Properties are Server-defined characteristics of Objects, DataVariables and other Nodes. Properties differ from Attributes in that they characterize what the Node represents, such as a device or a purchase order. Attributes define additional metadata that is instantiated for all Nodes from a NodeClass. Attributes are common to all Nodes of a NodeClass and only defined by this document whereas Properties can be Server-defined. For example, an Attribute defines the DataType of Variables whereas a Property can be used to specify the engineering unit of some Variables. To prevent recursion, Properties are not allowed to have Properties defined for them. To easily identify Properties, the BrowseName of a Property shall be unique in the context of the Node containing the Properties (see 5.6.3 for details). A Node and its Properties shall always reside in the same Server. 4.4.3 DataVariables DataVariables represent the content of an Object. For example, a file Object may be defined that contains a stream of bytes. The stream of bytes may be defined as a DataVariable that is an array of bytes. Properties may be used to expose the creation time and owner of the file Object.