1 Scope

This specification describes the OPC Unified Architecture (OPC UA) AddressSpace and its Objects. This Part is the OPC UA meta model on which OPC UA information models are based.

2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments and errata) applies.

OPC 10000-1, OPC Unified Architecture - Part 1: Overview and Concepts

3 Terms, definitions, abbreviations and conventions

3.1 Terms and definitions

For the purposes of this document, the terms and definitions given in OPC 10000-1 and the following apply.

3.1.1 AddIn

a widely applicable feature or feature-set represented by an ObjectType that will be applied using aggregation

3.1.2 DataType

instance of a DataType Node that is used together with the ValueRank Attribute to define the data type of a Variable

3.1.3 Built-in DataType

a fundamental building block DataType defined by this standard

3.1.4 Simple DataType

a DataType which is not a Structure, Enumeration or OptionSet

3.1.5 DataTypeId

NodeId of a DataType Node

3.1.6 DataTypeRefinement

refinement of a Structured DataType restricting the usage of fields and adding meta data to fields

3.1.7 DataVariable

Variables that represent values of Objects, either directly or indirectly for complex Variables, where the Variables are always the TargetNode of a HasComponent Reference

3.1.8 EventType

ObjectType Node that represents the type definition of an Event

3.1.9 Hierarchical Reference

Reference that is used to construct hierarchies in the AddressSpace

3.1.10 InstanceDeclaration

Node that is used by a complex TypeDefinitionNode to expose its complex structure

3.1.11 Interface

an abstract ObjectType used to specify a feature or feature-set that shall be implemented by the Object or ObjectType where it is applied to

3.1.12 ModellingRule

metadata of an InstanceDeclaration that defines how the InstanceDeclaration will be used for instantiation and also defines subtyping rules for an InstanceDeclaration

3.1.13 Property

Variables that are the TargetNode for a HasProperty Reference

3.1.14 SourceNode

Node having a Reference to another Node

3.1.15 SubtypeRestriction

restriction of the usage of a subtype of a DataType to only allow a subset of all its subtypes

3.1.16 TargetNode

Node that is referenced by another Node

3.1.17 TypeDefinitionNode

Node that is used to define the type of another Node

3.1.18 VariableType

Node that represents the type definition for a Variable

3.2 Abbreviated terms

• UA Unified Architecture

• UML Unified Modeling Language

• URI Uniform Resource Identifier as defined by RFC 3986

• W3C World Wide Web Consortium

• XML Extensible Markup Language

3.3 Conventions

3.3.1 Conventions for AddressSpace figures

Nodes and their References to each other are illustrated using figures. Figure 1 illustrates the conventions used in these figures.

In these figures, rectangles represent Nodes. Node rectangles may be titled with one or two lines of text. When two lines are used, the first text line in the rectangle identifies the NodeClass and the second line contains the BrowseName. When one line is used, it contains the BrowseName.

Node rectangles may contain boxes used to define their Attributes and References. Specific names in these boxes identify specific Attributes and References.

Shaded rectangles with rounded corners and with arrows passing through them represent References. The arrow that passes through them begins at the SourceNode and points to the TargetNode. References may also be shown by drawing an arrow that starts at the Reference name in the “References” box and ends at the TargetNode.

3.3.2 Conventions for defining NodeClasses

Clause 4.10 defines AddressSpace NodeClasses. Table 1 describes the format of the tables used to define NodeClasses.

The Name column contains the name of the Attribute, the name of the ReferenceType used to create a Reference or the name of a Property referenced using the HasProperty Reference.

The Use column defines whether the Attribute or Property is mandatory (M) or optional (O). When mandatory the Attribute or Property shall exist for every Node of the NodeClass. For References it specifies the cardinality. The following values may apply:

• “0..*” identifies that there are no restrictions, that is, the Reference does not have to be provided but there is no limitation how often it can be provided;

• “0..1” identifies that the Reference is provided at most once;

• “1” identifies that the Reference shall be provided exactly once.

The Data Type column contains the name of the DataType of the Attribute or Property. It is not used for References.

The Description column contains the description of the Attribute, the Reference or the Property.

Only this document may define Attributes. Thus, all Attributes of the NodeClass are specified in the table and may only be extended by other parts of this series of standards.

This document also defines ReferenceTypes, but ReferenceTypes may also be specified by a Server or by a client using the NodeManagement Services specified in OPC 10000-4. Thus, the NodeClass tables contained in this document 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 standard organizations or vendors and Nodes can have Properties that are not standardised. Properties defined in this standard 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 AddressSpace concepts

4.1 Overview

The remainder of Clause 4 defines the concepts of the AddressSpace. Clause 5 defines the NodeClasses of the AddressSpace representing the AddressSpace concepts. Clause 6 defines details on the type model for ObjectTypes and VariableTypes. Standard ReferenceTypes, DataTypes and EventTypes are defined in Clauses 7 to 8.61.

The informative Annex A describes general considerations on how to use the Address Space Model and the informative Annex B provides a UML Model of the Address Space Model. The normative Annex C defines a graphical notation for OPC UA data.

4.2 URIs

URIs provide a syntax for constructing unique identifiers for resources that are used in several different contexts with this specification. Three use cases where implementors may need to construct their own URIs are:

NamespaceUris used to identify an information model. NamespaceUris appear in UANodeSets (see OPC 10000-6) and in the NamespaceArray in a Server AddressSpace (see OPC 10000-5);

ApplicationUris identify an OPC UA Application running on a particular Device and are assigned by the OwnerOperator or automatically created by the application software. An ApplicationInstance Certificate has the ApplicationUri in the subjectAltName (see OPC 10000-6);

ProductInstanceUris identify a Device and are assigned by the Device Manufacturer (see OPC 10000-21). A DeviceIdentity Certificate has the ProductInstanceUri in the subjectAltName.

These URIs conform to RFC 3986, however, this specification is very open ended. This clause provides recommendations for constructing these URIs.

The recommendations help ensure that URIs are unique by providing a scope controlled by a single authority (e.g. the domain name) and are human readable. Programs shall always treat URIs as opaque strings that can only be tested for equality with a case sensitive string comparison.

URIs should be tag URIs (RFC 4151) or HTTP URLs (RFC 3986). URLs should only be used if they link to actual webpages.

The scheme and domain name portions of the URI are in lower case. URIs may include query subcomponents and/or fragments.

Below are recommended URI formats for different use cases.

1) NamespaceUri assigned by the creator of a InformationModel:

tag:<authority-domain-name>,<yyyy-MM>:UA:<model-short-name>

http://<authority-domain-name>/UA/<model-short-name>

Where the <authority-domain-name> is a domain name owned by the authority creating the information model. In many cases this will be ‘opcfoundation.org’. <yyyy-MM> is the year and month that the URI was created. The <model-short-name> is a short identifier for the InformationModel.

2) ApplicationUri assigned by the OwnerOperator

tag:<device-domain-name>,<yyyy-MM>:<product>

http://<device-domain-name>/<product>

Where the <device-domain-name> is a domain name or IP address for the Device that is unique within the OwnerOperator system. <yyyy-MM> is the year and month that the URI was created. The <product> is an identifier for the product which could contain many path segments.

3) ApplicationUri automatically generated by product software:

tag:<vendor-domain-name>,<yyyy-MM>:<product>:<guid>

http://<vendor-domain-name>/<product>/<guid>

Where <vendor-domain-name> is a domain name owned by the product vendor. <yyyy-MM> is the year and month that the URI was created. The <product> is an identifier for the product which could contain many path segments and <guid> is unique identifier generated when the product is first initialized on a particular Device.

4) ProductInstanceUri assigned by a Device Manufacturer:

tag:<manufacturer-domain-name>,<yyyy-MM>:<product>:<serial-number>

http://<manufacturer-domain-name>/<product>/<serial-number>

Where <manufacturer-domain-name> is a domain name owned by the Manufacturer. <yyyy-MM> is the year and month that the URI was created. The <product> is an identifier for the product which could contain many path segments and <serial-number> is unique identifier for the Device assigned by the Manufacturer.

4.3 Object Model

The primary objective of the OPC UA AddressSpace is to provide a standard way for Servers to represent Objects to Clients. The OPC UA Object Model has been designed to meet this objective. It defines Objects in terms of Variables and Methods. It also allows relationships to other Objects to be expressed. Figure 2 illustrates the model.

The elements of this model are represented in the AddressSpace as Nodes. Each Node is assigned to a NodeClass and each NodeClass represents a different element of the Object Model. Clause 4.10 defines the NodeClasses used to represent this model.

4.4 Node Model

4.4.1 General

The set of Objects and related information that the OPC UA Server makes available to Clients is referred to as its AddressSpace. The model for Objects is defined by the OPC UA Object Model (see 4.3).

Objects and their components are represented in the AddressSpace as a set of Nodes described by Attributes and interconnected by References. Figure 3 illustrates the model of a Node and the remainder of 4.3 discusses the details of the Node Model.

4.4.2 NodeClasses

NodeClasses are defined in terms of the Attributes and References that shall be instantiated (given values) when a Node is defined in the AddressSpace. Attributes are discussed in 4.4.3 and References in 4.4.4.

Clause 5 defines the NodeClasses for the OPC UA AddressSpace. These NodeClasses are referred to collectively as the metadata for the AddressSpace. Each Node in the AddressSpace is an instance of one of these NodeClasses. No other NodeClasses shall be used to define Nodes, and as a result, Clients and Servers are not allowed to define NodeClasses or extend the definitions of these NodeClasses.

4.4.3 Attributes

Attributes are data elements that describe Nodes. Clients can access Attribute values using Read, Write, Query, and Subscription/MonitoredItem Services. These Services are defined in OPC 10000-4.

Attributes are elementary components of NodeClasses. Attribute definitions are included as part of the NodeClass definitions in Clause 4.10 and, therefore, are not included in the AddressSpace.

Each Attribute definition consists of an attribute id (for attribute ids of Attributes, see OPC 10000-6), a name, a description, a data type and a mandatory/optional indicator. The set of Attributes defined for each NodeClass shall not be extended by Clients or Servers.

When a Node is instantiated in the AddressSpace, the values of the NodeClass Attributes are provided. The mandatory/optional indicator for the Attribute indicates whether the Attribute has to be instantiated.

4.4.4 References

References are used to relate Nodes to each other. They can be accessed using the browsing and querying Services defined in OPC 10000-4.

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 are 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.

The TargetNode of a Reference may be in the same AddressSpace or in the AddressSpace of another OPC UA Server. TargetNodes located in other Servers are identified in OPC UA Services using a combination of the remote Server name and the identifier assigned to the Node by the remote Server.

OPC UA does not require that the TargetNode exists, thus References may point to a Node that does not exist.

4.5 Variables

4.5.1 General

Variables are used to represent values. Two types of Variables are defined, Properties and DataVariables. They differ in the kind of data that they represent and whether they can contain other Variables.

4.5.2 Properties

Properties are Server-defined characteristics of Objects, DataVariables and other Nodes. Properties differ from Attributes in that they characterise 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 specification 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.

4.5.3 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.

For example, if a DataVariable is defined by a data structure that contains two fields, “startTime” and “endTime” then it might have a Property specific to that data structure, such as “earliestStartTime”.

As another example, function blocks in control systems might be represented as Objects. The parameters of the function block, such as its setpoints, may be represented as DataVariables. The function block Object might also have Properties that describe its execution time and its type.

DataVariables may have additional DataVariables, but only if they are complex. In this case, their DataVariables shall always be elements of their complex definitions. Following the example introduced by the description of Properties in 4.5.2, the Server could expose “startTime” and “endTime” as separate components of the data structure.

As another example, a complex DataVariable may define an aggregate of temperature values generated by three separate temperature transmitters that are also visible in the AddressSpace. In this case, this complex DataVariable could define HasComponent References from it to the individual temperature values that it is composed of.

4.6 TypeDefinitionNodes

4.6.1 General

OPC UA Servers shall provide type definitions for Objects and Variables. The HasTypeDefinition Reference shall be used to link an instance with its type definition represented by a TypeDefinitionNode. Type definitions are required; however, OPC 10000-5 defines a BaseObjectType, a PropertyType, and a BaseDataVariableType so a Server can use such a base type if no more specialised type information is available. Objects and Variables inherit the Attributes specified by their TypeDefinitionNode (see 6.4 for details).

In some cases, the NodeId used by the HasTypeDefinition Reference will be well-known to Clients and Servers. Organizations may define TypeDefinitionNodes that are well-known in the industry. Well-known NodeIds of TypeDefinitionNodes provide for commonality across OPC UA Servers and allow Clients to interpret the TypeDefinitionNode without having to read it from the Server. Therefore, Servers may use well-known NodeIds without representing the corresponding TypeDefinitionNodes in their AddressSpace. However, the TypeDefinition­Nodes shall be provided for generic Clients. These TypeDefinitionNodes may exist in another Server.

The following example, illustrated in Figure 5, describes the use of the HasTypeDefinition Reference. In this example, a setpoint parameter “SP” is represented as a DataVariable in the AddressSpace. This DataVariable is part of an Object not shown in the figure.

To provide for a common setpoint definition that can be used by other Objects, a specialised VariableType is used. Each setpoint DataVariable that uses this common definition will have a HasTypeDefinition Reference that identifies the common “SetPoint” VariableType.

4.6.2 Complex TypeDefinitionNodes and their InstanceDeclarations

TypeDefinitionNodes can be complex. A complex TypeDefinitionNode also defines References to other Nodes as part of the type definition. The ModellingRules defined in 6.4.4 specify how those Nodes are handled when creating an instance of the type definition.

A TypeDefinitionNode references instances instead of other TypeDefinitionNodes to allow unique names for several instances of the same type, to define default values and to add References for those instances that are specific to this complex TypeDefinitionNode and not to the TypeDefinitionNode of the instance. For example, in Figure 6 the ObjectType “AI_BLK_TYPE”, representing a function block, has a HasComponent Reference to a Variable “SP” of the VariableType “SetPoint”. “AI_BLK_TYPE” could have an additional setpoint Variable of the same type using a different name. It could add a Property to the Variable that was not defined by its TypeDefinitionNode “SetPoint”. And it could define a default value for “SP”, that is, each instance of “AI_BLK_TYPE” would have a Variable “SP” initially set to this value.

This approach is commonly used in object-oriented programming languages in which the variables of a class are defined as instances of other classes. When the class is instantiated, each variable is also instantiated, but with the default values (constructor values) defined for the containing class. That is, typically, the constructor for the component class runs first, followed by the constructor for the containing class. The constructor for the containing class may override component values set by the component class.

To distinguish instances used for the type definitions from instances that represent real data, those instances are called InstanceDeclarations. However, this term is used to simplify this specification, if an instance is an InstanceDeclaration or not is only visible in the AddressSpace by following its References. Some instances may be shared and therefore referenced by TypeDefinitionNodes, InstanceDeclarations and instances. This is similar to class variables in object-oriented programming languages.

4.6.3 Subtyping

This standard allows subtyping of type definitions. The subtyping rules are defined in Clause 6. Subtyping of ObjectTypes and VariableTypes allows:

• Clients that only know the supertype to handle an instance of the subtype as if it were an instance of the supertype;

• instances of the supertype to be replaced by instances of the subtype;

• specialised types that inherit common characteristics of the base type.

In other words, subtypes reflect the structure defined by their supertype but may add additional characteristics. For example, a vendor may wish to extend a general “TemperatureSensor” VariableType by adding a Property providing the next maintenance interval. The vendor would do this by creating a new VariableType which is a TargetNode for a HasSubtype reference from the original VariableType and adding the new Property to it.

4.6.4 Instantiation of complex TypeDefinitionNodes

The instantiation of complex TypeDefinitionNodes depends on the ModellingRules defined in 6.4.4. However, the intention is that instances of a type definition will reflect the structure defined by the TypeDefinitionNode. Figure 7 shows an instance of the TypeDefinitionNode “AI_BLK_TYPE”, where the ModellingRule Mandatory, defined in 6.4.4.4.1, was applied for its containing Variable. Thus, an instance of “AI_BLK_TYPE”, called AI_BLK_1”, has a HasTypeDefinition Reference to “AI_BLK_TYPE”. It also contains a Variable “SP” having the same BrowseName as the Variable “SP” used by the TypeDefinitionNode and thereby reflects the structure defined by the TypeDefinitionNode.

A client knowing the ObjectType “AI_BLK_TYPE” can use this knowledge to directly browse to the containing Nodes for each instance of this type. This allows programming against the TypeDefinitionNode. For example, a graphical element may be programmed in the client that handles all instances of “AI_BLK_TYPE” in the same way by showing the value of “SP”.

There are several constraints related to programming against the TypeDefinitionNode. A TypeDefinitionNode or an InstanceDeclaration shall never reference two Nodes having the same BrowseName using forward hierarchical References. Instances based on InstanceDeclarations shall always keep the same BrowseName as the InstanceDeclaration they are derived from. A special Service defined in OPC 10000-4 called TranslateBrowsePathsToNodeIds may be used to identify the instances based on the InstanceDeclarations. Using the simple Browse Service might not be sufficient since the uniqueness of the BrowseName is only required for TypeDefinitionNodes and InstanceDeclarations, not for other instances. Thus, “AI_BLK_1” may have another Variable with the BrowseName “SP”, although this one would not be derived from an InstanceDeclaration of the TypeDefinitionNode.

Instances derived from an InstanceDeclaration shall be of the same TypeDefinitionNode or a subtype of this TypeDefinitionNode.

A TypeDefinitionNode and its InstanceDeclarations shall always reside in the same Server. However, instances may point with their HasTypeDefinition Reference to a TypeDefinitionNode in a different Server.

4.7 Event Model

4.7.1 General

The Event Model defines a general purpose eventing system that can be used in many diverse vertical markets.

Events represent specific transient occurrences. System configuration changes and system errors are examples of Events. Event Notifications report the occurrence of an Event. Events defined in this document are not directly visible in the OPC UA AddressSpace. Objects and Views can be used to subscribe to Events. The EventNotifier Attribute of those Nodes identifies if the Node allows subscribing to Events. Clients subscribe to such Nodes to receive Notifications of Event occurrences.

Event Subscriptions use the Monitoring and Subscription Services defined in OPC 10000-4 to subscribe to the Event Notifications of a Node.

Any OPC UA Server that supports eventing shall expose at least one Node as EventNotifier. The Server Object defined in OPC 10000-5 is used for this purpose. Events generated by the Server are available via this Server Object. A Server is not expected to produce Events if the connection to the event source is down for some reason (i.e. the system is offline).

Events may also be exposed through other Nodes anywhere in the AddressSpace. These Nodes (identified via the EventNotifier Attribute) provide some subset of the Events generated by the Server. The position in the AddressSpace dictates what this subset will be. For example, a process area Object representing a functional area of the process would provide Events originating from that area of the process only. It should be noted that this is only an example and it is fully up to the Server to determine what Events should be provided by which Node.

4.7.2 EventTypes

Each Event is of a specific EventType. A Server may support many types. This part defines the BaseEventType that all other EventTypes derive from. It is expected that other companion specifications will define additional EventTypes deriving from the base types defined in this part.

The EventTypes supported by a Server are exposed in the AddressSpace of a Server. EventTypes are represented as ObjectTypes in the AddressSpace and do not have a special NodeClass associated to them. OPC 10000-5 defines how a Server exposes the EventTypes in detail.

EventTypes defined in this document are specified as abstract and therefore never instantiated in the AddressSpace. Event occurrences of those EventTypes are only exposed via a Subscription. Components of an EventType defined with ModellingRules other than Mandatory or Optional cannot be returned in an Event Subscription. EventTypes exist in the AddressSpace to allow Clients to discover the EventType. This information is used by a client when establishing and working with Event Subscriptions. EventTypes defined by other parts of this series of standards or companion specifications as well as Server specific EventTypes may be defined as not abstract and therefore instances of those EventTypes may be visible in the AddressSpace although Events of those EventTypes are also accessible via the Event Notification mechanisms.

Standard EventTypes are described in Clause 8.61. Their representation in the AddressSpace is specified in OPC 10000-5.

4.7.3 Event Categorization

Events can be categorised by creating new EventTypes which are subtypes of existing EventTypes but do not extend an existing type. They are used only to identify an event as being of the new EventType. For example, the EventType DeviceFailureEventType could be subtyped into TransmitterFailureEventType and ComputerFailureEventType. These new subtypes would not add new Properties or change the semantic inherited from the DeviceFailureEventType other than purely for categorization of the Events.

Event sources can also be organised into groups by using the Event ReferenceTypes described in 7.16 and 7.18. For example, a Server may define Objects in the AddressSpace representing Events related to physical devices, or Event areas of a plant or functionality contained in the Server. Event References would be used to indicate which Event sources represent physical devices and which ones represent some Server-based functionality. In addition, References can be used to group the physical devices or Server-based functionality into hierarchical Event areas. In some cases, an Event source may be categorised as being both a device and a Server function. In this case, two relationships would be established. Refer to the description of the Event ReferenceTypes for additional examples.

Clients can select a category or categories of Events by defining content filters that include terms specifying the EventType of the Event or a grouping of Event sources. The two mechanisms allow for a single Event to be categorised in multiple manners. A client could obtain all Events related to a physical device or all failures of a particular device.

4.8 Methods

Methods are “lightweight” functions, whose scope is bounded by an owning (see Note) Object, similar to the methods of a class in object-oriented programming or an owning ObjectType, similar to static methods of a class. Methods are invoked by a client, proceed to completion on the Server and return the result to the client. The lifetime of the Method’s invocation instance begins when the client calls the Method and ends when the result is returned.

While Methods may affect the state of the owning Object, they have no explicit state of their own. In this sense, they are stateless. Methods can have a varying number of input arguments and return resultant arguments. Each Method is described by a Node of the Method NodeClass. This Node contains the metadata that identifies the Method’s arguments and describes its behaviour.

Methods are invoked by using the Call Service defined in OPC 10000-4.

Clients discover the Methods supported by a Server by browsing for the owning Objects References that identify their supported Methods.

4.9 Roles

4.9.1 Overview

A Role is a function assumed by a Client when it accesses a Server. Roles are used to separate authentication (determining who a Client is) from authorization (determining what the Client is allowed to do). By separating these tasks Servers can allow centralized services to manage user identities and credentials while the Server only manages the Permissions on its Nodes assigned to Roles.

The set of Roles supported by a Server are published as components of the RoleSet Object defined in OPC 10000-18. Servers should define a base set of Roles and allow configuration Clients to add system specific Roles. Adding, deleting, and modifying Roles is restricted to callers with appropriate permissions.

When a Session is created, or a Session-less Service is called, the Server must determine what Roles are granted to that Session or Session-less Service invocation. This specification defines standard mapping rules which Servers may support. Servers may also use vendor specific mapping rules in addition to or instead of the standard rules.

The Anonymous Role is the default Role which is always assigned to all Sessions.

The AuthenticatedUser Role is always assigned when a Session has been authenticated with a UserIdentityToken other than the AnonymousIdentityToken (see OPC 10000-4).

The TrustedApplication Role is always assigned when a Session has been authenticated with a trusted ApplicationInstance Certificate (see OPC 10000-4) and uses at least a signed communication channel.

The standard mapping rules allow Roles to be granted based on:

User identity;

Application identity;

Endpoint;

User identity mappings can be based on user names, user certificates or user groups.

Application identity mappings are based on the ApplicationUri specified in the Client Certificate. Application identity can only be enforced if the Client proves possession of a trusted Certificate by using it to create a Secure Channel or by providing a signature in ActivateSession (see OPC 10000-4).

Endpoint identity mappings are based on the URL used to connect to the Server. Endpoint identity can be used to restrict access to Clients running on particular networks. Endpoint identity mappings should not be used as the only criteria unless access to the endpoint is restricted by the network infrastructure. For example, an endpoint on a loopback address is only accessible from the same machine.

OPC 10000-5 defines the Objects, Methods and DataTypes used to represent and manage these mapping rules in the Address Space.

4.9.2 Well Known Roles

The NodeIds for the well-known Roles are defined in OPC 10000-6. All Servers should support the well-known Roles which are defined in Table 2.

4.9.3 Evaluating Permissions with Roles

When a Client attempts to access a Node, the Server goes through the list of Roles granted to the Session and logically ORs the Permissions for the Role on the Node. If there are no Node specific Permissions then the default Permissions for the Role in the DefaultRolePermissions Property of the NamespaceMetadata for the namespace the Node belongs to are used (see OPC 10000-5). The resulting mask is the effective Permissions. If the bits corresponding to current operation are set, then the operation can proceed. If they are not set the Server returns Bad_UserAccessDenied.

Roles appear under the Roles Object in the Server Address Space. Each Role has mapping rules defined which appear as Properties of the Role Object (see OPC 10000-5). The examples shown in Table 3 illustrate how the standard mapping rules can be used to determine which Roles a Session has access to and, consequently, the Permissions that are granted to the Session.

The examples also make use of the Nodes defined in Table 4. The table specifies the value of the RolePermissions Attribute for each Node.

When a Client creates a Session the Roles assigned to the Session depend on the rules defined for each Role. Table 5 lists the assigned Roles for different Sessions created with different Users, Client applications and Endpoints.

When a Client application accesses a Node the RolePermissions for the Node are compared to the Roles assigned to the Session. Any Permissions available to at least one Role is granted to the Client. Table 6 provides a number of scenarios and examples and the resulting decision on access.

4.10 Interfaces and AddIns for Objects

4.10.1 Overview

OPC UA defines a type model supporting one object-oriented type hierarchy for ObjectTypes. Although the specification does not restrict those hierarchies to be single inheritance (i.e. a type can only have one super-type) it only specifies the semantic (inheritance rules) for single inheritance.

In general, good object-oriented design is accomplished by using composition to aggregate an object which provides several functions instead of over-using inheritance [GH95], [FF04] .

Interfaces and AddIns complement the type model and can be used when subtyping is not suitable for the required extension. They:

allow enhancing multiple types at arbitrary positions in the type hierarchy.

also allow enhancing just instances.

4.10.2 Interface Model

Interfaces are ObjectTypes that represent a generic feature (functionality), assumed to be usable by different ObjectTypes or Objects. The Interface model specifies the rules and mechanisms to achieve this.

The “InterfaceTypes” Object (see OPC 10000-5) has been defined so that all Interfaces of the Server are either directly or indirectly accessible browsing HierarchicalReferences starting from this Node.

Rules for the definition of Interfaces:

Interface ObjectTypes shall be abstract subtypes of the BaseInterfaceType ObjectType.

InstanceDeclarations on an Interface shall only have ModellingRules Optional or Mandatory.

Interfaces can be subtyped as specified in clause 6.3.

Interfaces shall not be the source of HasInterface References.

Recommended convention: The first letter of an Interface should be ‘I’. See examples below.

Rules for applying Interfaces:

When an ObjectType references an Interface with a HasInterface Reference or a subtype, the following rules apply:

Each mandatory InstanceDeclaration of the fully-inherited InstanceDeclaration­Hierarchy of the Interface shall have for each BrowsePath a similar Node (see 6.2.4) with the ModellingRule Mandatory using the same BrowsePath in the fully-inherited InstanceDeclarationHierarchy of the ObjectType. The rules for instantiating InstanceDeclarations defined in 6.2.6 shall be applied.

Each optional InstanceDeclaration of the fully-inherited InstanceDeclaration­Hierarchy of the Interface should have for each BrowsePath a similar Node (see 6.2.4) with the ModellingRule Mandatory or Optional using the same BrowsePath in the fully-inherited InstanceDeclarationHierarchy of the ObjectType. The rules for instantiating InstanceDeclarations defined in 6.2.6 shall be applied. If no similar Node with the same BrowsePath exists, the ObjectType and its sub-types shall not use the same BrowsePath for a different Node (e.g. with a different NodeClass).

If the rules cannot be fulfilled (e.g. name collisions) the ObjectType cannot apply the Interface, i.e. it shall not reference the Interface with a HasInterface Reference of a subtype.

The rules apply for each referenced Interface. As a consequence, an ObjectType cannot reference two Interfaces using the same BrowsePath for Nodes that are not similar Nodes or have TypeDefinitionNodes that are not compatible (compatible means they have either the same TypeDefinitionNode or one TypeDefinitionNode is the subtype of the other TypeDefinitionNode).

Subtypes should not have a HasInterface Reference to an Interface if it was already applied to a super-type.

When an Object references an Interface with a HasInterface Reference or a subtype, the following rules apply:

The Interface shall not be applied on the Object when the Interface cannot be applied on the TypeDefinitionNode of the Object.

The same rules on the Object apply as if the Interface would have been applied on the TypeDefinitionNode of the Object (e.g. all Mandatory InstanceDeclarations need to be applied).

The Nodes defined based on the Interface shall be handled as if they were defined by the TypeDefinitionNode. For example, the TranslateBrowsePathsToNodeIds Service shall return them first.

If several Interfaces should be applied to the Object, they should be treated as if they were all applied to the ObjectType of the Object at the same time. If this is not possible, the Interfaces cannot be applied to the Object together.

Instances should not have a HasInterface Reference to an Interface if it was already applied to the TypeDefinitionNode.

A BaseInterfaceType or any subtype of BaseInterfaceType shall not be the TargetNode of a HasTypeDefinition Reference.

Figure 8 illustrates example Interfaces:

ISerializeServiceType, an Interface to convert the Object and its components into a stream of bytes.

ITransactionServiceType, an Interface to perform a sequence of changes to the Object as a single operation.

ILocationType, an Interface to specify the installation location of the Object.

The following examples illustrate the application of these Interfaces. In Figure 9 the example Interface ILocationType is applied to the XYZ-DeviceType ObjectType. It also illustrates the overriding of Property “Address” by changing the ModellingRule from Optional to Mandatory. Figure 10 in addition shows how to use the ISerializeService Interface on the instance only. Figure 11 shows an Interface hierarchy where InstanceDeclarations of the referenced Interface and its parent type(s) are applied (the fully-inherited InstanceDeclarationHierarchy).

Clients can detect the implementation of Interfaces by filtering for the HasInterface Reference into the Browse Service request.

On instances, the Browse Service will return elements derived from an Interface together with elements of the Node’s base type. Clients can also use the TranslateBrowsePathsToNodeId Service with BrowseNames of Interface members to get the NodeId of these members directly.

In the example in Figure 10 “Address” with the starting node MD002 can be used to request the NodeId of this Property.

On Object instances, some Nodes of an Interface may not be available if defined with ModellingRule Optional.

4.10.3 AddIn model

AddIns associate a feature or feature-set, represented by an ObjectType to the Node (an Object or ObjectType) they are applied to. The Interface model is different than the AddIn model in that it is based on composition. An AddIn is applied to a Node by adding a Reference to the AddIn instance.

There are no restrictions for AddIn ObjectTypes and there is no special supertype for AddIns. To identify instances as an AddIn, the HasAddIn Reference or a subtype shall be used.

The AddIn ObjectType shall include the definition of a default BrowseName using the DefaultInstanceBrowseName Property. Instances of such an AddIn should use this default BrowseName. If an AddIn is instantiated multiple times in the same parent, only one instance can have the default BrowseName.

The definition of an AddIn and its use with a default BrowseName is illustrated in Figure 12.

As already described, an AddIn can be applied on types and instances. The use on an instance is shown in Figure 13.

Clients can detect the implementation of AddIns by passing the HasAddIn Reference as filter to the Browse Service request. If an AddIn has a default BrowseName, Clients can use the TranslateBrowsePathsToNodeId Service with the default BrowseName to get the NodeId of an AddIn.

In the example in Figure 12 the relative path “MyFeature/MyPropertyM” with the starting node MD002 can be used to request the NodeId of this Property and the relative path “MyFeature/MyMethodO” can be used for the respective Method.

5 Standard NodeClasses

5.1 Overview

Clause 5 defines the NodeClasses used to define Nodes in the OPC UA AddressSpace. NodeClasses are derived from a common Base NodeClass. This NodeClass is defined first, followed by those used to organise the AddressSpace and then by the NodeClasses used to represent Objects.

The NodeClasses defined to represent Objects fall into three categories: those used to define instances, those used to define types for those instances and those used to define data types. Subclause 6.3 describes the rules for subtyping and 6.4 the rules for instantiation of the type definitions.

5.2 Base NodeClass

5.2.1 General

The OPC UA Address Space Model defines a Base NodeClass from which all other NodeClasses are derived. The derived NodeClasses represent the various components of the OPC UA Object Model (see 4.2). The Attributes of the Base NodeClass are specified in Table 7. There are no References specified for the Base NodeClass.

5.2.2 NodeId

Nodes are unambiguously identified using a constructed identifier called the NodeId. Some Servers may accept alternative NodeIds in addition to the canonical NodeId represented in this Attribute. A Server shall persist the identifierType and identifier NodeId elements of a Node as well as the Namespace Uri which the namespaceIndex NodeId element references. A Server may change the namespaceIndex NodeId element of a Node with future Sessions and therefore a Client shall not assume the namespaceIndex will not change. The structure of the NodeId is defined in 8.2.

5.2.3 NodeClass

The NodeClass Attribute identifies the NodeClass of a Node. Its data type is defined in 8.29.

5.2.4 BrowseName

Nodes have a BrowseName Attribute that is used as a non-localised human-readable name when browsing the AddressSpace to create paths out of BrowseNames. The TranslateBrowsePathsToNodeIds Service defined in OPC 10000-4 can be used to follow a path constructed of BrowseNames.

A BrowseName should never be used to display the name of a Node. The DisplayName should be used instead for this purpose.

Unlike NodeIds, the BrowseName cannot be used to unambiguously identify a Node. Different Nodes may have the same BrowseName.

Subclause 8.3 defines the structure of the BrowseName. It contains a namespace and a string. The namespace is provided to make the BrowseName unique in some cases in the context of a Node (e.g. Properties of a Node) although not unique in the context of the Server. If different organizations define BrowseNames for Properties, the namespace of the BrowseName provided by the organization makes the BrowseName unique, although different organizations may use the same string having a slightly different meaning.

Servers may often choose to use the same namespace for the NodeId and the BrowseName. However, if they want to provide a standard Property, its BrowseName shall have the namespace of the standards body although the namespace of the NodeId reflects something else, for example the local Server.

Standards bodies defining standard type definitions shall use their namespace(s) for the NodeId of the TypeDefinitionNode as well as for the BrowseName of the TypeDefinitionNode. BrowseNames of TypeDefinitionNodes, ReferenceTypes, and DataTypes shall be unique. Any well-known instances used as entry points shall also be unique. For example, the Root Node defined in OPC 10000-5.

The string-part of the BrowseName is case sensitive. That is, Clients shall consider them case sensitive. Servers are allowed to handle BrowseNames passed in Service requests as case insensitive. Examples are the TranslateBrowsePathsToNodeIds Service or Event filter. If a Server accepts a case insensitive BrowseName it needs to ensure that the uniqueness of the BrowseName does not depend on case.

5.2.5 DisplayName

The DisplayName Attribute contains the localised name of the Node. Clients should use this Attribute if they want to display the name of the Node to the user. They should not use the BrowseName for this purpose. The Server may maintain one or more localised representations for each DisplayName. Clients negotiate the locale to be returned when they open a session with the Server. Refer to OPC 10000-4 for a description of session establishment and locales. Subclause 8.5 defines the structure of the DisplayName. The string part of the DisplayName is restricted to 512 characters.

5.2.6 Description

The optional Description Attribute shall explain the meaning of the Node in a localised text using the same mechanisms for localisation as described for the DisplayName in 5.2.5.

5.2.7 WriteMask

The optional WriteMask Attribute exposes the possibilities of a client to write the Attributes of the Node. The WriteMask Attribute does not take any user access rights into account, that is, although an Attribute is writeable this may be restricted to a certain user/user group.

If the OPC UA Server does not have the ability to get the WriteMask information for a specific Attribute from the underlying system, it should state that it is writeable. If a write operation is called on the Attribute, the Server should transfer this request and return the corresponding StatusCode if such a request is rejected. StatusCodes are defined in OPC 10000-4.

The AttributeWriteMask DataType is defined in 8.60.

5.2.8 UserWriteMask

The optional UserWriteMask Attribute exposes the possibilities of a client to write the Attributes of the Node taking user access rights into account. It uses the AttributeWriteMask DataType which is defined in 8.60.

The UserWriteMask Attribute can only further restrict the WriteMask Attribute, when it is set to not writeable in the general case that applies for every user.

Clients cannot assume an Attribute can be written based on the UserWriteMask Attribute.It is possible that the Server may return an access denied error due to some server specific change which was not reflected in the state of this Attribute at the time the Client accessed it.

5.2.9 RolePermissions

The optional RolePermissions Attribute specifies the Permissions that apply to a Node for all Roles which have access to the Node. The value of the Attribute is an array of RolePermissionType Structures (see Table 8).

Servers may allow administrators to write to the RolePermissions Attribute.

If not specified, the value of DefaultRolePermissions Property from the NamespaceMetadata Object associated with the Node shall be used instead. If the NamespaceMetadata Object does not define the Property or does not exist, then the Server should not publish any information about how it manages Permissions.

If a Server supports Permissions for a particular Namespace it shall add the DefaultRolePermissions Property to the NamespaceMetadata Object for that Namespace (see Figure 14). If a particular Node in the Namespace needs to override the default values, the Server adds the RolePermissions Attribute to the Node. The DefaultRolePermissions Property and RolePermissions Attribute shall only be readable by administrators. If a Server allows the Permissions to be changed these values shall be writeable. If the Server allows the Permissions to be overridden for a particular Node but does not currently have any Node Permissions configured, then the value of the Attribute shall be an empty array. If the administrator wishes to remove overridden Permissions, an empty array shall be written to this Attribute. Servers shall prevent Permissions from being changed in such a way as to render the Server inoperable.

If a Server allows writes to the RolePermissions it shall preserve all bits written by the Client even if they are not valid for the Node. When a Client reads the RolePermissions or UserRolePermissions it shall ignore bits that are not valid for the Node.

If a Server publishes information about the Roles for a Namespace assigned to the current Session, it shall add the DefaultUserRolePermissions Property to the NamespaceMetadata Object for that Namespace. The value of this Property shall be a readonly list of Permissions for each Role assigned to the current Session. If a particular Node in the Namespace overrides the default RolePermissions the Server shall also override the DefaultUserRolePermissions by adding the UserRolePermissions Attribute to the Node. If the Server allows the Permissions to be overridden for a particular Node but does not currently have any Node Permissions configured, then the Server shall return the value of the DefaultUserRolePermissions Property for the Node Namespace.

If a Server implements a vendor specific Role Permission model for a Namespace, it shall not add the DefaultRolePermissions or DefaultUserRolePermissions Properties to the Namespace
Metadata Object.

5.2.10 UserRolePermissions

The optional UserRolePermissions Attribute specifies the Permissions that apply to a Node for all Roles granted to current Session. The value of the Attribute is an array of RolePermissionType Structures (see Table 8).

Clients may determine their effective Permissions by performing a logical OR of Permissions for each Role in the array.

The value of this Attribute is derived from the rules used by the Server to map Sessions to Roles. This mapping may be vendor specific or it may use the standard Role model defined in 4.9.

This Attribute shall not be writeable. When a Client reads the UserRolePermissions it shall ignore bits that are not valid for the Node.

If not specified, the value of DefaultUserRolePermissions Property from the Namespace Metadata Object associated with the Node is used instead. If the NamespaceMetadata Object does not define the Property or does not exist, then the Server does not publish any information about Roles mapped to the current Session.

5.2.11 AccessRestrictions

The optional AccessRestrictions Attribute specifies the AccessRestrictions that apply to a Node. Its data type is defined in 8.56. If a Server supports AccessRestrictions for a particular Namespace it adds the DefaultAccessRestrictions Property to the NamespaceMetadata Object for that Namespace (see Figure 14). If a particular Node in the Namespace needs to override the default value the Server adds the AccessRestrictions Attribute to the Node.

If a Server implements a vendor specific access restriction model for a Namespace, it does not add the DefaultAccessRestrictions Property to the NamespaceMetadata Object.

5.3 ReferenceType NodeClass

5.3.1 General

References are defined as instances of ReferenceType Nodes. ReferenceType Nodes are visible in the AddressSpace and are defined using the ReferenceType NodeClass as specified in Table 9. In contrast, a Reference is an inherent part of a Node and no NodeClass is used to represent References.

This standard defines a set of ReferenceTypes provided as an inherent part of the OPC UA Address Space Model. These ReferenceTypes are defined in Clause 7 and their representation in the AddressSpace is defined in OPC 10000-5. Servers may also define ReferenceTypes. In addition, OPC 10000-4 defines NodeManagement Services that allow Clients to add ReferenceTypes to the AddressSpace.

5.3.2 Attributes

The ReferenceType NodeClass inherits the base Attributes from the Base NodeClass defined in 5.2. The inherited BrowseName Attribute is used to specify the meaning of the ReferenceType as seen from the SourceNode. For example, the ReferenceType with the BrowseName “Contains” is used in References that specify that the SourceNode contains the TargetNode. The inherited DisplayName Attribute contains a translation of the BrowseName.

The BrowseName of a ReferenceType shall be unique in a Server. It is not allowed that two different ReferenceTypes have the same BrowseName.

The IsAbstract Attribute indicates if the ReferenceType is abstract. Abstract ReferenceTypes cannot be instantiated and are used only for organizational reasons, for example to specify some general semantics or constraints that its subtypes inherit.

The Symmetric Attribute is used to indicate whether or not the meaning of the ReferenceType is the same for both the SourceNode and TargetNode.

If a ReferenceType is symmetric, the InverseName Attribute shall be omitted. Examples of symmetric ReferenceTypes are “Connects To” and “Communicates With”. Both imply the same semantic coming from the SourceNode or the TargetNode. Therefore both directions are considered to be forward References.

If the ReferenceType is non-symmetric the InverseName Attribute shall be set. The InverseName Attribute specifies the meaning of the ReferenceType as seen from the TargetNode. Examples of non-symmetric ReferenceTypes include “Contains” and “Contained In”, and “Receives From” and “Sends To”.

Any subtype, either directly or indirectly of a concrete ReferenceType shall not change the Symmetric Attribute definition of its parent type.

References that use the InverseName, such as “Contained In” References, are referred to as inverse References.

Figure 15 provides examples of symmetric and non-symmetric References and the use of the BrowseName and the InverseName.

It might not always be possible for Servers to instantiate both forward and inverse References for non-symmetric ReferenceTypes as shown in Figure 15. When they do, the References are referred to as bidirectional. Although not required, it is recommended that all hierarchical References be instantiated as bidirectional to ensure browse connectivity. A bidirectional Reference is modelled as two separate References.

As an example of a unidirectional Reference, it is often the case that a signal sink knows its signal source, but this signal source does not know its signal sink. The signal sink would have a “Sourced By” Reference to the signal source, without the signal source having the corresponding “Sourced To” inverse References to its signal sinks.

The DisplayName and the InverseName are the only standardised places to indicate the semantic of a ReferenceType. There may be more complex semantics associated with a ReferenceType than can be expressed in those Attributes (e.g. the semantic of HasSubtype). This standard does not specify how this semantic should be exposed. However, the Description Attribute can be used for this purpose. This standard provides a semantic for the ReferenceTypes specified in Clause 7.

A ReferenceType can have constraints restricting its use. For example, it can specify that starting from Node A and only following References of this ReferenceType or one of its subtypes, it shall never be able to return to A, that is, a “No Loop” constraint.

This standard does not specify how those constraints could or should be made available in the AddressSpace. Nevertheless, for the standard ReferenceTypes, some constraints are specified in Clause 7. This standard does not restrict the kind of constraints valid for a ReferenceType. It can, for example, also affect an ObjectType. The restriction that a ReferenceType can only be used by relating Nodes of some NodeClasses with a defined cardinality is a special constraint of a ReferenceType.

5.3.3 References

5.3.3.1 General

HasSubtype References and HasProperty References are the only ReferenceTypes that may be used with ReferenceType Nodes as SourceNode. ReferenceType Nodes shall not be the SourceNode of other types of References.

5.3.3.2 HasProperty References

HasProperty References are used to identify the Properties of a ReferenceType and shall only refer to Nodes of the Variable NodeClass.

The Property NodeVersion is used to indicate the version of the ReferenceType.

There are no additional Properties defined for ReferenceTypes in this standard. Additional parts this series of standards may define additional Properties for ReferenceTypes.

5.3.3.3 HasSubtype References

HasSubtype References are used to define subtypes of ReferenceTypes. It is not required to provide the HasSubtype Reference for the supertype, but it is required that the subtype provides the inverse Reference to its supertype. The following rules for subtyping apply.

1. The semantic of a ReferenceType (e.g. “spans a hierarchy”) is inherited to its subtypes and can be refined there (e.g. “spans a special hierarchy”). The DisplayName, and also the InverseName for non-symmetric ReferenceTypes, reflect the specialization.

2. If a ReferenceType specifies some constraints (e.g. “allow no loops”) this is inherited and can only be refined (e.g. inheriting “no loops” could be refined as “shall be a tree – only one parent”) but not lowered (e.g. “allow loops”).

3. The constraints concerning which NodeClasses can be referenced are also inherited and can only be further restricted. That is, if a ReferenceType “A” is not allowed to relate an Object with an ObjectType, this is also true for its subtypes.

4. A ReferenceType shall have exactly one supertype, except for the References ReferenceType defined in 7.2 as the root type of the ReferenceType hierarchy. The ReferenceType hierarchy does not support multiple inheritances.

5.4 View NodeClass

Underlying systems are often large and Clients often have an interest in only a specific subset of the data. They do not need, or want, to be burdened with viewing Nodes in the AddressSpace for which they have no interest.

To address this problem, this standard defines the concept of a View. Each View defines a subset of the Nodes in the AddressSpace. The entire AddressSpace is the default View. Each Node in a View may contain only a subset of its References, as defined by the creator of the View. The View Node acts as the root for the Nodes in the View. Views are defined using the View NodeClass, which is specified in Table 10.

All Nodes contained in a View shall be accessible starting from the View Node when browsing in the context of the View. It is not expected that all containing Nodes can be browsed directly from the View Node but rather browsed from other Nodes contained in the View.

A View Node may not only be used as additional entry point into the AddressSpace but as a construct to organize the AddressSpace and thus as the only entry point into a subset of the AddressSpace. Therefore, Clients shall not ignore View Nodes when exposing the AddressSpace. Simple Clients that do not deal with Views for filtering purposes can, for example, handle a View Node like an Object of type FolderType (see 5.5.3).

The View NodeClass inherits the base Attributes from the Base NodeClass defined in 5.2. It also defines two additional Attributes.

The mandatory ContainsNoLoops Attribute is set to FALSE if the Server is not able to identify if the View contains loops or not.

The mandatory EventNotifier Attribute identifies if the View can be used to subscribe to Events that either occur in the content of the View or as ModelChangeEvents (see 9.32) of the content of the View or to read / write the history of the Events. A View that supports Events shall provide all Events that occur in any Object used as EventNotifier that is part of the content of the View. In addition, it shall provide all ModelChangeEvents that occur in the context of the View.

To avoid recursion, i.e. getting all Events of the Server, the Server Object defined in OPC 10000-5 shall never be part of any View since it provides all Events of the Server.

Views are defined by the Server. The browsing and querying Services defined in OPC 10000-4 expect the NodeId of a View Node to provide these Services in the context of the View.

HasProperty References are used to identify the Properties of a View. The Property NodeVersion is used to indicate the version of the View Node. The ViewVersion Property indicates the version of the content of the View. In contrast to the NodeVersion, the ViewVersion Property is updated even if Nodes not directly referenced by the View Node are added to or deleted from the View. This Property is optional because it might not be possible for Servers to detect changes in the View contents. Servers may also generate a ModelChangeEvent, described in 9.32, if Nodes are added to or deleted from the View. There are no additional Properties defined for Views in this document. Additional parts of this series of standards may define additional Properties for Views.

Views can be the SourceNode of any hierarchical Reference. They shall not be the SourceNode of any NonHierarchical Reference.

5.5 Objects

5.5.1 Object NodeClass

Objects are used to represent systems, system components, real-world objects and software objects. Objects are defined using the Object NodeClass, specified in Table 11.

The Object NodeClass inherits the base Attributes from the Base NodeClass defined in 5.2.

The mandatory EventNotifier Attribute identifies whether the Object can be used to subscribe to Events or to read and write the history of the Events.

The Object NodeClass uses the HasComponent Reference to define the DataVariables, Objects and Methods of an Object.

It uses the HasProperty Reference to define the Properties of an Object. The Property NodeVersion is used to indicate the version of the Object. The Property Icon provides an icon of the Object. There are no additional Properties defined for Objects in this document. Additional parts of this series of standards may define additional Properties for Objects.

To specify its ModellingRule, an Object can use at most one HasModellingRule Reference pointing to a ModellingRule Object. ModellingRules are defined in 6.4.4.

HasNotifier and HasEventSource References are used to provide information about eventing and can only be applied to Objects used as event notifiers. Details are defined in 7.16 and 7.18.

The HasTypeDefinition Reference points to the ObjectType used as type definition of the Object.

Objects may use any additional References to define relationships to other Nodes. No restrictions are placed on the types of References used or on the NodeClasses of the Nodes that may be referenced. However, restrictions may be defined by the ReferenceType excluding its use for Objects. Standard ReferenceTypes are described in Clause 7.

If the Object is used as an InstanceDeclaration (see 4.6) then all Nodes referenced with forward hierarchical References direction shall have unique BrowseNames in the context of this Object.

If the Object is created based on an InstanceDeclaration then it shall have the same BrowseName as its InstanceDeclaration.

5.5.2 ObjectType NodeClass

ObjectTypes provide definitions for Objects. ObjectTypes are defined using the ObjectType NodeClass, which is specified in Table 12.

The ObjectType NodeClass inherits the base Attributes from the Base NodeClass defined in 5.2. The additional IsAbstract Attribute indicates if the ObjectType is abstract or not.

The ObjectType NodeClass uses the HasComponent References to define the DataVariables, Objects, and Methods for it.

The HasProperty Reference is used to identify the Properties. The Property NodeVersion is used to indicate the version of the ObjectType. The Property Icon provides an icon of the ObjectType. There are no additional Properties defined for ObjectTypes in this document. Additional parts of this series of standards may define additional Properties for ObjectTypes.

HasSubtype References are used to subtype ObjectTypes. ObjectType subtypes inherit the general semantics from the parent type. The general rules for subtyping apply as defined in Clause 6. It is not required to provide the HasSubtype Reference for the supertype, but it is required that the subtype provides the inverse Reference to its supertype.

GeneratesEvent References identify the type of Events that instances of the ObjectType may generate. These Objects may be the source of an Event of the specified type or one of its subtypes. Servers should make GeneratesEvent References bidirectional References. However, it is allowed to be unidirectional when the Server is not able to expose the inverse direction pointing from the EventType to each ObjectType supporting the EventType. Note that the EventNotifier Attribute of an Object and the GeneratesEvent References of its ObjectType are completely unrelated. Objects that can generate Events might not be used as Objects to which Clients subscribe to get the corresponding Event notifications.

GeneratesEvent References are optional, i.e. Objects may generate Events of an EventType that is not exposed by its ObjectType.

ObjectTypes may use any additional References to define relationships to other Nodes. No restrictions are placed on the types of References used or on the NodeClasses of the Nodes that may be referenced. However, restrictions may be defined by the ReferenceType excluding its use for ObjectTypes. Standard ReferenceTypes are described in Clause 7.

All Nodes referenced with forward hierarchical References shall have unique BrowseNames in the context of an ObjectType (see 4.6).

5.5.3 Standard ObjectType FolderType

The ObjectType FolderType is formally defined in OPC 10000-5. Its purpose is to provide Objects that have no other semantic than organizing of the AddressSpace. A special ReferenceType is introduced for those Folder Objects, the Organizes ReferenceType. The SourceNode of such a Reference should always be a View or an Object of the ObjectType FolderType; the TargetNode can be of any NodeClass. Organizes References can be used in any combination with HasChild References (HasComponent, HasProperty, etc.; see 7.5) and do not prevent loops. Thus, they can be used to span multiple hierarchies.

5.5.4 Client-side creation of Objects of an ObjectType

Objects are always based on an ObjectType, i.e. they have a HasTypeDefinition Reference pointing to its ObjectType.

Clients can create Objects using the AddNodes Service defined in OPC 10000-4. The Service requires specifying the TypeDefinitionNode of the Object. An Object created by the AddNodes Service contains all components defined by its ObjectType dependent on the ModellingRules specified for the components. However, the Server may add additional components and References to the Object and its components that are not defined by the ObjectType. This behaviour is Server dependent. The ObjectType only specifies the minimum set of components that shall exist for each Object of an ObjectType.

In addition to the AddNodes Service ObjectTypes may have a special Method with the BrowseName “Create”. This Method is used to create an Object of this ObjectType. This Method may be useful for the creation of Objects where the semantic of the creation should differ from the default behaviour expected in the context of the AddNodes Service. For example, the values should directly differ from the default values or additional Objects should be added, etc. The input and output arguments of this Method depend on the ObjectType; the only commonality is the BrowseName identifying that this Method will create an Object based on the ObjectType. Servers should not provide a Method on an ObjectType with the BrowseName “Create” for any other purpose than creating Objects of the ObjectType.

5.6 Variables

5.6.1 General

Two types of Variables are defined, Properties and DataVariables. Although they differ in the way they are used as described in 4.5 and have different constraints described in the remainder of 5.6 they use the same NodeClass described in 5.6.2. The constraints of Properties based on this NodeClass are defined in 5.6.3, the constraints of DataVariables in 5.6.4.

5.6.2 Variable NodeClass

Variables are used to represent values which may be simple or complex. Variables are defined by VariableTypes, as specified in 5.6.5.

Variables are always defined as Properties or DataVariables of other Nodes in the AddressSpace. They are never defined by themselves. A Variable is always part of at least one other Node, but may be related to any number of other Nodes. Variables are defined using the Variable NodeClass, specified in Table 13.

The Variable NodeClass inherits the base Attributes from the Base NodeClass defined in 5.2.

The Variable NodeClass also defines a set of Attributes that describe the Variable’s Runtime value. The Value Attribute represents the Variable value. The DataType, ValueRank and ArrayDimensions Attributes provide the capability to describe simple and complex values.

To maximize interoperability with Clients, when a Server instantiates Variable Nodes the DataType, ValueRank and ArrayDimension Attributes should be set to the most restrictive values for the instance. For example, if a Variable Node will produce a 1-dimension array of floats with a maximum array length of 10, set DataType = Float, ValueRank = 1, ArrayDimensions = [10]. If the array length has no fixed upper limit, then set ArrayDimensions = [0].

The AccessLevel Attribute indicates the accessibility of the Value of a Variable not taking user access rights into account. If the OPC UA Server does not have the ability to get the AccessLevel information from the underlying system then it should state that it is readable and writeable. If a read or write operation is called on the Variable then the Server should transfer this request and return the corresponding StatusCode even if such a request is rejected. StatusCodes are defined in OPC 10000-4.

The SemanticChange flag of the AccessLevel Attribute is used for Properties that may change and define semantic aspects of the parent Node. For example, the EngineeringUnits Property describes the semantic of a DataVariable, whereas the Icon Property does not. In this example, if the EngineeringUnits Property may change while the Server is running, the SemanticChange flag shall be set for it.

Servers that support Event Subscriptions shall generate a SemanticChangeEvent whenever a Property with SemanticChange flag set changes.

If a Variable having a Property with SemanticChange flag set is used in a Subscription and the Property value changes, then the SemanticsChanged bit of the StatusCode shall be set as defined in OPC 10000-4. Clients subscribing to a Variable should look at the StatusCode to identify if the semantic has changed and retrieve the relevant Properties before processing the value returned from the Subscription.

The UserAccessLevel Attribute indicates the accessibility of the Value of a Variable taking user access rights into account. If the OPC UA Server does not have the ability to get any user access rights related information from the underlying system then it should use the same bit mask as used in the AccessLevel Attribute. The UserAccessLevel Attribute can restrict the accessibility indicated by the AccessLevel Attribute, but not exceed it. Clients should not assume access rights based on the UserAccessLevel Attribute. For example it is possible that the Server returns an error due to some server specific change which was not reflected in the state of this Attribute at the time the Client accessed the Variable.

The MinimumSamplingInterval Attribute specifies how fast the Server can reasonably sample the value for changes. The accuracy of this value (the ability of the Server to attain “best case” performance) can be greatly affected by system load and other factors.

The Historizing Attribute indicates whether the Server is actively collecting data for the history of the Variable. See OPC 10000-11 for details on historizing Variables.

Clients may read or write Variable values, or monitor them for value changes, as specified in OPC 10000-4. OPC 10000-8 defines additional rules when using the Services for automation data.

To specify its ModellingRule, a Variable can use at most one HasModellingRule Reference pointing to a ModellingRule Object. ModellingRules are defined in 6.4.4.

If the Variable is created based on an InstanceDeclaration (see 4.6) it shall have the same BrowseName as its InstanceDeclaration.

The other References are described separately for Properties and DataVariables in the remainder of 5.6.

5.6.3 Properties

Properties are used to define the characteristics of Nodes. Properties are defined using the Variable NodeClass, specified in Table 13. However, they restrict their use.

Properties are the leaf of any hierarchy; therefore they shall not be the SourceNode of any hierarchical References. This includes the HasComponent or HasProperty Reference, that is, Properties do not contain Properties and cannot expose their complex structure. However, they may be the SourceNode of any NonHierarchical References.

The HasTypeDefinition Reference points to the VariableType of the Property. Since Properties are uniquely identified by their BrowseName, all Properties shall point to the PropertyType defined in OPC 10000-5.

Properties shall always be defined in the context of another Node and shall be the TargetNode of at least one HasProperty Reference. To distinguish them from DataVariables, they shall not be the TargetNode of any HasComponent Reference. Thus, a HasProperty Reference pointing to a Variable Node defines this Node as a Property.

The BrowseName of a Property is always unique in the context of a Node. It is not permitted for a Node to refer to two Variables using HasProperty References having the same BrowseName.

5.6.4 DataVariable

DataVariables represent the content of an Object. DataVariables are defined using the Variable NodeClass, specified in Table 13.

DataVariables identify their Properties using HasProperty References. Complex DataVariables use HasComponent References to expose their component DataVariables.

The Property NodeVersion indicates the version of the DataVariable.

The Property LocalTime indicates the difference between the SourceTimestamp of the value and the standard time at the location in which the value was obtained.

The Property AllowNulls indicates if null values are allowed for the Value Attribute.

The Property ValueAsText provides a localized text representation for enumeration values.

The Property MaxStringLength indicates the maximum number of bytes of a String or the text field of a LocalizedText value. If a Server does not impose a maximum number of bytes or is not able to determine the maximum number of bytes this Property shall not be provided. If this Property is provided, then the MaxCharacters Property shall not be provided.

The Property MaxCharacters indicates the maximum number of Unicode characters of a String or the text field of a LocalizedText value. If a Server does not impose a maximum number of Unicode characters or is not able to determine the maximum number of Unicode characters this Property shall not be provided. If this Property is provided then the MaxStringLength Property shall not be provided.

The Property MaxByteStringLength indicates the maximum number of bytes of a ByteString value. If a Server does not impose a maximum number of bytes or is not able to determine the maximum number of bytes this Property shall not be provided.

The Property MaxArrayLength indicates the maximum allowed array length of the value.

The Property EngineeringUnits indicates the engineering units of the value. There are no additional Properties defined for DataVariables in this part of this document. Additional parts of this series of standards may define additional Properties for DataVariables. OPC 10000-8 defines a set of Properties that can be used for DataVariables.

The Property CurrencyUnit represents the currency of the value. The information in the structure is designed to be suited for human users and for automated systems.

DataVariables may use additional References to define relationships to other Nodes. No restrictions are placed on the types of References used or on the NodeClasses of the Nodes that may be referenced. However, restrictions may be defined by the ReferenceType excluding its use for DataVariables. Standard ReferenceTypes are described in Clause 7.

A DataVariable is intended to be defined in the context of an Object. However, complex DataVariables may expose other DataVariables, and ObjectTypes and complex VariableTypes may also contain DataVariables. Therefore each DataVariable shall be the TargetNode of at least one HasComponent Reference coming from an Object, an ObjectType, a DataVariable or a VariableType. DataVariables shall not be the TargetNode of any HasProperty References. Therefore, a HasComponent Reference pointing to a Variable Node identifies it as a DataVariable.

The HasTypeDefinition Reference points to the VariableType used as type definition of the DataVariable.

If the DataVariable is used as InstanceDeclaration (see 4.6) all Nodes referenced with forward hierarchical References shall have unique BrowseNames in the context of this DataVariable.

5.6.5 VariableType NodeClass

VariableTypes are used to provide type definitions for Variables. VariableTypes are defined using the VariableType NodeClass, as specified in Table 14.

The VariableType NodeClass inherits the base Attributes from the Base NodeClass defined in 5.2. The VariableType NodeClass also defines a set of Attributes that describe the default or initial value of its instance Variables. The Value Attribute represents the default value. The DataType, ValueRank and ArrayDimensions Attributes provide the capability to describe simple and complex values. The IsAbstract Attribute defines if the type can be directly instantiated.

The VariableType NodeClass uses HasProperty References to define the Properties and HasComponent References to define DataVariables. Whether they are instantiated depends on the ModellingRules defined in 6.4.4.

The Property NodeVersion indicates the version of the VariableType. There are no additional Properties defined for VariableTypes in this document. Additional parts of this series of standards may define additional Properties for VariableTypes. OPC 10000-8 defines a set of Properties that can be used for VariableTypes.

HasSubtype References are used to subtype VariableTypes. VariableType subtypes inherit the general semantics from the parent type. The general rules for subtyping are defined in Clause 6. It is not required to provide the HasSubtype Reference for the supertype, but it is required that the subtype provides the inverse Reference to its supertype.

GeneratesEvent References identify that Variables of the VariableType may be the source of an Event of the specified EventType or one of its subtypes. Servers should make GeneratesEvent References bidirectional References. However, it is allowed to be unidirectional when the Server is not able to expose the inverse direction pointing from the EventType to each VariableType supporting the EventType.

GeneratesEvent References are optional, i.e. Variables may generate Events of an EventType that is not exposed by its VariableType.

VariableTypes may use any additional References to define relationships to other Nodes. No restrictions are placed on the types of References used or on the NodeClasses of the Nodes that may be referenced. However, restrictions may be defined by the ReferenceType excluding its use for VariableTypes. Standard ReferenceTypes are described in Clause 7.

All Nodes referenced with forward hierarchical References shall have unique BrowseNames in the context of the VariableType (see 4.6).

5.6.6 Client-side creation of Variables of an VariableType

Variables are always based on a VariableType, i.e. they have a HasTypeDefinition Reference pointing to its VariableType.

Clients can create Variables using the AddNodes Service defined in OPC 10000-4. The Service requires specifying the TypeDefinitionNode of the Variable. A Variable created by the AddNodes Service contains all components defined by its VariableType dependent on the ModellingRules specified for the components. However, the Server may add additional components and References to the Variable and its components that are not defined by the VariableType. This behaviour is Server dependent. The VariableType only specifies the minimum set of components that shall exist for each Variable of a VariableType.

5.7 Methods

5.7.1 Method NodeClass

Methods define callable functions. Methods are invoked using the Call Service defined in OPC 10000-4. Method invocations are not represented in the AddressSpace. Method invocations always run to completion and always return responses when complete. Methods are defined using the Method NodeClass, specified in Table 15.

The Method NodeClass inherits the base Attributes from the Base NodeClass defined in 5.2. The Method NodeClass defines no additional Attributes.

The Executable Attribute indicates whether the Method is executable, not taking user access rights into account. If the OPC UA Server cannot get the Executable information from the underlying system, it should state that it is executable. If a Method is called then the Server should transfer this request and return the corresponding StatusCode even if such a request is rejected. StatusCodes are defined in OPC 10000-4.

The UserExecutable Attribute indicates whether the Method is executable, taking user access rights into account. If the OPC UA Server cannot get any user rights related information from the underlying system, it should use the same value as used in the Executable Attribute. The UserExecutable Attribute can be set to FALSE, even if the Executable Attribute is set to TRUE, but it shall be set to FALSE if the Executable Attribute is set to FALSE. Clients cannot assume a Method can be executed based on the UserExecutable Attribute. It is possible that the Server may return an access denied error due to some Server specific change which was not reflected in the state of this Attribute at the time the Client accessed it.

Properties may be defined for Methods using HasProperty References. The Properties InputArguments and OutputArguments specify the input arguments and output arguments of the Method. Both contain an array of the DataType Argument as specified in 8.6. An empty array or a Property that is not provided indicates that there are no input arguments or output arguments for the Method.

The Property NodeVersion indicates the version of the Method. There are no additional Properties defined for Methods in this document. Additional parts of this series of standards may define additional Properties for Methods.

To specify its ModellingRule, a Method can use at most one HasModellingRule Reference pointing to a ModellingRule Object. ModellingRules are defined in 6.4.4.

GeneratesEvent References identify that Methods may generate an Event of the specified EventType or one of its subtypes for every call of the Method. A Server may generate one Event for each referenced EventType when a Method is successfully called.

AlwaysGeneratesEvent References identify that Methods will generate an Event of the specified EventType or one of its subtypes for every call of the Method. A Server shall always generate one Event for each referenced EventType when a Method is successfully called.

Servers should make GeneratesEvent References bidirectional References. However, it is allowed to be unidirectional when the Server is not able to expose the inverse direction pointing from the EventType to each Method generating the EventType.

GeneratesEvent References are optional, i.e. the call of a Method may produce Events of an EventType that is not referenced with a GeneratesEvent Reference by the Method.

Methods may use additional References to define relationships to other Nodes. No restrictions are placed on the types of References used or on the NodeClasses of the Nodes that may be referenced. However, restrictions may be defined by the ReferenceType excluding its use for Methods. Standard ReferenceTypes are described in Clause 7.

A Method shall always be the TargetNode of at least one HasComponent Reference. The SourceNode of these HasComponent References shall be an Object or an ObjectType. If a Method is called, then the NodeId of one of those Nodes shall be put into the Call Service defined in OPC 10000-4 as parameter to detect the context of the Method operation.

If the Method is used as InstanceDeclaration (see 4.6) all Nodes referenced with forward hierarchical References shall have unique BrowseNames in the context of this Method.

The Variable referenced by a HasArgumentDescription ReferenceType shall use a BrowseName equal to the name of the Argument it describes. The Namespace of the BrowseName shall be ignored by a Client when performing an equality check with an Argument name. For each Argument there shall be at most one Variable referenced by a HasArgumentDescription ReferenceType. The Variable referenced by the HasArgumentDescription ReferenceType shall have the same DataType as the Argument’s DataType.

Argument names shall be unique within the scope of the Method.

An example use of the HasArgumentDescription Reference is illustrated in Figure 16. In this example an ObjectType defines a Method which illustrates the following:

Output1 as a discrete output argument with a HasArgumentDescription Reference to a TwoStateDiscreteType Variable which provides descriptions of the states of the output argument.

Input1 as a numeric input argument with a HasArgumentDescription Reference to a DataVariable Variable which provides the default value 42.

Input2 as a numeric input argument with a HasArgumentDescription Reference to an AnalogItemType Variable which provides an engineering units range of 0 kPa to 100 kPa.

Input3 as an optional discrete input argument with a HasOptionalInputArgumentDescription Reference to a TwoStateDiscreteType Variable which provides descriptions of the states of the input argument and a default value of TRUE.

Object1 as an instance of MyObjectType with an instance specific HasArgumentDescription Reference to an instance specific AnalogItemType Variable which provides an instance specific engineering range of 0 kPa to 200 kPa and a default value of 75 kPa for the Input2 argument.

5.7.2 HasArgumentDescription ReferenceType

The HasArgumentDescription ReferenceType is a concrete ReferenceType that is a subtype of the HasComponent ReferenceType.

The semantic of the HasArgumentDescription ReferenceType – extends the semantic of the HasComponent ReferenceType to reference argument Metadata of a Method NodeClass.

The SourceNode of this ReferenceType shall be a Method and the TargetNode of this ReferenceType shall be a Variable.

5.7.3 HasOptionalInputArgumentDescription ReferenceType

The HasOptionalInputArgumentDescription ReferenceType is a concrete ReferenceType that can be used directly. It is a subtype of the HasArgumentDescription ReferenceType.

The semantic of the HasOptionalInputArgumentDescription ReferenceType – extends the semantic of the HasArgumentDescription ReferenceType to reference optional input arguments of a Method NodeClass. Optional input arguments shall always follow any non-optional input arguments in the InputArguments array. For example, if a method has 3 arguments with 1 being optional then the 3^rd argument shall be the optional one.

There are no additional constraints defined for this ReferenceType.

5.8 DataTypes

5.8.1 DataType Model

The DataType Model is used to define simple and structured data types. Data types are used to describe the structure of the Value Attribute of Variables and their VariableTypes. Therefore each Variable and VariableType is pointing with its DataType Attribute to a Node of the DataType NodeClass as shown in Figure 17.

In many cases, the NodeId of the DataType Node – the DataTypeId – will be well-known to Clients and Servers. Clause 8 defines DataTypes and OPC 10000-6 defines their DataTypeIds. In addition, other organizations may define DataTypes that are well-known in the industry. Well-known DataTypeIds provide for commonality across OPC UA Servers and allow Clients to interpret values without having to read the type description from the Server. Therefore, Servers may use well-known DataTypeIds without representing the corresponding DataType Nodes in their AddressSpaces.

In other cases, DataTypes and their corresponding DataTypeIds may be vendor-defined. Servers should attempt to expose the DataType Nodes and the information about the structure of those DataTypes for Clients to read, although this information might not always be available to the Server.

Figure 18 illustrates the Nodes used in the AddressSpace to describe the structure of a DataType. The DataType points to an Object of type DataTypeEncodingType. Each DataType can have several DataTypeEncoding, for example “Default”, “UA Binary” and “XML” encoding. Services in OPC 10000-4 allow Clients to request an encoding or choosing the “Default” encoding. Each DataTypeEncoding is used by exactly one DataType, that is, it is not permitted for two DataTypes to point to the same DataTypeEncoding.

Since the NodeId of the DataTypeEncoding will be used in some Mappings to identify the DataType and it’s encoding as defined in OPC 10000-6, those NodeIds may also be well-known for well-known DataTypeIds.

5.8.2 Encoding Rules for different kinds of DataTypes

Different kinds of DataTypes are handled differently regarding their encoding and according to whether this encoding is represented in the AddressSpace.

Built-in DataTypes are a fixed set of DataTypes (see OPC 10000-6 for a complete list of Built-in DataTypes). They have no encodings visible in the AddressSpace since the encoding should be known to all OPC UA products. Examples of Built-in DataTypes are Int32 (see 8.26) and Double (see 8.12).

Simple DataTypes are subtypes of the Built-in DataTypes, which are handled on the wire like the Built-in DataType, i.e. they cannot be distinguished on the wire from their Built-in supertypes. Since they are handled like Built-in DataTypes regarding the encoding they cannot have encodings defined in the AddressSpace. Clients can read the DataType Attribute of a Variable or VariableType to identify the Simple DataType of the Value Attribute. An example of a Simple DataType is Duration. It is handled on the wire as a Double but the Client can read the DataType Attribute and thus interpret the value as defined by Duration (see 8.13).

Structured DataTypes are DataTypes that represent structured data and are not defined as Built-in DataTypes. Structured DataTypes inherit directly or indirectly from the DataType Structure defined in 8.32. Structured DataTypes may have several encodings and the encodings are exposed in the AddressSpace. How the encoding of Structured DataTypes is handled on the wire is defined in OPC 10000-6. The encoding of the Structured DataType is transmitted with each value, thus Clients are aware of the DataType without reading the DataType Attribute. The encoding has to be transmitted so the Client is able to interpret the data. An example of a Structured DataType is Argument (see 8.6).

Enumeration DataTypes are DataTypes that represent discrete sets of named values. Enumerations are always encoded as Int32 on the wire as defined in OPC 10000-6. Enumeration DataTypes inherit directly or indirectly from the DataType Enumeration defined in 8.14. Enumerations have no encodings exposed in the AddressSpace. To expose the human-readable representation of an enumerated value the DataType Node may have the EnumStrings Property that contains an array of LocalizedText. The Integer representation of the enumeration value points to a position within that array. EnumValues Property can be used instead of the EnumStrings to support integer representation of enumerations that are not zero-based or have gaps. It contains an array of a Structured DataType containing the integer representation as well as the human-readable representation. An example of an enumeration DataType containing a sparse list of Integers is NodeClass which is defined in 8.29.

An OptionSet can be defined in one of two ways. An OptionSet which is 64 bits or less may be defined as an UInteger DataType and always encoded on the wire as defined in OPC 10000-6. An OptionSet may be defined as an OptionSet DataType which is defined in 8.40 and is encoded on the wire as a Structured DataType. To expose the human-readable representation of an OptionSet the DataType Node shall have the OptionSetValues Property that contains an array of LocalizedText.

In addition to the DataTypes described above, abstract DataTypes are also supported, which do not have any encodings and cannot be exchanged on the wire. Variables and VariableTypes use abstract DataTypes to indicate that their Value may be any one of the subtypes of the abstract DataType. An example of an abstract DataType is Integer which is defined in 8.24.

OPC 10000-6 defines a number of DataEncodings which specify how to serialize DataTypes. Some of these DataEncodings are text based and make use of Name portion the DataType BrowseName. For this reason, the BrowseName for all DataTypes should be Strings that start with a letter and contain only letters, digits or the underscore (_). If a DataType has a BrowseName that does not meet these requirements it will be transformed using the Name encoding rules defined in OPC 10000-6 into a String that meets the requirements. This will result in text based DataEncodings with Names that are not friendly to human readers.

5.8.3 DataType NodeClass

The DataType NodeClass describes the syntax of a Variable Value. DataTypes are defined using the DataType NodeClass, as specified in Table 16.

The DataType NodeClass inherits the base Attributes from the Base NodeClass defined in 5.2. The IsAbstract Attribute specifies if the DataType is abstract or not. Abstract DataTypes can be used in the AddressSpace, i.e. Variables and VariableTypes can point with their DataType Attribute to an abstract DataType. However, concrete values can never be of an abstract DataType and shall always be of a concrete subtype of the abstract DataType.

HasProperty References are used to identify the Properties of a DataType. The Property NodeVersion is used to indicate the version of the DataType. The Property EnumStrings contains human-readable representations of enumeration values and is only applied to Enumeration DataTypes. Instead of the EnumStrings Property an Enumeration DataType can also use the EnumValues Property to represent Enumerations with integer values that are not zero-based or containing gaps. There are no additional Properties defined for DataTypes in this standard. Additional parts of this series of standards may define additional Properties for DataTypes.

HasSubtype References may be used to expose a data type hierarchy in the AddressSpace. The semantic of subtyping is only defined to the point, that a Server may provide instances of the subtype instead of the DataType. Clients should not make any assumptions about any other semantic with that information. For example, it might not be possible to cast a value of one data type to its base data type. Servers need not provide HasSubtype References, even if their DataTypes span a type hierarchy, however it is required that the subtype provides the inverse Reference to its supertype. Some restrictions apply for subtyping enumeration DataTypes as defined in 8.14.

HasEncoding References point from the DataType to its DataTypeEncodings. Each concrete Structured DataType can point to many DataTypeEncodings, but each DataTypeEncoding shall belong to one DataType, that is, it is not permitted for two DataType Nodes to point to the same DataTypeEncoding Object using HasEncoding References. The DataTypeEncoding Node shall provide the inverse HasEncoding Reference to its DataType.

An abstract DataType is not the SourceNode of a HasEncoding Reference. The DataTypeEncoding of an abstract DataType is provided by its concrete subtypes.

DataType Nodes shall not be the SourceNode of other types of References. However, they may be the TargetNode of other References.

5.8.4 DataTypeEncoding and Encoding Information

If a DataType Node is exposed in the AddressSpace, it shall provide its DataTypeEncodings using HasEncoding References. These References shall be bi-directional. Figure 19 provides an example how DataTypes are modelled in the AddressSpace.

The information on how to encode the DataType is provided in the Attribute DataTypeDefinition of the DataType Node. The content of this Attribute shall not be changed once it had been provided to Clients since Clients might persistently cache this information. If the encoding of a DataType needs to be changed conceptually a new DataType needs to be provided, meaning that a new NodeId shall be used for the DataType. Since Clients identify the DataType via the DataTypeEncodings, also the NodeIds for the DataTypeEncodings of the DataType shall be changed, when the encoding changes.

5.8.5 DataTypeRefinement

5.8.5.1 Overview

A DataTypeRefinement refines the usage of a concrete Structured DataType. It can restrict the usage of fields of the Structured DataType and add meta data to fields of the Structured DataType, like providing the EngineeringUnits or EURange.

In order to restrict the usage for each field of a Structured DataType, the following can be defined:

What DataType can be used, including the maximum array length and maximum string length

If an optional field shall become mandatory or shall not be provided

If a certain subtype of the DataType is allowed or not

The general rules for inheritance apply, as defined in 6.3, i.e., the DataType of a field can only be restricted to subtypes of the original DataType, mandatory fields cannot become optional, etc. If a concrete maximum array length or maximum string length has already been defined, it shall not be changed. If a field is defined that does not allow subtypes, this shall not be changed. If a field is defined that does allow subtypes, this can be changed to disallow subtypes.

Any restrictions on the Structured DataType only affect the usage, not the encoding of the DataType. It is still encoded according to the rules of the Structured DataType.

In addition to the restrictions, the DataTypeRefinement also allows meta data for fields to be added to a Structured DataType.

5.8.5.2 DataTypeRefinement Objects

A DataTypeRefinement is represented by an Object of DataTypeRefinementType (see OPC 10000-5) or a subtype. A DataTypeRefinement Object shall always be referenced using a HasDataTypeRefinement Reference (see 7.30) or subtype from exactly one DataType Node representing a Structured DataType that is being refined.

For each field having a refinement, a Variable of BaseDataVariableType or subtype is referenced using a Reference of HasFieldDescription ReferenceType (see 7.25) or a subtype. The BrowseName of the Variable shall use a BrowseName equal to the name of the field. The Namespace of the BrowseName shall be ignored by a Client when performing an equality check with a field name.

A DataTypeRefinement Object shall not reference any Node with a HasFieldDescription or subtype which is not representing a field of the DataType it refines and shall reference at most one Variable for each field with a HasFieldDescription or subtype.

For the restriction of the field, the rules defined in 5.8.5.1 apply. In Table 17, the mapping of the fields in the StructureField of the DataTypeDefinition Attribute to concepts of a Variable is defined.

By using the subtypes HasFieldDescriptionSetMandatory (see 7.26) a field becomes mandatory, and by IsDisabledOptionalField (see 7.27) an optional field shall not be present. In the AccessLevelEx Attribute, the NoSubDataTypes bit can be used to express that a field where subtypes are allowed, no longer allows subtypes.

Depending on the Structured DataType, some limitations on the DataTypeRefinement occur.

If the DataType of a field being refined is BaseDataType, Structure, Number, Integer or UInteger, the DataType of the field can be restricted by using a subtype or SubtypeRestriction (see 5.8.6) on the Variable refining the field. If the StructureType of the DataType being refined is StructureWithSubtypedValues or UnionWithSubtypedValues and IsOptional of the field is set true, the DataType of that field may be restricted by using a subtype or SubtypeRestriction on the Variable refining the field. In all other cases, it is not allowed to use subtypes on the field and therefore restricting the DataType of the field to subtypes is not allowed.

If the DataType of a Variable refining a field is BaseDataType, Structure, Number, Integer or UInteger, then the AccessLevelEx bit NoSubDataTypes of the Variable shall be true. If the DataType of a field being refined is BaseDataType, Structure, Number, Integer or UInteger, then the AccessLevelEx bit NoSubDataTypes of the Variable may be true or false. If the StructureType of a DataType is StructureWithSubtypedValues or UnionWithSubtypedValues, and IsOptional of the field is true, the AccessLevelEx bit NoSubDataTypes may be true or false. In all other cases the AccessLevelEx bit NoSubDataTypes of a Variable refining a field of a Structured DataType shall be false, as it is not allowed to use subtypes based on the Structured DataType being refined.

If the StructureType of a DataType is Structure or StructureWithSubtypedValues, any refinement of a field of the DataType shall not be referenced with IsDisabledOptionalField or HasFieldDescriptionSetMandatory, as there are no optional fields defined for the DataType. If the StructureType is StructureWithOptionalFields, then the IsDisabledOptionalField or HasFieldDescriptionSetMandatory may be used on a field when IsOptional of that field is true. If the StructureType is Union or UnionWithSubtypedValues then the IsDisabledOptionalField may be used on any field and the HasFieldDescriptionSetMandatory shall not be used. In this case, the semantic of IsDisabledOptionalField implies that the DataType of the field shall not be used in the Union using the DataTypeRefinement.

In Figure 20, an example is given. The structure of X:SuperDataType and X:SubDataType is provided in pseudo-code. The Y:Refinement_1 is refining the X:SubDataType and referenced from the DataType with a HasDataTypeRefinement Reference.

The field1 is defined in the DataType as Number, the DataTypeRefinement restricts this to an Int32. As meta data, the EngineeringUnits is defined as Kelvin, therefore the AnalogUnitType is used as VariableType.

The optional field2 is refined to “not used” by referencing the corresponding Variable with IsDisabledOptionalField.

The field3 is defined in the DataType as Number and a one-dimensional array without a size limitation. In the DataTypeRefinement it is refined as an array of Int32 with the maximum length of 5.

The optional field4, added in the subtype X:SubDataType, of String without a maximum length, becomes in the DataTypeRefinement a mandatory field of String with the MaxStringLength of 50.

The field5 is not refined and stays as an Int32.

As fields of Structured DataTypes may use Structured DataTypes, it may be desirable to refine fields of those fields as well. This can be done as shown in Figure 21. In this example, the Y:Refinement_2 provides similar refinements as Y:Refinement_1, but makes the optional field4 “not used”.

The new optional field6, introduced by X:SubSubDataType, uses the X:SubDataType as DataType. The Y:Refinement_2 makes this optional field mandatory, and by letting the X:field6 Variable reference the Y:Refinement_1 with UsesDataTypeRefinement (see 7.24), this DataTypeRefinement is applied to field6.

5.8.5.3 Usage of DataTypeRefinements

Each DataTypeRefinement Object shall be referenced from exactly one DataType Node using HasDataTypeRefinement or a subtype.

DataTypeRefinements are expressed in an OPC UA information model, by using the non-hierarchical Reference UsesDataTypeRefinement (see 7.24) or a subtype. Those References always starts from an instance of a Variable, where the Structured DataType is used. They point to an Object of DataTypeRefinementType or a subtype, which contains the restrictions and meta data of the DataTypeRefinement.

DataTypeRefinements are only allowed to be used on the concrete occurrence of the usage of the Structured DataType, that is on a Variable.

DataTypeRefinements shall not be used from any DataType Node, from any VariableType, or any InstanceDeclaration, only from concrete Variables. This does include Variables describing a DataTypeRefinement, as shown in Figure 21.

Note, that Method Arguments can be further described using Variables referenced with a HasArgumentDescription. If the Method Argument is a Structured DataType, this Variable can be used as SourceNode for the UsesDataTypeRefinement to further refine the Structured DataType.

5.8.5.4 Example of usage of DataTypeRefinements

An example of DataTypeRefinements is given in Figure 22. The X:SuperDataType has two DataTypeRefinements, its subtype has an additional one. Note that the details of the DataTypeRefinements is not shown in the figure. There are two Objects of Z:SuperObjectType, both using DataTypeRefinements.

Z:Object1 has the Property Z:Prop1 and the DataVariables Z:Var1 and Z:Var2, as defined on the X:SuperObjectType. Although all three Variables use the same DataType, the Z:Prop1 is refined by Y:Refinement_4 and Z:Var2 by Y:Refinement_3. Z:Var1 is not further refined.

Z:Object2 has Z:Prop1 and Z:Var1 as defined on the ObjectType, but refines the DataType from Z:Var2 to X:SubDataType and adds another Variable Z:Var3. Like in Z:Object1, the Z:Prop1 is refined by Y:Refinement_4, and both Z:Var2 and Z:Var3 are refined by Y:Refinement_1.

5.8.6 SubtypeRestriction on a DataType

5.8.6.1 Overview

When a Variable or VariableType is defined, the DataType of the Value is defined by the DataType, ValueRank and ArrayDimensions Attributes. In general, it is always allowed for the Value to use a subtype of the DataType unless restricted in the NoSubDataTypes bit of the AccessLevelEx Attribute. In fields of a structure, using subtypes is allowed when IsOptional is set and it is a StructureWithOptionalFields or the DataType is an abstract DataType like the BaseDataType. In many cases where it is allowed to use subtypes, not all possible subtypes are allowed, but the DataType hierarchy does not allow to restrict this by one supertype. For example, when an Int16 and Int32 are allowed, their common supertype Integer would also allow SByte and Int64.

A SubtypeRestriction limits the usage of subtypes to a subset of all subtypes of a DataType. It is represented by an Object of SubtypeRestrictionType defined in OPC 10000-5. A SubtypeRestriction Object is bound to a DataType by a HasDataTypeRefinement Reference from the DataType to the SubtypeRestriction Object. Each SubtypeRestriction Object shall be referenced from exactly one DataType Node using a HasDataTypeRefinement Reference. A SubtypeRestriction Object is referencing Variables with an AllowedSubtype Reference (see 7.29), and thereby restricting that the DataType including ValueRank and ArrayDimensions of the Variable is allowed to be used as subtype. The general rules of inheritance defined in 6.3 apply, i.e., the DataType shall be the same or a subtype of the DataType, the ValueRank can only be changed according to the subtyping rules, etc.

A SubtypeRestriction Object should have at least two AllowedSubtype References to Variables defining different variations of subtypes. Otherwise, a Variable using the SubtypeRestriction should just use the concrete subtype and does not need a SubtypeRestriction.

In Figure 23, an example of a SubtypeRestriction Object is given. The Y:Restriction1 Object is referenced from Number and therefore can be used by any Variable using the Number DataType. It references two Variables, Y:UInt32 with the DataType UInt32, the ValueRank Scalar which does not allow subtypes and X:IntegerArray, of Integer, ValueRank OneDimension that does allow subtypes (which is required since Integer is an abstract DataType). Thereby, any Variable using the SubtypeRestriction is limiting its values to scalar UInt32s or arrays of Integer or any subtypes of Integer.

In the example, Z:SampleVar is using the Y:Restriction1 by referencing it with UsesSubtypeRestriction. This is allowed since the DataType of Z:SampleVar is Number. The DataTypes always have to match exactly. The NoSubDataTypes is set to true, which is required, since Number is abstract, but would also be required for a concrete DataType, since setting it to true is already restricting it to disallow subtypes.

In order to use a SubtypeRestriction, the ValueRank and ArrayDimensions Attribute of the Variable using the SubtypeRestriction shall be compatible to all referenced Variables using AllowedSubtype, i.e., it is required that based on the definitions of the Variable using the SubtypeRestriction a value based on any of the referenced Variables can be used.

A SubtypeRestriction shall only be used on Variables used as instances, never on InstanceDeclarations or VariableTypes.

For Method Arguments, it can be used in combination with the HasArgumentDescription.

5.8.6.2 SubtypeRestriction on fields of Structured DataTypes

Using a SubtypeRestriction, it is possible to restrict the usage of subtypes on fields of a Structured DataType, by combining it with DataTypeRefinements (see 5.8.5). The Variable used to refine the field in a DataTypeRefinement can be the SourceNode of a UsesSubtypeRestriction, thereby limiting the subtypes that can be used in the field.

In Figure 24, an example is given. The X:SuperDataType contains field1 of a scalar Number. The Y:Refinement_3 of X:SuperDataType defines the field1 to either use a scalar UInt32 or Int32 by referencing the Z:Restriction1. No other subtypes of Number are allowed.

5.9 Summary of Attributes of the NodeClasses

Table 18 summarises all Attributes defined in this document and points out which NodeClasses use them either in an optional (O) or mandatory (M) way.

6 Type Model for ObjectTypes and VariableTypes

6.1 Overview

In the remainder of Clause 6 the type model of ObjectTypes and VariableTypes is defined regarding subtyping and instantiation.

6.2 Definitions

6.2.1 InstanceDeclaration

An InstanceDeclaration is an Object, Variable or Method that references a ModellingRule with a HasModellingRule Reference and is the TargetNode of a hierarchical Reference from a TypeDefinitionNode or another InstanceDeclaration. There shall be no two TypeDefinitionNodes referencing the same InstanceDeclaration with a hierarchical Reference, either directly or from another InstanceDeclaration of that TypeDefinitionNode, i.e. an InstanceDeclaration belongs to exactly one TypeDefinitionNode.

The type of an InstanceDeclaration may be abstract, however the instance must be of a concrete type.

6.2.2 Instances without ModellingRules

If no ModellingRule exists then the Node is neither considered for instantiation of a type nor for subtyping.

If a Node referenced by a TypeDefinitionNode does not reference a ModellingRule it indicates that this Node only belongs to the TypeDefinitionNode and not to the instances. For example, an ObjectType Node may contain a Property that describes scenarios where the type could be used. This Property would not be considered when creating instances of the type. This is also true for subtyping, that is, subtypes of the type definition would not inherit the referenced Node.

6.2.3 InstanceDeclarationHierarchy

The InstanceDeclarationHierarchy of a TypeDefinitionNode contains the TypeDefinitionNode and all InstanceDeclarations that are directly or indirectly referenced from the TypeDefinitionNode using forward hierarchical References.

6.2.4 Similar Node of InstanceDeclaration

A similar Node of an InstanceDeclaration is a Node that has the same BrowseName and NodeClass as the InstanceDeclaration and in cases of Variables and Objects the same TypeDefinitionNode or a subtype of it. In the case of a Method a similar Node of an InstanceDeclaration is a Node that also has the same arguments of the InstanceDeclaration, however it may append additional optional arguments and it may specialize the DataType of arguments defined with an abstract DataType to a subtype of the abstract DataType.

6.2.5 BrowsePath

A BrowsePath is a sequence of BrowseNames used to describe a path between Nodes related with a Reference.

6.2.6 BrowseName within a TypeDefinitionNode

A BrowsePath within a TypeDefinitionNode which include targets of forward hierarchical References shall have a BrowseName that is unique within the TypeDefinitionNode, this same restriction applies to the targets of forward hierarchical References from any InstanceDeclaration. This means that any InstanceDeclaration within the InstanceDeclarationHierarchy can be uniquely identified by a sequence of BrowseNames.

6.2.7 Attribute Handling of InstanceDeclarations

Some restrictions exist regarding the Attributes of InstanceDeclarations when the InstanceDeclaration is overridden or instantiated. The BrowseName and the NodeClass shall never change and always be the same as the original InstanceDeclaration.

In addition, the rules defined in 6.2.8 apply for InstanceDeclarations of the NodeClass Variable.

6.2.8 Attribute Handling of Variable and VariableTypes

Some restrictions exist regarding the Attributes of a VariableType or a Variable used as an InstanceDeclaration with regard to the data type of the Value Attribute.

When a Variable used as InstanceDeclaration or a VariableType is overridden or instantiated the following rules apply:

1. The DataType Attribute can only be changed to a new DataType if the new DataType is a subtype of the DataType originally used.

2. The ValueRank Attribute may only be further restricted

3. ‘Any’ may be set to any other value;

4. ‘ScalarOrOneDimension’ may be set to ‘Scalar’ or ‘OneDimension’;

5. ‘OneOrMoreDimensions’ may be set to a concrete number of dimensions (value > 0).

6. All other values of this Attribute shall not be changed.

7. The ArrayDimensions Attribute may be added if it was not provided or when modifying the value of an entry in the array from 0 to a different value. All other values in the array shall remain the same.

6.2.9 NodeIds of InstanceDeclarations

InstanceDeclarations are identified by their BrowsePath. Different Servers might use different NodeIds for the InstanceDeclarations of common TypeDefinitionNodes, unless the definition of the TypeDefinitionNode already defines a NodeId for the InstanceDeclaration. All TypeDefinitionNodes defined in OPC 10000-5 already define the NodeIds for their InstanceDeclarations and therefore shall be used in all Servers.

6.3 Subtyping of ObjectTypes and VariableTypes

6.3.1 Overview

The HasSubtype ReferenceType defines subtypes of types. Subtyping can only occur between Nodes of the same NodeClass. Rules for subtyping ReferenceTypes are described in 5.3.3.3. There is no common definition for subtyping DataTypes, as described in 5.8.3. The remainder of 6.3 specify subtyping rules for single inheritance on ObjectTypes and VariableTypes.

6.3.2 Attributes

Subtypes inherit the parent type’s Attribute values, except for the NodeId. Inherited Attribute values may be overridden by the subtype, the BrowseName and DisplayName values should be overridden. Special rules apply for some Attributes of VariableTypes as defined in 6.2.8. Optional Attributes, not provided by the parent type, may be added to the subtype.

6.3.3 InstanceDeclarations

6.3.3.1 Overview

Subtypes inherit the fully-inherited parent type’s InstanceDeclarations.

As long as those InstanceDeclarations are not overridden they are not referenced by the subtype. InstanceDeclarations can be overridden by adding References, changing References to reference different Nodes, changing References to be subtypes of the original ReferenceType, changing values of the Attributes or adding optional Attributes. In order to get the full information about a subtype, the inherited InstanceDeclarations have to be collected from all types that can be found by recursively following the inverse HasSubtype References from the subtype. This collection of InstanceDeclarations is called the fully-inherited InstanceDeclarationHierarchy of a subtype.

The remainder of 6.3.3 define how to construct the fully-inherited InstanceDeclarationHierarchy and how InstanceDeclarations can be overridden.

6.3.3.2 Fully-inherited InstanceDeclarationHierarchy

An instance of a TypeDefinitionNode is described by the fully-inherited InstanceDeclaration­Hierarchy of the TypeDefinitionNode. The fully-inherited InstanceDeclarationHierarchy can be created by starting with the InstanceDeclarationHierarchy of the TypeDefinitionNode and merging the fully-inherited InstanceDeclarationHierarchy of its parent type.

The process of merging InstanceDeclarationHierarchies is straightforward and can be illustrated with the example shown in Figure 25 which specifies a TypeDefinitionNode “BetaType” which is a subtype of “AlphaType”. The name in each box is the BrowseName and the number is the NodeId.

An InstanceDeclarationHierarchy can be fully described as a table of Nodes identified by their BrowsePaths with a corresponding table of References. The InstanceDeclarationHierarchy for “BetaType” is described in Table 19 where the top half of the table is the table of Nodes and the bottom half is the table of References (the HasModellingRule references have been omitted from the table for the sake of clarity; all Nodes except for 1, 6, and 5 have ModellingRules). All InstanceDeclarations of the InstanceDeclarationHierarchy and all Nodes referenced with a NonHierarchical Reference from such an InstanceDeclaration are added to the table. Hierarchical References to Nodes without a ModellingRule are not considered.

Multiple BrowsePaths to the same Node shall be treated as separate Nodes. An Instance may provide different Nodes for each BrowsePath.

The fully-inherited InstanceDeclarationHierarchy for “BetaType” can now be constructed by merging the InstanceDeclarationHierarchy for “AlphaType”. The result is shown in Table 20 where the entries added from “AlphaType” are shaded with grey.

The BrowsePath “/B” already exists in the table so it does not need to be added. However, the HasNotifier reference from “/” to “/B” does not exist and was added.

The Nodes and References defined in Table 20 can be used to create the fully-inherited InstanceDeclarationHierarchy shown in Figure 26. The fully-inherited InstanceDeclarationHierarchy contains all necessary information about a TypeDefinitionNode regarding its complex structure without needing any additional information from its supertypes.

6.3.3.3 Overriding InstanceDeclarations

A subtype overrides an InstanceDeclaration by specifying an InstanceDeclaration with the same BrowsePath. An overridden InstanceDeclaration shall have the same NodeClass and BrowseName. The TypeDefinitionNode of the overridden InstanceDeclaration shall be the same or a subtype of the TypeDefinitionNode specified in the supertype.

When overriding an InstanceDeclaration it is necessary to provide hierarchical References that link the new Node back to the subtype (the References are used to determine the BrowsePath of the Node).

It is only possible to override InstanceDeclarations that are directly referenced from the TypeDefinitionNode. If an indirect referenced InstanceDeclaration, such as “J” in Figure 26, has to be overridden, then the directly referenced InstanceDeclarations that includes “J”, in that case “B”, have to be overridden first and then “J” can be overridden in a second step.

A Reference is replaced if it goes between two overridden Nodes and has the same ReferenceType as a Reference defined in the supertype. The Reference specified in the subtype may be a subtype of the ReferenceType used in the parent type.

Any NonHierarchical References specified for the overridden InstanceDeclaration are treated as new References unless the ReferenceType only allows a single Reference per SourceNode. If this situation exists the subtype can change the target of the Reference but the new target shall have the same NodeClass and for Objects and Variables also the same type or a subtype of the type specified in the parent.

The overriding Node may specify new values for the Node Attributes other than the NodeClass or BrowseName, however, the restrictions on Attributes specified in 6.2.7 apply. Any Attribute provided by the overridden InstanceDeclaration has to be provided by the overriding InstanceDeclaration, additional optional Attributes may be added.

A subtype shall not override an argument of its supertype’s Method InstanceDeclaration which is defined with a concrete DataType. Method arguments defined with an abstract DataType may be overridden. A subtype shall not remove an argument of its supertype’s Method InstanceDeclaration. A subtype shall not add mandatory additional arguments however it may append optional arguments after all existing arguments of the supertype’s Method InstanceDeclaration.

The ModellingRule of the overriding InstanceDeclaration may be changed as defined in 6.4.4.2.

Each overriding InstanceDeclaration needs its own HasModellingRule and HasTypeDefinition References, even if they have not been changed.

A subtype should not override a Node unless it needs to change it.

The semantics of certain TypeDefinitionNodes and ReferenceTypes may impose additional restrictions with regard to overriding Nodes.

6.4 Instances of ObjectTypes and VariableTypes

6.4.1 Overview

Any Instance of a TypeDefinitionNode will be the root of a hierarchy which mirrors the InstanceDeclarationHierarchy for the TypeDefinitionNode. Each Node in the hierarchy of the Instance will have a BrowsePath which may be the same as the BrowsePath for one of the InstanceDeclarations in the hierarchy of the TypeDefinitionNode. The InstanceDeclaration with the same BrowsePath is called the InstanceDeclaration for the Node. If a Node has an InstanceDeclaration then it shall have the same BrowseName and NodeClass as the InstanceDeclaration and, in cases of Variables and Objects, the same TypeDefinitionNode or a subtype of it.

Instances may reference several Nodes with the same BrowsePath. Clients that need to distinguish between the Nodes based on the InstanceDeclarationHierarchy and the Nodes that are not based on the InstanceDeclarationHierarchy can accomplish this using the TranslateBrowsePathsToNodeIds service defined in OPC 10000-4.

6.4.2 Creating an Instance

Instances inherit the initial values for the Attributes that they have in common with the TypeDefinitionNode from which they are instantiated, with the exceptions of the NodeClass and NodeId.

When a Server creates an instance of a TypeDefinitionNode it shall create the same hierarchy of Nodes beneath the new Object or Variable depending on the ModellingRule of each InstanceDeclaration. Standard ModellingRules are defined in 6.4.4.4. The Nodes within the newly created hierarchy may be copies of the InstanceDeclarations, the InstanceDeclaration itself or another Node in the AddressSpace that has the same TypeDefinitionNode and BrowseName. If new copies are created, then the Attribute values of the InstanceDeclarations are used as the initial values. An instance shall not be a SourceNode of a hierarchical Reference that has the same BrowsePath as an InstanceDeclaration of its TypeDefinition which has an optional or mandatory ModellingRule except for the one based on the InstanceDeclaration.

Figure 27 provides a simple example of a TypeDefinitionNode and an Instance. Nodes referenced by the TypeDefinitionNode without a ModellingRule do not appear in the instance. Instances may have children with duplicate BrowseNames; however, only one of those children will correspond to the InstanceDeclaration.

It is up to the Server to decide which InstanceDeclarations appear in any single instance. In some cases, the Server will not define the entire instance and will provide remote references to Nodes in another Server. The ModellingRules described in 6.4.4.4 allow Servers to indicate that some Nodes are always present; however, the Client shall be prepared for the case where the Node exists in a different Server.

A Client can use the information of TypeDefinitionNodes to access Nodes which are in the hierarchy of the instance. It shall pass the NodeId of the instance and the BrowsePath of the child Nodes based on the TypeDefinitionNode to the TranslateBrowsePathsToNodeIds service (see OPC 10000-4). This Service returns the NodeId for each of the child Nodes. If a child Node exists then the BrowseName and NodeClass shall match the InstanceDeclaration. In the case of Objects or Variables, also the TypeDefinitionNode shall either match or be a subtype of the original TypeDefinitionNode.

6.4.3 Constraints on an Instance

Objects and Variables may change their Attribute values after being created. Special rules apply for some Attributes as defined in 6.2.7.

Additional References may be added to the Nodes, and References may be deleted as long as the ModellingRules defined on the InstanceDeclarations of the TypeDefinitionNode are still fulfilled.

For Variables and Objects the HasTypeDefinition Reference shall always point to the same TypeDefinitionNode as the InstanceDeclaration or a subtype of it.

If two InstanceDeclarations of the fully-inherited InstanceDeclarationHierarchy have been connected directly with several References, all those References shall connect the same Nodes. An example is given in Figure 28. The instances A1 and A2 are allowed since B1 references the same Node with both References, whereas A3 is not allowed since two different Nodes are referenced. Note that this restriction only applies for directly connected Nodes. For example, A2 references a C1 directly and a different C1 via B1.

6.4.4 ModellingRules

6.4.4.1 General

For a definition of ModellingRules, see 6.4.4.4. Other parts of this series of standards may define additional ModellingRules. ModellingRules are an extendable concept in OPC UA; therefore, vendors may define their own ModellingRules.

Note that the ModellingRules defined in this standard do not define how to deal with NonHierarchical References between InstanceDeclarations, i.e. it is Server-specific if those References exist in an instance hierarchy or not. Other ModellingRules may define behaviour for NonHierarchical References between InstanceDeclaration as well.

ModellingRules are represented in the AddressSpace as Objects of the ObjectType ModellingRuleType. There are some Properties defining common semantic of ModellingRules. This edition of this standard only specifies one Property for ModellingRules. Future editions may define additional Properties for ModellingRules. OPC 10000-5 specifies the representation of the ModellingRule Objects, their Properties and their type in the AddressSpace.

Subclause 6.4.4.3 defines how the ModellingRule may be changed when instantiating InstanceDeclarations with respect to the Properties. Subclause 6.4.4.2 defines how the ModellingRule may be changed when overriding InstanceDeclarations in subtypes with respect to the Properties.

6.4.4.2 Subtyping Rules for Properties of ModellingRules

It is allowed that subtypes override ModellingRules on their InstanceDeclarations. As a general rule for subtyping, constraints shall only be tightened, not loosened. Therefore, it is not allowed to specify on the supertype that an instance shall exist with the ModellingRule Mandatory and on the subtype make this ModellingRule Optional. Table 21 specifies the allowed changes on the Properties when overriding the ModellingRules in the subtype.

6.4.4.3 Instantiation Rules for Properties of ModellingRules

There are two different use cases when creating an instance ‘A’ based on a TypeDefinitionNode ‘A_Type’. Either ‘A’ is used as normal instance or it is used as an InstanceDeclaration of another TypeDefinitionNode.

In the first case, it is not required that newly created or referenced instances based on InstanceDeclarations have a ModellingRule, however, it is allowed that they have any ModellingRule independent of the ModellingRule of their InstanceDeclaration.

In Figure 29 an example is given. The instances A1, A2, and A3 are all valid instances of Type_A, although B of A1 has no ModellingRule and B of A3 has a different ModellingRule than B of Type_A.

In the second case, all instances that are referenced directly or indirectly from ‘A’ based on InstanceDeclarations of ‘A_Type’ initially maintain the same ModellingRule as their InstanceDeclarations. The ModellingRules may be updated; the allowed changes to the ModellingRules of these Nodes are the same as those defined for subtyping in 6.4.4.2.

In Figure 30 an example of such a scenario is given. Type_B uses an InstanceDeclaration based on Type_A (upper part of the Figure). Later on the ModellingRule of the InstanceDeclaration A1 is changed (lower part of the Figure). A1 has become the NamingRule of Mandatory (changed from Optional).

6.4.4.4 Standard ModellingRules

6.4.4.4.1 Mandatory

An InstanceDeclaration marked with the ModellingRule Mandatory means that for each existing BrowsePath on the instance a similar Node shall exist, but it is not defined whether a new Node is created or an existing Node is referenced.

For example, the TypeDefinitionNode of a functional block “AI_BLK_TYPE” will have a setpoint “SP1”. An instance of this type “AI_BLK_1” will have a newly-created setpoint “SP1” as a similar Node to the InstanceDeclaration SP1. Figure 31 illustrates the example.

In 6.4.4.4.2 a complex example combining the Mandatory and Optional ModellingRules is given.

6.4.4.4.2 Optional

An InstanceDeclaration marked with the ModellingRule Optional means that for each existing BrowsePath on the instance a similar Node may exist, but it is not defined whether a new Node is created or an existing Node is referenced.

In Figure 32 an example using the ModellingRules Optional and Mandatory is shown. The example contains an ObjectType Type_A and all valid combinations of instances named A1 to A13. Note that if the optional B is provided, the mandatory E has to be provided as well, otherwise not. F is referenced by C and D. On the instance, this can be the same Node or two different Nodes with the same BrowseName (similar Node to InstanceDeclaration F). Not considered in the example is if the instances have ModellingRules or not. It is assumed that each F is similar to the InstanceDeclaration F, etc.

If there would be a NonHierarchical Reference between E and F in the InstanceDeclaration­Hierarchy, it is not specified if it occurs in the instance hierarchy or not. In the case of A10, there could be a reference from E to one F but not to the other F, or to both or none of them.

6.4.4.4.3 ExposesItsArray

The ExposesItsArray ModellingRule exposes a special semantic on VariableTypes having a single- or multidimensional array as the data type. It indicates that each value of the array will also be exposed as a Variable in the AddressSpace.

The ExposesItsArray ModellingRule can only be applied on InstanceDeclarations of NodeClass Variable that are part of a VariableType having a single- or multidimensional array as its data type.

The Variable A having this ModellingRule shall be referenced by a forward hierarchical Reference from a VariableType B. B shall have a ValueRank value that is equal to or larger than zero. A should have a data type that reflects at least parts of the data that is managed in the array of B. Each instance of B shall reference one instance of A for each of its array elements. The used Reference shall be of the same type as the hierarchical Reference that connects B with A or a subtype of it. If there are more than one forward hierarchical References between A and B, then all instances based on B shall be referenced with all those References.

Figure 33 gives an example. A is an instance of Type_A having two entries in its value array. Therefore it references two instances of the same type as the InstanceDeclaration ArrayExpose. The BrowseNames of those instances are not defined by the ModellingRule. In general, it is not possible to get a Variable representing a specific entry in the array (e.g. the second). Clients will typically either get the array or access the Variables directly, so there is no need to provide that information.

It is allowed to reference A by other InstanceDeclarations as well. Those References have to be reflected on each instance based on A.

Figure 34 gives an example. The Property EUUnit is referenced by ArrayExpose and therefore each instance based on ArrayExpose references the instance based on the InstanceDeclaration EUUnit.

6.4.4.4.4 OptionalPlaceholder

For Object and Variable the intention of the ModellingRule OptionalPlaceholder is to expose the information that a complex TypeDefinition expects from instances of the TypeDefinition to add instances with specific References without defining BrowseNames for the instances. For example, a Device might have a Folder for DeviceParameters, and the DeviceParameters should be connected with a HasComponent Reference. However, the names of the DeviceParameters are specific to the instances. The example is shown in Figure 35, where an instance Device A adds two DeviceParameters in the Folder.

The ModellingRule OptionalPlaceholder adds no additional constraints on instances of the TypeDefinition. It just provides useful information when exposing a TypeDefinition. When the InstanceDeclaration is complex, i.e. it references other InstanceDeclarations with hierarchical References, these InstanceDeclarations are not further considered for instantiating the TypeDefinition.

It is recommended that the BrowseName and the DisplayName of InstanceDeclarations having the OptionalPlaceholder ModellingRule should be enclosed within angle brackets.

When overriding the InstanceDeclaration, the ModellingRule shall remain OptionalPlaceholder.

For Methods, the ModellingRule OptionalPlaceholder is used to define the BrowseName where subtypes and instances provide more information. The Method definition with the OptionalPlaceholder only defines the BrowseName. An instance or subtype defines the InputArguments and OutputArguments. A subtype shall also change the ModellingRule to Optional or Mandatory. The Method is optional for instances. For example, a Device might have a Method to perform calibration however the specific arguments for the Method depend on the instance of the Device. In this example Device A does not implement the Method, Device B implements the Method with no arguments and Device C implements the Method accepting a mode argument to select how the calibration is to be performed. The example is shown in Figure 36.

6.4.4.4.5 MandatoryPlaceholder

For Object and Variable the ModellingRule MandatoryPlaceholder has a similar intention as the ModellingRule OptionalPlaceholder. It exposes the information that a TypeDefinition expects of instances of the TypeDefinition to add instances defined by the InstanceDeclaration. However, MandatoryPlaceholder requires that at least one of those instances shall exist.

For example, when the DeviceType requires that at least one DeviceParameter shall exist without specifying the BrowseName for it, it uses MandatoryPlaceholder as shown in
Figure 37. Device A is a valid instance as it has the required DeviceParameter. Device B is not valid as it uses the wrong ReferenceType to reference a DeviceParameter (Organizes instead of HasComponent) and Device C is not valid because it does not provide a DeviceParameter at all.

The ModellingRule MandatoryPlaceholder requires that each instance provides at least one instance with the TypeDefinition of the InstanceDeclaration or a subtype, and is referenced with the same ReferenceType or a subtype as the InstanceDeclaration. It does not require a specific BrowseName and thus cannot be used for the TranslateBrowsePathsToNodeIds Service (see OPC 10000-4).

When the InstanceDeclaration is complex, i.e. it references other InstanceDeclarations with hierarchical References, these InstanceDeclarations are not further considered for instantiating the TypeDefinition.

It is recommended that the BrowseName and the DisplayName of InstanceDeclarations having the MandatoryPlaceholder ModellingRule should be enclosed within angle brackets.

When overriding the InstanceDeclaration, the ModellingRule shall remain MandatoryPlaceholder.

For Methods, the ModellingRule MandatoryPlaceholder is used to define the BrowseName where subtypes and instances provide more information. The Method definition with the MandatoryPlaceholder only defines the BrowseName. An instance or subtype defines the InputArguments and OutputArguments. A subtype shall also change the ModellingRule to Mandatory. The Method is mandatory for instances.

6.5 Changing Type Definitions that are already used

1. There is no behaviour specified regarding subtypes and instances when changing ObjectTypes and VariableTypes. It is Server-dependent, if those changes are reflected on the subtypes and instances of the types. However, all constraints defined for subtypes and instances shall be fulfilled. For example, it is not allowed to add a Property using the ModellingRule Mandatory on a type if instances of this type exist without the Property. In that case, the Server either has to add the Property to all instances of the type or adding the Property on the type has to be rejected.

7 Standard ReferenceTypes

7.1 General

This standard defines ReferenceTypes as an inherent part of the OPC UA Address Space Model. Figure 38 informally describes the hierarchy of these ReferenceTypes. Other parts of this series of standards may specify additional ReferenceTypes. The remainder of 7 defines the ReferenceTypes. OPC 10000-5 defines their representation in the AddressSpace.

7.2 References ReferenceType

The References ReferenceType is an abstract ReferenceType; only concrete subtypes of it shall be used as a Reference between Nodes in an OPC UA AddressSpace.

There is no semantic associated with this ReferenceType. This is the base type of all ReferenceTypes. All ReferenceTypes shall be a subtype of this base ReferenceType – either direct or indirect. The main purpose of this ReferenceType is allowing simple filter and queries in the corresponding Services of OPC 10000-5.

There are no constraints defined for this abstract ReferenceType.

7.3 HierarchicalReferences ReferenceType

The HierarchicalReferences ReferenceType is an abstract ReferenceType; only subtypes of it can be used and they shall be a non-symmetric Reference.

The semantic of HierarchicalReferences is to denote that References of HierarchicalReferences span a hierarchy. It means that it may be useful to present Nodes related with References of this type in a hierarchical-like way. HierarchicalReferences does not forbid loops. For example, starting from Node “A” and following HierarchicalReferences it may be possible to browse to Node “A”, again.

It is not permitted to have a Property as SourceNode of a Reference of any subtype of this abstract ReferenceType.

It is not allowed that the SourceNode and the TargetNode of a Reference of the ReferenceType HierarchicalReferences are the same, that is, it is not allowed to have self-references using HierarchicalReferences.

7.4 NonHierarchicalReferences ReferenceType

The NonHierarchicalReferences ReferenceType is an abstract ReferenceType; only subtypes of it can be used.

The semantic of NonHierarchicalReferences is to denote that its subtypes do not span a hierarchy and should not be followed when trying to present a hierarchy. To distinguish Hierarchical and NonHierarchical References, all concrete ReferenceTypes shall inherit from either hierarchical References or Non-hierarchical References, either direct or indirect.

There are no constraints defined for this abstract ReferenceType.

7.5 HasChild ReferenceType

The HasChild ReferenceType is an abstract ReferenceType; only subtypes of it can be used. It is a subtype of HierarchicalReferences.

The semantic is to indicate that References of this type span a non-looping hierarchy.

Starting from Node “A” and only following References of the subtypes of the HasChild ReferenceType it shall never be possible to return to “A”. But it is allowed that following the References there may be more than one path leading to another Node “B”.

7.6 Aggregates ReferenceType

The Aggregates ReferenceType is an abstract ReferenceType; only subtypes of it can be used. It is a subtype of HasChild.

The semantic is to indicate a part (the TargetNode) belongs to the SourceNode. It does not specify the ownership of the TargetNode.

There are no constraints defined for this abstract ReferenceType.

7.7 HasComponent ReferenceType

The HasComponent ReferenceType is a concrete ReferenceType that can be used directly. It is a subtype of the Aggregates ReferenceType.

The semantic is a part-of relationship. The TargetNode of a Reference of the HasComponent ReferenceType is a part of the SourceNode. This ReferenceType is used to relate Objects or ObjectTypes with their containing Objects, DataVariables, and Methods. This ReferenceType is also used to relate complex Variables or VariableTypes with their DataVariables.

Like all other ReferenceTypes, this ReferenceType does not specify anything about the ownership of the parts, although it represents a part-of relationship semantic. That is, it is not specified if the TargetNode of a Reference of the HasComponent ReferenceType is deleted when the SourceNode is deleted.

The TargetNode of this ReferenceType shall be a Variable, an Object or a Method.

If the TargetNode is a Variable, the SourceNode shall be an Object, an ObjectType, a DataVariable or a VariableType. By using the HasComponent Reference, the Variable is defined as DataVariable.

If the TargetNode is an Object or a Method, the SourceNode shall be an Object or ObjectType.

7.8 HasProperty ReferenceType

The HasProperty ReferenceType is a concrete ReferenceType that can be used directly. It is a subtype of the Aggregates ReferenceType.

The semantic is to identify the Properties of a Node. Properties are described in 4.5.2.

The SourceNode of this ReferenceType can be of any NodeClass. The TargetNode shall be a Variable. By using the HasProperty Reference, the Variable is defined as Property. Since Properties shall not have Properties, a Property shall never be the SourceNode of a HasProperty Reference.

7.9 HasOrderedComponent ReferenceType

The HasOrderedComponent ReferenceType is a concrete ReferenceType that can be used directly. It is a subtype of the HasComponent ReferenceType.

The semantic of the HasOrderedComponent ReferenceType – besides the semantic of the HasComponent ReferenceType – is that when browsing from a Node and following References of this type or its subtype all References are returned in the Browse Service defined in OPC 10000-4 in a well-defined order. The order is Server-specific, but the Client can assume that the Server always returns them in the same order.

There are no additional constraints defined for this ReferenceType.

7.10 HasSubtype ReferenceType

The HasSubtype ReferenceType is a concrete ReferenceType that can be used directly. It is a subtype of the HasChild ReferenceType.

The semantic of this ReferenceType is to express a subtype relationship of types. It is used to span the ReferenceType hierarchy, whose semantic is specified in 5.3.3.3; a DataType hierarchy is specified in 5.8.3, and other subtype hierarchies are specified in Clause 6.

The SourceNode of References of this type shall be an ObjectType, a VariableType, a DataType or a ReferenceType and the TargetNode shall be of the same NodeClass as the SourceNode. Each ReferenceType shall be the TargetNode of at most one Reference of type HasSubtype.

7.11 Organizes ReferenceType

The Organizes ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of HierarchicalReferences.

The semantic of this ReferenceType is to organise Nodes in the AddressSpace. It can be used to span multiple hierarchies independent of any hierarchy created with the non-looping Aggregates References.

The SourceNode of References of this type shall be an Object, ObjectType or a View. If it is an Object then it should be an Object of the ObjectType FolderType or one of its subtypes (see 5.5.3).

The TargetNode of this ReferenceType can be of any NodeClass.

7.12 HasModellingRule ReferenceType

The HasModellingRule ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of NonHierarchicalReferences.

The semantic of this ReferenceType is to bind the ModellingRule to an Object, Variable or Method. The ModellingRule mechanisms are described in 6.4.4.

The SourceNode of this ReferenceType shall be an Object, Variable or Method. The TargetNode shall be an Object of the ObjectType “ModellingRule” or one of its subtypes.

Each Node shall be the SourceNode of at most one HasModellingRule Reference.

7.13 HasTypeDefinition ReferenceType

The HasTypeDefinition ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of NonHierarchicalReferences.

The semantic of this ReferenceType is to bind an Object or Variable to its ObjectType or VariableType, respectively. The relationships between types and instances are described in 4.6.

The SourceNode of this ReferenceType shall be an Object or Variable. If the SourceNode is an Object, then the TargetNode shall be an ObjectType; if the SourceNode is a Variable, then the TargetNode shall be a VariableType.

Each Variable and each Object shall be the SourceNode of exactly one HasTypeDefinition Reference.

7.14 HasEncoding ReferenceType

The HasEncoding ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of NonHierarchicalReferences.

The semantic of this ReferenceType is to reference DataTypeEncodings of a subtype of the Structure DataType.

The SourceNode of References of this type shall be a subtype of the Structure DataType.

The TargetNode of this ReferenceType shall be an Object of the ObjectType DataTypeEncodingType or one of its subtypes (see 5.8.4).

7.15 GeneratesEvent

The GeneratesEvent ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of NonHierarchicalReferences.

The semantic of this ReferenceType is to identify the types of Events instances of ObjectTypes or VariableTypes may generate and Methods may generate on each Method call.

The SourceNode of References of this type shall be an ObjectType, a VariableType or a Method InstanceDeclaration.

The TargetNode of this ReferenceType shall be an ObjectType representing EventTypes, that is, the BaseEventType or one of its subtypes.

7.16 AlwaysGeneratesEvent

The AlwaysGeneratesEvent ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of GeneratesEvent.

The semantic of this ReferenceType is to identify the types of Events Methods have to generate on each Method call.

The SourceNode of References of this type shall be a Method InstanceDeclaration.

The TargetNode of this ReferenceType shall be an ObjectType representing EventTypes, that is, the BaseEventType or one of its subtypes.

7.17 HasEventSource

The HasEventSource ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of HierarchicalReferences.

The semantic of this ReferenceType is to relate event sources in a hierarchical, non-looping organization. This ReferenceType and any subtypes are intended to be used for discovery of Event generation in a Server. They are not required to be present for a Server to generate an Event from its source (causing the Event) to its notifying Nodes. In particular, the root notifier of a Server, the Server Object defined in OPC 10000-5, is always capable of supplying all Events from a Server and as such has implied HasEventSource References to every event source in a Server.

The SourceNode of this ReferenceType shall be an Object or View that is a source of Event Subscriptions. A source of Event Subscriptions is an Object or View that has its “SubscribeToEvents” bit set within the EventNotifier Attribute. The SourceNode may also be an ObjectType when referencing an InstanceDeclaration where an instance of the ObjectType containing the InstanceDeclaration generates events. Note the ObjectType is not considered a source of Event Subscriptions.

The TargetNode of this ReferenceType can be a Node of any NodeClass that can generate event notifications via a subscription to the reference source.

Starting from Node “A” and only following References of the HasEventSource ReferenceType or of its subtypes it shall never be possible to return to “A”. But it is permitted that, following the References, there may be more than one path leading to another Node “B”.

7.18 HasNotifier

The HasNotifier ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of HasEventSource.

The semantic of this ReferenceType is to relate Object Nodes that are notifiers with other notifier Object Nodes. The ReferenceType is used to establish a hierarchical organization of event notifying Objects. It is a subtype of the HasEventSource ReferenceType defined in 7.16.

The TargetNode of this ReferenceType shall be Objects that are a source of Event Subscriptions.

If the TargetNode of a Reference of this type generates an Event, then this Event shall also be provided in the SourceNode of the Reference.

An example of a possible organization of Event References is represented in Figure 39. In this example an unfiltered Event subscription directed to the “Pump” Object will provide the Event sources “Start” and “Stop” to the subscriber. An unfiltered Event subscription directed to the “Area 1” Object will provide Event sources from “Machine B”, “Tank A” and all notifier sources below “Tank A”.

A second example of a more complex organization of Event References is represented in Figure 40. In this example, explicit References are included from the Server’s Server Object, which is a source of all Server Events. A second Event organization has been introduced to collect the Events related to “Tank Farm 1”. An unfiltered Event subscription directed to the “Tank Farm 1” Object will provide Event sources from “Tank B”, “Tank A” and all notifier sources below “Tank B” and “Tank A”.

7.19 HasInterface ReferenceType