(PDF) OPC UA Address Space Interchange XML

MANUAL

ADDRESS SPACE INTERCHANGE XML

Copyright CAS



2016

translated, or transmitted in any form or by any means, electronic, mechanical, photocopying,

recording, or otherwise, without prior written permission of CAS.

Product

Address Space Model Designer

release

1.1

Document

P-150101E02-AddressSpaceInterchangeXML

Date

13 April 2016

Authors

Mariusz Postół, PhD. Eng. (Project Manager)

Company

CAS

Poland 90-527 Łódź, ul. Wólczańska 124/134

tel/fax: (42) 686 25 47; (42) 686 50 28

mpostol@cas.eu

http://www.cas.eu

http://www.commsvr.com

Table of Contents

Overview.................................................................................................................................................. 2

Address Space Model .......................................................................................................................... 2

Address Space Representation ............................................................................................................ 2

Conventions ......................................................................................................................................... 3

UANodeSet ........................................................................................................................................... 3

UAVariable and UAVariableType ................................................................................................ 5

Introduction ......................................................................................................................................... 5

XML representation ............................................................................................................................. 6

Value ................................................................................................................................................ 6

DataType ......................................................................................................................................... 6

ValueRank ........................................................................................................................................ 6

ArrayDimensions ............................................................................................................................. 7

TranslationType ............................................................................................................................... 7

AccessLevel ...................................................................................................................................... 8

UserAccessLevel .............................................................................................................................. 9

MinimumSamplingInterval ............................................................................................................ 10

Historizing ...................................................................................................................................... 10

UAObject and UAObjectType ................................................................................................................ 10

UAMethod ............................................................................................................................................. 10

MethodDeclarationId ........................................................................................................................ 11

Executable ......................................................................................................................................... 11

UserExecutable .................................................................................................................................. 11

UADataType ........................................................................................................................................... 11

UAView .................................................................................................................................................. 13

UAReferenceType .................................................................................................................................. 14

InverseName ..................................................................................................................................... 15

Symmetric .......................................................................................................................................... 15

Base types .............................................................................................................................................. 16

UAType .............................................................................................................................................. 16

UAInstance ........................................................................................................................................ 16

UANode ............................................................................................................................................. 16

NodeId ........................................................................................................................................... 17

BrowseName ................................................................................................................................. 17

SymbolicName ............................................................................................................................... 18

WriteMask ..................................................................................................................................... 18

Overview

UserWriteMask .............................................................................................................................. 18

DisplayName .................................................................................................................................. 19

Description .................................................................................................................................... 19

Category ........................................................................................................................................ 19

Documentation .............................................................................................................................. 19

References ..................................................................................................................................... 19

Extensions ...................................................................................................................................... 20

EventNotifier ..................................................................................................................................... 20

Variant ............................................................................................................................................... 20

Data encoding........................................................................................................................................ 21

General .............................................................................................................................................. 21

Overview ........................................................................................................................................ 21

Built-in Types ................................................................................................................................. 21

Built-in Types Syntax ..................................................................................................................... 21

Example ................................................................................................................................................. 30

UANodeSetChanges .............................................................................................................................. 32

Address Space Interchange XML

Overview

Address Space Model

Each OPC UA Server interchanges data with OPC UA Clients engaging the Address Space concept.

More details about information modeling are captured in the whitepaper, which can be found here:

OPC UA Information Model Deployment

Address Space instantiated by a server allows publication of the underlying process data in a

standard way as a collection of NodeClass instances created according to the strictly observed rules

defined by a not expandable Address Space Model. Each node in the Address Space is an instance of

one of these NodeClasses.

As it was described in the article mentioned above the content of the particular Address Space is

formally determined by a set of selected Information Models, which control:

 What Nodes are created;

 Nodes relationship (interconnections);

 Value of selected attributes of the Nodes.

Information Model is an expandable concept, hence the basic Information Model has been

already expanded by some companion specifications and it is expected that this process will continue

in the future as a result of adopting the standard to diverse application domains. Each Information

Model is assigned a unique identifier to precisely distinguish a set of definitions contained in it. Any

definition must be part of one Information Model only.

Address Space Representation

To instantiate the Address Space according to a selected collection of Information Models a

reusable OPC UA Server needs a description of it. The OPC UA standard defines a formal definition of

the syntax and semantics of documents used for this purpose. An XML Schema is defined to describe

syntax. The semantics of a compliant with this schema document is explained in this document.

The latest definition of the schema can be found at:

http://opcfoundation.org/UA/2011/03/UANodeSet.xsd

Generally speaking the document compliant with the schema provides two kinds of information:

 Address Space metadata – list of nodes, their relations and attribute values;

 Instruction how to create and maintain the Address Space.

UANodeSet

3

The XML document compliant with the schema shall contain definitions of all the nodes including

instance declarations defined by a selected Information Model. All the values must be evaluated as a

result of inheritance chains processing. More can be found in an eBook available at:

http://www.commsvr.com/UAModelDesigner/Index.aspx

The XML document compliant with the schema may contain only a partial representation of a

selected Address Space. For example this schema can be used to import or export an arbitrary set of

nodes in the server Address Space. This partial representation can be used to save the server state

for use by the server later on or to exchange with other applications, e.g. to:

 support offline configuration by a client,

 configure design tools.

Conventions

For convenience the common “field” term is also used as reference to elements or attributes

defined by the schema.

 NodeId: XML identifiers are emphasized using bold blue color.

 NodeId: Address Space Model terms emphasized using italic grey color.

UANodeSet

The UANodeSet is the root element of the XML document. The fields of the element are shown

in Table 1.

UANodeSet

4

Table 1 UANodeSet

Field

XML Type

Description

NamespaceUris

UriTable

A list of namespaces represented by URI referred in the document. External references

are allowed.

ServerUris

UriTable

A list of ServerUri entries used in the UANodeSet document.

Models

ModelTableEntry[]

A list of Models that are defined in the UANodeSet along with any dependencies

these models have.

ModelUri

string

The URI for the model. This ModelUri should be one of the entries in the

NamespaceUris table.

Version

string

The version of the model defined in the UANodeSet document. This is a human

readable string and not intended for programmatic comparisons.

PublicationDate

dateTime

When the model was published. This value is used for comparisons if the Information

Model is defined in multiple UANodeSet documents.

RequiredModels

ModelTableEntry[]

A list of dependencies for the model. If the model requires a minimum version the

PublicationDate shall be specified. Tools which attempt to resolve these

dependencies may accept any PublicationDate after this date.

Aliases

AliasTable

A list of aliases used in the UANodeSet document.

Extensions

xs:any

An element containing any vendor defined extensions to the UANodeSet.

LastModified

dateTime

The last time a document was modified.

UAObject

UAVariable

UAMethod

UAView

UAObjectType

UAVariableType

UADataType

UAReferenceType

The Address Space nodes collection representation in the UANodeSet.

The NamespaceUri is a list of URIs for namespaces referred to a set of selected OPC UA

Information Models used by the UANodeSet. The NamespaceIndex used in NodeId,

ExpandedNodeIds and QualifiedNames identifies an element in the list. The first index is

always 1 (0 is always the OPC UA namespace). OPC UA namespace is not added to the table. At index

0 the table contains the namespace referred to the Information Model domain included in the XML

document.

The ServerUris is a list of URIs for servers referenced in the UANodeSet. The

ServerIndex in ExpandedNodeIds identifies an element in that list. The first index is always 1

(0 is always the current server).

The Mmdels element (added to the schema rel. 1.03) specifies the models, which are formally

defined by the UANodeSet. It includes version information as well as information about any

dependencies the model may have.

The Aliases are a list of string elements that can be used to substitute the numeric part (of

type i) of NodeId (section NodeId). Aliases can be used to make the file more readable by allowing

a string like ‘HasProperty’ to be used in place of a numeric NodeId (i=46). Aliases are optional.

UAObject, UAVariable, UAMethod, UAView are derived from UAInstance described

in the section UAInstance. UAObjectType, UAVariableType, UADataType,

UAReferenceType are derived from UAType described in the section UAType.

The Extensions enables to extend the XML document with elements not specified by the

schema that can be used to attach custom vendor data to the UANodeSet.

UAVariable and UAVariableType

5

UAVariable and UAVariableType

Introduction

In the Address Space Variable NodeClass is used to provide a value, which may be simple or

complex. In the UANodeSet document this NodeClass is represented by UAVariable type. A

UAVariable is a subtype of the UAInstance defined in UAInstance.

The type of the Variable is defined by VariableType NodeClass that is represented in the

UANodeSet document by the UAVariableType. A UAVariableType is a subtype of the

UAType defined in the section UAType.

Variables are always defined as Properties or DataVariables and are exposed as components of

other nodes in the Address Space. They are never defined by themselves. A Variable node is always

part of at least one other node, but may be related to any number of other nodes. These rules must

be also observed in the UANodeSet document.

Properties are used to define the characteristics of nodes. Properties are instantiated using the

Variable NodeClass.

Properties are the leaf of any hierarchy; therefore they shall not be the source node of any

hierarchical references. This includes the references HasComponent or HasProperty types, that is,

Properties do not contain Properties and, hence, cannot have complex structure. However, they may

be the source node of any non-hierarchical references.

Properties shall always be defined in the context of another node and shall be the target node of

at least one HasProperty reference. To distinguish them from DataVariables, they shall not be the

target node of any reference of the HasComponent type. Thus, a reference of the HasComponent

type 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. Any node is not

permitted to refer to two Variables using references of the HasProperty type having the same

BrowseName.

DataVariables represent the content of an object. DataVariables are instantiated using the

Variable NodeClass.

DataVariables identify their Properties using references of the HasProperty type. Complex

DataVariables use HasComponent references to expose their component DataVariables.

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.

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. Each DataVariable shall, therefore, be the target node of at least one

HasComponent reference coming from an Object, an ObjectType, a DataVariable or a VariableType.

UAVariable and UAVariableType

6

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 reference of HasTypeDefinition type points to the VariableType used as type definition of the

DataVariable.

If the DataVariable is used as instance declaration all nodes referenced with hierarchical

references in the forward direction shall have unique BrowseNames in the context of this

DataVariable.

XML representation

The fields in the UAVariable and UAVariableType types are defined in Table 2.

Table 2 – UAVariable and UAVariableType fields

Field

XML Type

Description

Value

Variant

The Value of the node encoding using the UA XML wire encoding.

DataType

NodeId

The data type of the value.

ValueRank

The value rank.

ArrayDimensions

The number of dimensions in an array value.

UAVariable only

Translation

TranslationType[]

A list of translations for the Value if the Value is a LocalizedText or a

structure containing LocalizedTexts.

See the TranslationType section for more information.

AccessLevel

The access level.

UserAccessLevel

AccessLevel

The access level for the current user.

MinimumSamplingInterval

Duration

The minimum sampling interval.

Historizing

Boolean

Whether history is being archived.

ArrayDimensions is ignored if ValueRank is not equal to the OneOrMoreDimensions.

Value

The most recent value of the Variable node that the server has. Its data type is defined by the

DataType field.

DataType

NodeId of the DataType definition for the Value attribute of the represented NodeClass. The

value of this field must be a valid NodeId (see the section NodeId). It is not required that the

pointed out element is defined in the same NodeSet document. If that is the case many documents

must be combined to resolve and validate this reference.

ValueRank

This field indicates whether the Value attribute of the Variable NodeClass is an array and how

many dimensions the array has.

It may have the following values:

 n > 1: the Value is an array with the specified number of dimensions.

 OneDimension (1): The value is an array with one dimension.

UAVariable and UAVariableType

7

 OneOrMoreDimensions (0): The value is an array with one or more dimensions.

 Scalar (−1): The value is not an array.

 Any (−2): The value can be a scalar or an array with any number of dimensions.

 ScalarOrOneDimension (−3): The value can be a scalar or a one dimensional array.

NOTE: All DataTypes are considered to be scalar, even if they have array-like semantics like

ByteString and String.

ArrayDimensions

This field specifies the length of each dimension for an array value. It is intended to describe the

capability of the Variable, not the current size.

The number of elements shall be equal to the value defined by the ValueRank. It shall be null if

ValueRank ≤ 0. The value of 0 for an individual dimension indicates that the dimension has a

variable length. For example, if a Variable is defined by the following C array:

Int32 myArray[346];

then the DataType would point to an Int32, the ValueRank has the value 1 and the

ArrayDimensions is an array with one entry having the value 346.

Note that the maximum length of an array transferred on the wire is 2147483647 (max Int32)

and a multidimensional array is encoded as a one dimensional array.

TranslationType

This element applies only to UAVariable element and may be omitted. If the Value is an

array the number of elements in this array shall match the number of elements in the Value. Extra

elements are ignored.

If the Value is a scalar, there is one element in this array.

If the Value is a structure, each element contains translations for one or more fields identified

by a name.

The TranslationType contains additional translations for LocalizedText entities used in

the Value of the Variable NodeClass. The fields in the TranslationType are defined in Table 3. If

multiple arguments exist there will be a Translation element for each argument.

The type can have two forms depending on whether the Value is a LocalizedText or a Structure

DataType containing a LocalizedText list. If it is a LocalizedText it contains a simple list of translations.

If it is a Structure it contains a list of fields which each contain a list of translations. Each field is

identified by a Name which is a unique name of a field within the Structure. The mapping between

the Name and the Structure requires an understanding of the Structure encoding. If the Structure

field is encoded as a LocalizedText with UA XML, the name is an unqualified path to the XML

element where names in the path are separated by ‘/’. For example, a structure with a nested

structure containing a LocalizedText could have a path like “Server/ApplicationName”.

UAVariable and UAVariableType

8

The following example illustrates how translations for the Description field in the Argument

Structure are represented in XML:

<Value>

</TypeId>

<Body>

<Name>ConfigData</Name>

</DataType>

<Text>[English Translation for Description]</Text>

</Description>

</Argument>

</Body>

</ExtensionObject>

</ListOfExtensionObject>

</Value>

<Text Locale="de-DE">[German Translation for Description]</Text>

<Text Locale="fr-FR">[French Translation for Description]</Text>

</Field>

</Translation>

More information about the ExtensionObject element is captured in the section

ExtensionObject.

Table 3 – TranslationType

Element

Type

Description

Text

LocalizedText[]

An array of translations for the Value.

It only appears if the Value is a LocalizedText or an array of

LocalizedText.

Field

StructureTranslationType[]

An array of structure fields which have translations.

It only appears if the Value is a Structure or an array of Structures.

Name

string

The name of the field.

This uniquely identifies the field within the structure.

The exact mapping depends on the encoding of the structure.

Text

LocalizedText[]

An array of translations for the structure field.

AccessLevel

The AccessLevel attribute indicates the accessibility of the Value of a Variable node not

taking user access rights into account and applies only to a UAVariable element. The

AccessLevel attribute is used to indicate how the Value of a Variable node can be accessed

(read/write) and if it contains current and/or historic data. The AccessLevel does not take any

user access rights into account, i.e. although the Variable is writable this may be restricted to a

certain user / user group.

The AccessLevel is an 8-bit unsigned integer with the structure defined in the Table 4.

UAVariable and UAVariableType

9

Table 4 – AccessLevel bits meaning

Field

Bit

Description

CurrentRead

0

Indicates if the current value is readable; (0 means not readable, 1 means readable).

CurrentWrite

1

Indicates if the current value is writable (0 means not writable, 1 means writable).

HistoryRead

2

Indicates if the history of the value is readable (0 means not readable, 1 means readable).

HistoryWrite

3

Indicates if the history of the value is writable (0 means not writable, 1 means writable).

SemanticChange

4

Indicates if the Variable used as Property generates SemanticChangeEvents (see 9.33).

StatusWrite

5

Indicates if the current StatusCode of the value is writable (0 means not writable, 1 means writable).

TimestampWrite

6

Indicates if the current SourceTimestamp is writable (0 means not writable, 1 means writable).

Reserved

7

Reserved for future use. Shall always be zero.

The first two bits also indicate if a current value of this Variable is available and the other two bits

indicate if the history of the Variable is available in the OPC UA Server Address Space.

The SemanticChange bit of the AccessLevel shall be set when the Property describes the

semantics of the node that owns the Property and changes of the Property value will generate

SemanticChangeEvents. For example, a Property describing the engineering unit of a DataVariable

will have a bit set, whereas a Property containing an Icon of the DataVariable will not. This behavior

is exactly the same as described by the SemanticsChanged bit of the StatusCode. However,

subscribing to a Variable one should look at the StatusCode to identify if the semantics has changed

in order to receive this information before processing the value of the Variable.

UserAccessLevel

The UserAccessLevel attribute is used to indicate how the Value attribute of a Variable

NodeClass can be accessed (read/write) and if it contains current or historic data taking user access

rights into account. It applies only to a UAVariable element. If the OPC UA Server does not have

the ability to get any user access rights related information from the underlying system it should use

the same bit mask as used in the AccessLevel attribute. The UserAccessLevel attribute can

restrict the accessibility indicated by the AccessLevel, but not exceed it.

The UserAccessLevel is an 8-bit unsigned integer with the structure defined in the table

Table 5.

Table 5 – UserAccessLevel bits meaning.

Field

Bit

Description

CurrentRead

0

Indicates if the current value is readable

(0 means not readable, 1 means readable).

CurrentWrite

1

Indicates if the current value is writable

(0 means not writable, 1 means writable).

HistoryRead

2

Indicates if the history of the value is readable

(0 means not readable, 1 means readable).

HistoryWrite

3

Indicates if the history of the value is writable (0 means not writable, 1 means writable).

Reserved

4

Reserved for future use. Shall always be zero.

StatusWrite

5

Indicates if the current StatusCode of the value is writable

(0 means not writable, 1 means writable).

TimestampWrite

6

Indicates if the current SourceTimestamp of the value is writable

(0 means not writable, 1 means writable).

Reserved

7

Reserved for future use. Shall always be zero.

The first two bits also indicate if a current value of this Variable is available and the other two bits

indicate if the history of the Variable is available via the OPC UA Server.

UAObject and UAObjectType

10

MinimumSamplingInterval

The MinimumSamplingInterval attribute indicates how “current” the Value of the Variable

NodeClass will be kept. It specifies (in milliseconds) 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 the system load and other factors.

A MinimumSamplingInterval of 0 indicates that the server is to monitor the item

continuously. A MinimumSamplingInterval of -1 means indeterminate value.

Historizing

The Historizing attribute indicates whether the server is actively collecting data for the

history of the Variable node.

This differs from the AccessLevel which identifies if the Variable has any historical data. A

value of TRUE indicates that the server is actively collecting data. A value of FALSE indicates the

server is not actively collecting data. Default value is FALSE.

UAObject and UAObjectType

Objects are used to represent systems, system components, real-world objects and software

objects.

In the Address Space Object NodeClass is used to represent systems, system components, real-

world objects and software objects. In the UANodeSet document this NodeClass is represented by a

UAObject type. A UAObject is a subtype of the UAInstance defined in the section

UAInstance.

In the Address Space the type of the Object NodeClass is defined by ObjectType NodeClass that is

represented in the UANodeSet document by the UAObjectType. The UAObjectType type is a

subtype of the UAType defined in the section UAType and does not extend this definition.

The UAObject defines an additional attribute, EventNotifier. The EventNotifier

attribute is used to indicate if the node can be used to subscribe to events or the read/write historic

events.

The EventNotifier is an 8-bit unsigned integer with the structure defined in Section

EventNotifier.

UAMethod

In the Address Space the Methods NodeClass defines callable functions. Methods are invoked

remotely from the client using the call service. The invocations always run to completion and always

return responses when complete.

UADataType

11

Methods are represented in the UANodeSet document using the UAMethod type. UAMethod

is a subtype of the UAInstance. It represents a Method NodeClass. The fields in the UAMethod

type are defined in Table 6.

Table 6 – UAMethod

Element

Type

Description

MethodDeclarationId

NodeId

The NodeId of a UAMethod in the subtype of the parent defiition.

Executable

boolean

Indicates whether the Method node is executable (False means not executable, True means

executable)

UserExecutable

boolean

Indicates whether the Method node is executable (False means not executable, True means

executable) taking user access rights into account.

MethodDeclarationId

This attribute may be specified for a Method node that is a target of a HasComponent reference

from a single Object node. It is the NodeId of the UAMethod with the same BrowseName

contained in the TypeDefinition associated with the Object node. If the TypeDefinition

overrides a Method inherited from a base ObjectType,this attribute shall reference the Method node

in the subtype.

Executable

The Executable attribute indicates whether the Method node is executable (“False” means

not executable, “True” means executable), not taking user access rights into account. If the server

cannot get the executable information from the underlying system, it should state that it is

executable. If a Method is called, the server should transfer this request and return the

corresponding StatusCode if such a request is rejected.

UserExecutable

The UserExecutable attribute indicates if the Method is currently executable taking user

access rights into account (“False” means not executable, “True” means executable).

UADataType

In the Address Space the DataType node 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.

Each Variable and VariableType is, therefore, pointing with its DataType attribute to a node of the

DataType NodeClass.

Nodes of the DataType NodeClass are represented in the USNodeSet document using the

UADataType element. A UADataType is a subtype of the UAType defined in the section UAType.

The fields in the UADataType type are defined in Table 7.

UADataType

12

Table 7 – UADataType

Field

Type

Description

Definition

DataTypeDefinition

A definition of the data type.

The Definition element is a definition of the type that can be used by design tools to create

code that can serialize any compatible value using OPC UA XML and/or OPC UA Binary encodings. It

does not appear in the Address Space. This is only used to define subtypes of the Structure or

Enumeration standard DataTypes.

A DataTypeDefinition defines an abstract representation of a UADataType that can be

used by design tools to automatically create serialization code. The fields in the

DataTypeDefinition type are defined in Table 8.

Table 8 – DataTypeDefinition

Field

XML Type

Description

Name

QualifiedName

A unique name for the data type encoded usin the syntax described in the section

QualifiedName.

This field is only specified for nested DataTypeDefinitions.

The BrowseName of the DataType node is used otherwise.

SymbolicName

A symbolic name for the data type that can be used as a class/structure name in

autogenerated code. It should only be specified if the Name cannot be used for this purpose.

Only letters, digits or the underscore (‘_’) are permitted.

This field is only specified for nested DataTypeDefinitions.

The SymbolicName of the DataType node is used otherwise.

BaseType

QualifiedName

The name of any base type.

Note that the BaseType can refer to types defined in other files.

The NamespaceUri associated with the Name should indicate where to look for the

BaseType definition.

This field is only specified for nested DataTypeDefinitions.

The HasSubtype reference of the DataType NodeClass is used otherwise.

IsUnion

Boolean

This flag indicates if the data type represents a union.

Only one of the Fields defined for the data type is encoded into a value.

This field is optional. The default value is false.

If this value is true the first field is the switch value.

Field

DataTypeField[]

The list of fields that make up the data type.

This definition assumes the structure has a sequential layout.

For enumerations the fields are simply a list of values.

The Field element of the DataTypeField type defines an abstract representation of a field

within a UADataType that can be used by design tools to automatically create serialization code.

The fields in the DataTypeField type are defined in Table 9.

UAView

13

Table 9 – DataTypeField

Element

Type

Description

Name

string

A name for the field that is unique within the DataTypeDefinition.

SymbolicName

string

A symbolic name for the field that can be used in autogenerated code.

It should only be specified if the Name cannot be used for this purpose.

Only letters, digits or the underscore (‘_’) are permitted.

DataType

NodeId

The NodeId of the DataType for the field.

This NodeId can refer to another Node with its own DataTypeDefinition.

This field is not specified for subtypes of Enumeration.

ValueRank

The value rank for the field.

It shall be Scalar (-1) or a fixed rank Array (>=1).

This field is not specified for subtypes of Enumeration.

Description

LocalizedText[]

A description for the field in multiple locales.

Definition

DataTypeDefinition

The field is a structure with a layout specified by the definition.

This field is optional.

This field allows designers to create nested structures without defining a new

DataType Node for each structure.

This field is not specified for subtypes of Enumeration.

Value

int

The value associated with the field.

This field is only specified for subtypes of Enumeration.

IsOptional

boolean

The field indicates if a data type field in a structure is optional.

This field is optional. The default value is false.

This field is not specified for subtypes of Enumeration and Union.

UAView

Underlying systems are often large and clients could 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 Address Space for

which they have no interest.

To address this problem, the OPC UA standard defines the concept of a view. Each view defines

a subset of the nodes in the Address Space. The entire Address Space 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, and are

represented in the UANodeSet document by the UAView type. This type is specified in Table 10.

All nodes contained in a view shall be accessible starting from the root View node when browsing

in the context of the view. It is not expected that all contained 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 an additional entry point into the Address Space but also as

a construct to organize the Address Space and thus as the only entry point into a subset of the

Address Space. Therefore, clients shall not ignore view nodes when browsing the Address Space.

Simple clients that do not deal with views for filtering purposes can, for example, handle a View node

like an object of the type FolderType.

Table 10 – UAView

Element

Type

Description

ContainsNoLoops

boolean

Indication if following the references in the context of the view there are no loops.

EventNotifier

Indication if the node can be used to subscribe to events or the read / write historic events.

The ContainsNoLoops attribute is set to “true” to indicate that by following the References

in the context of the view there are no loops, i.e. starting from node “A” contained in the view and

following the forward references in the context of the view node “A” will not be reached again. It

UAReferenceType

14

does not specify that there is only one path starting from the view node to reach a node contained in

the view.

If set to “false” this attribute indicates that following references in the context of the view may

lead to loops.

The EventNotifier is used to indicate if the node can be used to subscribe to events or the

read / write historic events. The EventNotifier is an 8-bit unsigned integer. A detailed

description of the EventNotifier type encoding is covered in the section EventNotifier.

UAReferenceType

References are defined as instances of ReferenceType NodeClasses. ReferenceTypes are visible in

the Address Space and are defined using the ReferenceType NodeClass. By contrast, a reference is an

inherent part of a node and no NodeClass is used to represent references.

The OPC UA standard predefines a set of ReferenceTypes provided as an inherent part of the OPC

UA Information Model. Servers may define custom ReferenceTypes. In addition, the OPC UA standard

defines node management services that allow clients to add ReferenceTypes to the Address Space.

The inherited BrowseName attribute is used to specify the meaning of the ReferenceType as seen

from the source node. For example, the ReferenceType with the BrowseName “Contains” is used in

References which specify that the source node contains the target node. 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.

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.

The OPC UA standard does not specify how those constraints could or should be made available

in the Address Space. Nevertheless, for the ReferenceTypes defined by the standard, some

constraints are specified. 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.

ReferenceType NodeClasses is represented by the UAReferenceType in the UANodeSet

document. A UAReferenceType is a subtype of the UAType defined in the section UAType. The

fields in the UAMethod type are defined in Table 6.

UAReferenceType

15

Table 11 - UAReferenceType

Element

XML Type

Description

InverseName

LocalizedText

The inverse name of the Reference, that is the meaning of the ReferenceType

as seen from the TargetNode

Symmetric

boolean

The value is true if the meaning of the ReferenceType is the same as seen

from both the source node and the target node, otherwise false.

InverseName

InverseName attribute of the LocalizedText type defines inverse name of the reference

that is the meaning of the ReferenceType as seen from the target node.

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

semantics coming from the source node or the target node. Both directions are, therefore,

considered to be forward references.

If the ReferenceType is non-symmetric and not abstract, the InverseName attribute shall be

set. The InverseName specifies the meaning of the ReferenceType as seen from the target node.

Examples of non-symmetric ReferenceTypes include “Contains” and “Contained In”, and “Receives

From” and “Sends To”.

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

inverse References.

As an example of a unidirectional reference, it is often the case that a subscriber knows its

publisher, but the publisher does not know its subscribers. The subscriber would have a “Subscribes

To” Reference to the publisher, without the publisher having the corresponding “Publishes To”

inverse References to its subscribers.

The DisplayName attribute of the UAType and the InverseName are the only standardized

places to indicate the semantics of a ReferenceType. There may be more complex semantics

associated with a ReferenceType than can be expressed in those attributes (e.g. the semantics of

HasSubtype). The OPC UA specification does not define how this semantics should be exposed.

However, the Description attribute of the ReferenceType NodeClass can be used for this purpose.

Symmetric

The Symmetric attribute is used to indicate whether or not the meaning of the represented

ReferenceType is the same for both the source node and target node. Symmetric attribute of

the boolean has the following values:

 TRUE - the meaning of the ReferenceType is the same as seen from both the source node and the

target node.

 FALSE - the meaning of the ReferenceType as seen from the target node is the inverse of that

seen from the source node.

Base types

16

Base types

UAType

A UAType is a subtype of the UANode defined in the section UANode. It is the base type for the

XML types listed in Table 12.

Table 12 - UANodeSet Subtypes

Subtype

Description

UAObjectType

Defines an ObjectType Node as described in ...

UAVariableType

Defines a VariableType Node as described ...

UADataType

Defines a DataType Node as described ...

UAReferenceType

Defines a ReferenceType Node as described...

UAInstance

A UAInstance is a subtype of the UANode defined in the section UANode. It is the base type

for the types defined in Table 13.

Table 13 - UAInstance Subtypes

Subtype

Description

UAObject

Defines an Object node as described in...

UAVariable

Defines a Variable node as described in...

UAMethod

Defines a Variable node as described..

UAView

Defines a View node as described in...

The fields in the UAInstance type are defined in Table 14.

Table 14 – UAInstance

Element

Type

Description

ParentNodeId

NodeId

The NodeId of the parent.

ParentNodeId of NodeId type of the node contains a value pointing to the parent node

within the Address Space portion represented by the UANodeSet document. This field is used to

indicate that a tight coupling exists between the node and its parent (e.g. when the parent is deleted

the child is deleted as well). This information is not exposed in the Address Space, i.e. it does not

appear in the Address Space and is intended for use by design tools.

UANode

The OPC UA Address Space Model defines a Base NodeClass from which all other NodeClasses are

derived. The derived NodeClasses represent various components of the OPC UA Object Model. A

UANode XML type is an abstract base type that represents the Base NodeClasse attributes. It defines

a set of fields representing attributes and references of any node in the Address Space. There are the

following XML subtypes: UAObject, UAVariable, UAMethod, UAView, UAObjectType,

UAVariableType, UADataType, UAReferenceType; each one for an appropriate NodeClass

defined in the UANodeSet schema. Each of these subtypes defines XML fields for the OPC UA

attributes specific to the selected NodeClass. The fields in the UANode type are defined in Table 15.

Base types

17

Table 15 – UANode Type

Field

XML Type

Description

NodeId

It holds the value of the NodeId attribute of modes in the Address Space.

BrowseName

QualifiedName

It holds the value of the BrowseName attribute of modes in the Address Space .

SymbolicName

string

A symbolic name for the node.

WriteMask

The value of the WriteMask node attribute.

UserWriteMask

WriteMask

The value of the UserWriteMask node attribute .

DisplayName

LocalizedText[]

A list of DisplayNames for the node in different locales .

Description

LocalizedText[]

A list of the Description entries for the Node in different locales.

OPC UA Address Space Interchange XML

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