The WoT specification defines both a TD and a Protocol Binding.

The WoT TD is defined here:

https://www.w3.org/TR/wot-thing-description/#introduction-td

The WoT Protocol Binding is defined here:

https://www.w3.org/TR/wot-binding-templates/#binding-overview

The goal of this Companion Specification is to create a thin interface for configuring industrial connectivity software in a standardized way. By keeping the interface to a minimum, the chances of adoption by the industry are increased.

An example WoT TD is provided below:

{

"@context": [

"https://www.w3.org/2022/wot/td/v1.1"

],

"id": "urn:pac4200",

"securityDefinitions": {

"nosec_sc": {

"scheme": "nosec"

}

},

"security": [

"nosec_sc"

],

"@type": [

"Thing"

],

"name": "modbus-pac4200-sn324",

"base": "modbus+tcp://192.168.178.94:502/1",

"title": "Siemens SENTRON PAC4200",

"properties": {

"Voltage": {

"type": "number",

"readOnly": true,

"observable": true,

"forms": [

{

"href": "7?quantity=2",

"op": [

"readproperty",

"observeproperty"

],

"modv:type": "xsd:float",

"modv:entity": "holdingRegister",

"modv:pollingTime": 2000

}

]

}

}

}

Open-source WoT Thing Description parsers are available at: https://www.w3.org/WoT/developers/#runtime-consume.

The entry point of this specification in a Server’s object hierarchy is called “WoTAssetConnectionManagement” it is an instance of WoTAssetConnectionManagementType which is defined in 6.3.1

The AddressSpace for the entry point is shown in Figure 6.

Figure 6 – WoTAssetConnectionManagement Address Space

The WoTAssetConnectionManagement Node is formally defined in Table 4.

Table 4 – WoTAssetConnectionManagement definition

Attribute	Value
BrowseName	WoTAssetConnectionManagement
References	NodeClass	BrowseName
OrganizedBy by the 0:Objects Folder defined in OPC 10000-5
0:HasTypeDefinition	ObjectType	WoTAssetConnectionManagementType
Conformance Units
WoT Connectivity Base Functionality

The WoTAssetConnectionManagementType ObjectType represents a set of WoTAssets that may be managed by a Server. It is defined in Table 5.

Table 5 – WoTAssetConnectionManagementType Definition

Attribute	Value
BrowseName	WoTAssetConnectionManagementType
IsAbstract	False
References	NodeClass	BrowseName	DataType	TypeDefinition	Other
Subtype of the 0:BaseObjectType defined in OPC 10000-5.
0:Organizes	Object	<WoTAssetName>		BaseObjectType	OP
0:HasComponent	Method	CreateAsset	Defined in 6.3.2		M
0:HasComponent	Method	DeleteAsset	Defined in 6.3.1.3.		M
Conformance Units
WoT Connectivity Base Functionality

Any instance of the <WoTAssetName> Object shall implement the IWoTAssetType Interface.

The CreateAsset Method creates a new Object that implements the IWoTAssetType Interface organized by the WoTAssetConnectionManagement Object.

The DeleteAsset Method shall remove the Organizes Reference from the WoTAssetConnectionManagement Object and may delete the IWoTAssetType instance.

The components of the WoTAssetConnectionManagementType have additional references which are defined in Table 6.

Table 6 – WoTAssetConnectionManagementType Additional References

SourceBrowsePath	Reference Type	Is Forward	TargetBrowsePath
<WoTAssetName>	0:HasInterface	True	IWoTAssetType

CreateAsset creates a new Object that implements the IWoTAssetType Interface that can be used to upload the WoT TD file to the server. Once the upload is complete, the industrial asset becomes reachable via the OPC UA server. The WoT TD file uploaded fully defines the endpoint, protocol, and all asset tags to be mapped to OPC UA.

CreateAsset, when successful, returns the NodeId of the newly created Object and adds an Organizes Reference from the WoTAssetConnectionManagement Object.

Signature

CreateAsset(

[in] 0:String AssetName,

[out] 0:NodeId AssetId);

Table 7 – CreateAsset Method Arguments

Argument	Description
AssetName	A unique name for the asset.
AssetId	The NodeId of the WoTAsset Object, if call was successful.

Method Result Codes

Result Code Symbolic Id	Description
Bad_BrowseNameDuplicated	The AssetName is already used.
Bad_UserAccessDenied	The session does not have the permissions required.

DeleteAsset deletes a previously configured industrial asset. It takes the NodeId of a WoTAsset Object input, previously returned by CreateAsset. These NodeIds can also be browsed from the WoTAssetConnectionManagement instance.

Signature

DeleteAsset(

[in] 0:NodeId AssetId);

Table 8 – DeleteAsset Method Arguments

Argument	Description
AssetId	The NodeId of the WoTAsset Object.

Method Result Codes

Result Code Symbolic Id	Description
Bad_NotFound	The asset was not found.
Bad_InvalidArgument	The NodeId provided was invalid.
Bad_UserAccessDenied	The session does not have the permissions required.

The IWoTAssetType is an Interface representing the WoT asset functionality. It has a component representing a File Object that can be used to upload the WoT TD file. It is defined in Table 9.

Table 9 – IWoTAssetType Definition

Attribute	Value
BrowseName	IWoTAssetType
IsAbstract	True
References	NodeClass	BrowseName	DataType	TypeDefinition	Other
Subtype of the 0:BaseInterfaceType defined in OPC 10000-5.
1:HasWoTComponent	Variable	<WoTPropertyName>	0:BaseDataType	0:BaseDataVariableType	OP
0:HasComponent	Object	WoTFile		WoTAssetFileType	M
Conformance Units
WoT Connectivity Base Functionality

When the asset configuration is complete, the Server may update the internal configuration of existing Variable Nodes or it may add new Variable Nodes to represent the asset. Each WoT property in the forms section of the TD shall have exactly one VariableNode associated with it.

The WoTAsset Object must have hierarchical HasWoTComponent references to the newly created Variable Nodes, one for each WoT property.

The BrowseName of the Variable Node shall have a name equal to the WoT property name.

If a mapping to a WoT property cannot be done (e.g. the data cannot be read from the connected asset), the built-in status code Bad_ConfigurationError will be returned when the Value of the Variable is read.

Each WoT property type is mapped to the corresponding OPC UA built-in DataType using the following mapping table:

Table 10 – WoT property type mapping

WoT Property Type	OPC UA Data Type
Boolean	Boolean
Number	Double
Integer	Int64
Object	No mapping
String	String
Null	No mapping
Array[Items]	Array[of one of above defined types]

The WoTAssetFileType ObjectType represents a set of TD File that is used to define an WoTAsset. It is defined in Table 11.

All instances shall support a mode of Write + EraseExisting when Open is called. They may support mode of Read. Other modes are not allowed.

When writing, if Close is called the file is discard and nothing changes. If CloseAndUpdate is called the file is parsed and the variables are updated to match the WoT TD file.

Table 11 – WoTAssetFileType Definition

Attribute	Value
BrowseName	WoTAssetFileType
IsAbstract	False
References	NodeClass	BrowseName	DataType	TypeDefinition	Modelling Rule
Subtype of the 0:FileType defined in OPC 10000-5.
0:HasComponent	Method	CloseAndUpdate	Defined in 6.3.3.2		Mandatory
Conformance Units
WoT Connectivity Base Functionality

The CloseAndUpdate Method parses the TD and creates Variables from the TD properties.

CloseAndUpdate Method parses the TD and creates Variables from the TD properties.

Signature

CloseAndUpdate (

[in] 0:UInt32 FileHandle

);

Table 12 – CloseAndUpdate Method Arguments

Argument	Description
FileHandle	The handle for a file opened for writing.

Method Result Codes

Result Code Symbolic Id	Description
Bad_InvalidState	The file was not opened for writing.
Bad_DecodingError	The WoT Thing Description could not be parsed.
Bad_NotSupported	The TD is valid but cannot be mapped to OPC UA.
Bad_NotFound	The asset was not found on the network.
Bad_UserAccessDenied	The session does not have the permissions required.

The HasWoTComponent is a concrete ReferenceType used to link WoT asset Objects to the WoT properties. The source node shall be an instance of an Object implementing the IWoTAssetType Interface. The target Node shall be an instance or a subtype of a BaseDataVariableType.

The HasWoTComponent representation in the AddressSpace is specified in Table 13.

Table 13 – HasWoTComponent ReferenceType

Attributes	Value
BrowseName	HasWoTComponent
InverseName	WoTComponentOf
Symmetric	False
IsAbstract	False
References	NodeClass	BrowseName	Comment
Subtype of 0:HasComponent ReferenceType defined in OPC 10000-5.
Conformance Units
WoT Connectivity Base Functionality