The components of this parameter are defined in Table 171.

Table 171– ReadValueId

Name

Type

Description

ReadValueId

structure

Identifier for an item to read or to monitor.

nodeId

NodeId

NodeIdof a Node.

attributeId

IntegerId

Id of the Attribute. This shall be a valid Attributeid. The IntegerId is defined in 7.19. The IntegerIds for the Attributes are defined in OPC 10000-6.

indexRange

NumericRange

This parameter is used to identify a single element of an array, or a single range of indexes for arrays. If a range of elements is specified, the values are returned as a composite. The first element is identified by index 0 (zero). The NumericRangetype is defined in 7.27.

This parameter is null or empty if the specified Attributeis not an array. However, if the specified Attributeis an array, and this parameter is null or empty, then all elements are to be included in the range.

dataEncoding

QualifiedName

This parameter specifies the BrowseNameof the DataTypeEncodingthat the Servershould use when returning the Value Attributeof a Variable. It is an error to specify this parameter for other Attributes.

This parameter only applies if the DataTypeof the Variableis a subtype of Structure. It is an error to specify this parameter if the DataTypeof the Variableis not a subtype of Structure.

A Clientcan discover what DataTypeEncodings are available by following the HasEncoding Referencefrom the DataType Nodefor a Variable.

OPC UA defines BrowseNameswhich Serversshall recognize even if the DataType Nodesare not visible in the Server AddressSpace. These BrowseNamesare:

Default Binary

The default or native binary (or non-XML) encoding.

Default XML

The default XML encoding.

Default JSON

The default JSON encoding

Each DataTypeshall support at least one of these encodings. DataTypesthat do not have a true binary encoding (e.g. they only have a non-XML text encoding) should use the Default Binary name to identify the encoding that is considered to be the default non-XML encoding. DataTypesthat support at least one XML-based encoding shall identify one of the encodings as the Default XML encoding. Other standards bodies may define other well-known data encodings that could be supported.

If this parameter is null or empty then the Servershall choose the default according to what Messageencoding (see OPC 10000-6) is used for the Session. If the Serverdoes not support the encoding that matches the Messageencoding then the Servershall choose the default encoding that it does support.