7 Common parameter type definitions ToC Previous Next

7.24 ReadValueId ToC Previous Next

The components of this parameter are defined in Table 166.

Table 166 – ReadValueId

Name Type Description
ReadValueId structure Identifier for an item to read or to monitor.
   nodeId NodeId NodeId   of a Node.
   attributeId IntegerId Id of the Attribute. This shall be a valid Attribute id. The IntegerId is defined in 7.14. 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 NumericRange type is defined in 7.22.
This parameter is null if the specified Attribute is not an array. However, if the specified Attribute is an array, and this parameter is null, then all elements are to be included in the range.
   dataEncoding QualifiedName This parameter specifies the BrowseName of the DataTypeEncoding that the Server should use when returning the Value Attribute of a Variable. It is an error to specify this parameter for other Attributes.
This parameter only applies if the DataType of the Variable is a subtype of Structure. It is an error to specific this parameter if the DataType of the Variable is not a subtype of Structure.
A Client can discover what DataTypeEncodings are available by following the HasEncoding Reference from the DataType Node for a Variable.
OPC UA defines BrowseNames which Servers shall recognize even if the DataType Nodes are not visible in the Server AddressSpace. These BrowseNames are:

   Default Binary   The default or native binary (or non-XML) encoding.
   Default XML   The default XML encoding.
   Default JSON   The default JSON encoding
   Each DataType shall support at least one of these encodings. DataTypes that 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. DataTypes that 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 not specified then the Server shall choose the default according to what Message encoding (see OPC 10000-6) is used for the Session. If the Server does not support the encoding that matches the Message encoding then the Server shall choose the default encoding that it does support.

Previous Next