Variable Node Class

This topic contains the following sections:

• Property
• DataVariable
• See Also

Variables are used to represent values which may be simple or complex.

Variables are always defined as properties or data variables of other nodes in the Address Space. They are never defined by themselves.

Note
The Variable node 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 node class.

The Variable node class inherits the base attributes from the Basenode class.

The Variable node class 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.

The AccessLevel attribute indicates the accessibility of the value of a variable not taking user access rights into account. If the OPC Unified Architecture server does not have the ability to get the AccessLevel information from the underlying system, it should state that it is read and writable. If a read or write operation is called on the Variable, the server should transfer this request and return the corresponding StatusCode if such a request is rejected. Status codes are described in section Services.

The SemanticChange bit of the AccessLevel attribute shall be set when the property describes the semantic of the node that owns the property and changes of the property value will generate semantic change events

For example, a property describing the engineering unit of a DataVariable has the bit set, whereas a property containing an icon of the DataVariable will not. This behaviour is exactly the same as described by the SemanticsChanged bit of the StatusCode described in section Services. However, if you subscribe to a Variable you should look at the StatusCode to identify if the semantic has changed in order to receive this information before you are processing the value of the Variable.

The UserAccessLevel attribute indicates the accessibility of the Value of a variable taking user access rights into account. If the OPC Unified Architecture 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 attribute, but not exceed it.

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 section Historical Access for details on historizing Variables.

Clients may read or write Variable values, or monitor them for value changes, as described in section Services. Section Data Access describes additional rules when using the services for automation data.

To specify its modelling rule, a variable can use at most one HasModellingRule reference pointing to a ModellingRule node.

A variable shall use at most one HasModelParent reference to specify its model parent.

If the variable is created based on an instance declaration (see Type Definition ) it shall have the same BrowseName as its instance declaration.

The other references are described separately for properties and data variables in the following subclauses.