The Variablespecific Attributesof OPC UA Address Space Nodesand their mapping from IEC 61131-3 are defined in Table 33.

Table 33– Variable Node Attributes

Attribute

Use

DataType

Description

Value

Mandatory

Defined by DataType

The most recent value of the Variablethat the server has. Its data type is defined by the DataType, ValueRank andArrayDimensionAttribute.

DataType

Mandatory

NodeId

The DataTypeof the Variable Value. It defines the type specific content of the Value together with the ValueRankand the ArrayDimension Attributes,

The mapping is defined in 9.2.

ValueRank

Mandatory

Int32

This Attribute indicates whether the Value of the Variable is an

array and how many dimensions the array has.

Ctrl Variablesdeclared as scalar type have the ValueRank-1.

Ctrl Variablesdeclared with the key word ARRAY…OF have a ValueRankthat indicates the number of dimension of the array declared for the Ctrl Variable.

ArrayDimensions

Optional

UInt32[]

This Attributespecifies the length of each dimension for an array value. The Attributeis intended to describe the capability of the Variable, not the current size.

The dimension entries have the length defined with the key word ARRAY…OF.

AccessLevel

Mandatory

Byte

The AccessLevel Attribute is used to indicate how the Value of a Variablecan be accessed (read/write) and if it contains current and/or historic data.

The handling of access to historic data is server specific and is not part of this specification.

The mapping of the read and write access part of the AccessLevelis defined in 9.3.2

UserAccessLevel

Mandatory

Byte

The user specific settings for the AccessLevel.

MinimumSamplingInterval

Optional

Duration

The MinimumSamplingInterval Attributeindicates how “current” the Valueof the Variablewill be kept. It specifies (in milliseconds) how fast the server can reasonably sample the value for changes.

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

A MinimumSamplingInterval of -1 means indeterminate.

The value of this Attribute is server specific.

Historizing

Mandatory

Boolean

Indicates if the server is currently collecting history for the Variable Value. The support of value history is server specific.

If the IEC attribute CONSTANT is set, the Access Level shall be read only.

The IEC standard does not define a key word to set the Access Level for a Ctrl Variable. The configuration in a programming system is vendor specific but it is recommended to provide a configuration option for the OPC UA Access Level.

When using the PLCopen XML format the AccessLevel shall be provided in PLCopen XML Additional Datain the XML element addDatausing the XML element UaAccessLevel as part of the XML element representing the variable. The value is a bit mask where the first bit indicates the read access and the second bit indicates the write access.