This VariableTypedefines the general characteristics of a DataItem. All other DataItemTypes derive from it. The DataItemTypederives from the BaseDataVariableTypeand therefore shares the variable model as described in OPC 10000-3and OPC 10000-5. It is formally defined in Table 1.

Table 1– DataItemType definition

Attribute

Value

BrowseName

DataItemType

IsAbstract

False

ValueRank

−2 (−2 = ‘Any’)

DataType

BaseDataType

References

NodeClass

BrowseName

DataType

TypeDefinition

ModellingRule

Subtype of the BaseDataVariableTypedefined in OPC 10000-5; i.e the Propertiesof that type are inherited.

HasSubtype

VariableType

AnalogItemType

Defined in 0

HasSubtype

VariableType

DiscreteItemType

Defined in 5.3.3

HasSubtype

VariableType

ArrayItemType

Defined in 5.3.4

HasProperty

Variable

Definition

String

PropertyType

Optional

HasProperty

Variable

ValuePrecision

Double

PropertyType

Optional

Definition is a vendor-specific, human readable string that specifies how the value of this DataItemis calculated. Definitionis non-localized and will often contain an equation that can be parsed by certain clients.

Example: Definition::= “(TempA – 25) + TempB”

ValuePrecision specifies the maximum precision that the Servercan maintain for the item based on restrictions in the target environment.

ValuePrecision can be used for the following DataTypes:

  • For Float and Double values it specifies the number of digits after the decimal place.
  • For DateTime values it indicates the minimum time difference in nanoseconds. For example, a ValuePrecision of 20 000 000 defines a precision of 20 ms.

The ValuePrecision Propertyis an approximation that is intended to provide guidance to a Client. A Serveris expected to silently round any value with more precision that it supports. This implies that a Clientmay encounter cases where the value read back from a Serverdiffers from the value that it wrote to the Server. This difference shall be no more than the difference suggested by this Property.