This VariableType defines the general characteristics of a DataItem. All other DataItem Types derive from it. The DataItemType derives from the BaseDataVariableType and therefore shares the variable model as described in OPC 10000-3 and OPC 10000-5. It is formally defined in Table 1.
Table 1 – DataItemType definition
|ValueRank||−2 (−2 = ‘Any’)|
Subtype of the BaseDataVariableType defined in OPC 10000-5; i.e the Properties of that type are inherited.
|HasSubtype||VariableType||BaseAnalogType||Defined in 220.127.116.11.|
|HasSubtype||VariableType||DiscreteItemType||Defined in 5.3.3|
|HasSubtype||VariableType||ArrayItemType||Defined in 5.3.4|
Definition is a vendor-specific, human readable string that specifies how the value of this DataItem is calculated. Definition is 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 Server can 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 Property is an approximation that is intended to provide guidance to a Client. A Server is expected to silently round any value with more precision that it supports. This implies that a Client may encounter cases where the value read back from a Server differs from the value that it wrote to the Server. This difference shall be no more than the difference suggested by this Property.