Errata exists for this version of the document.
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
|
Attribute |
Value |
||||
|
BrowseName |
DataItemType |
||||
|
IsAbstract |
False |
||||
|
ValueRank |
−2 (−2 = ‘Any’) |
||||
|
DataType |
BaseDataType |
||||
|
References |
NodeClass |
BrowseName |
DataType |
TypeDefinition |
ModellingRule |
|
Subtype of the BaseDataVariableType defined in OPC 10000-5; i.e the Properties of 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 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.