IEC 62541-8-2020 OPC unified architecture – Part 8: Data access.
5.2 SemanticsChanged
The StatusCode also contains an informational bit called SemanticsChanged. Servers that implement Data Access shall set this Bit in notifications if certain Properties defined in this standard change. The corresponding Properties are specified individually for each VariableType.
Clients that use any of these Properties should re-read them before they process the data value.
5.3 Variable Types
5.3.1 DataItemType
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 IEC 62541 -3 and IEC 62541 -5. It is formally defined in Table 1 .
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.
5.3.2 AnalogItem VariableTypes General
The VariableTypes in this subclause define the characteristics of AnalogItems. The types have identical semantics and Properties but with diverging ModellingRules for individual Properties.
The Properties are only described once – in The descriptions apply to the Properties for the other VariableTypes as well. BaseAnalogType
This VariableType is the base type for analog items. All Properties are optional. Subtypes of this base type will mandate some of the Properties. The BaseAnalogType derives from the DataItemType. It is formally defined in Table 2.IEC 62541-8 pdf download.