Annex C (informative) – Parameter Representation ToC Previous Next

C.1 Simple Parameters ToC Previous Next

Parameters which hold simple data like a single numerical value, string value or a time-stamp value are represented by BaseDataVariableType defined in [OPC 10000-5] or one of its subtypes.

Variables which hold simple data like a single numerical value, string value or a time-stamp value are represented by BaseDataVariableType defined in [OPC 10000-5]. Those Variables typically represent some configuration Parameters, status, states or acquisition results of an analyser.

If a Variable represents simple data which is obtained “live” from an analyser, DataItem VariableType or one of its subtypes will be used. For example, AnalogItemType shall be used when there is a need for a specific Property of the AnalogItemType such as EURange and EngineeringUnits [OPC 10000-8].

Table 72 describes how each Attribute of super/sub class of DataItem is used in the ADI context.

Table 113 - ADI DataItem Attributes

Attributes/Properties Description
Base NodeClass  
         DisplayName Localized user readable name of this DataItem
         BrowseName The programmatic name of this DataItem
         Description Localized user readable description
         WriteMask Supports access control implementation
         UserWriteMask Supports access control implementation
Variable NodeClass  
         Value The Parameter value itself
         DataType DataType of Value
         ValueRank The number of dimensions of value
         ArrayDimensions The size of each value dimensions
         AccessLevel Supports access control implementation of Value
         UserAccessLevel Supports access control implementation of Value
         MinimumSamplingInterval Defined how fast Value may be updated
DataItemType  
         Definition 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 The maximum precision that the Server can maintain for the item based on restrictions in the target environment
AnalogItemType  
         InstrumentRange The value range that can be returned by the instrument
         EURange The value range likely to be obtained in normal operation. It is intended for such use as automatically scaling a bar graph display
         EngineeringUnits The units for the DataItem’s value (e.g., DEGC, hertz, seconds )
TwoStateDiscreteItemType  
         TrueState String to be associated with this DataItem when it is TRUE. This is typically used for a contact when it is in the closed (non-zero) state. e.g. “RUN”, “CLOSE”, “ENABLE”, “SAFE“, etc.
         FalseState String to be associated with this DataItem when it is FALSE. This is typically used for a contact when it is in the open (zero) state. e.g. “STOP”, “OPEN”, “DISABLE”, “UNSAFE“, etc.
MultiStateDiscreteItemType  
         EnumStrings EnumStrings which is a string lookup table corresponding to sequential numeric values (0, 1, 2, etc.)
Example:
   ”OPEN”
   ”CLOSE”
   ”IN TRANSIT” etc

The other source of information is the OPC UA Read Service described in [OPC 10000-4]. It provides:

  • The current value itself
  • Quality of the value
  • The time of the last update The SemanticsChanged bit in StatusCode

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