In most cases Variables have the TypeDefinition 0:DataItemType or one of its subtypes. The optional Property Definition can be added to a Variable that uses such a TypeDefinition. This allows manufacturers to store a specific definitions for each Variable.
Variables defined in this specification that have the TypeDefinition 0:BaseAnalogType or one of its subtypes, usually have a predefined unit for the 0:EngineeringUnits Property. If no value is specified, the 0:EngineeringUnits Property should not be instantiated, or the Value Attribute shall be Null. To comply with this Companion Specification, the default values specified should be used for the 0:EngineeringUnits Property. The 0:EngineeringUnits should be sensible to the use of the application.
Variables that use the DataType Boolean are modelled with the TypeDefinition 0:TwoStateDiscreteType. Such Variables have the TrueState and FalseState Properties which shall be used for defining the actual states.
Variables that are children of the SupvervisionType or one of its subtypes represent supervision states. Such a supervision state is active if the Boolean value is True (see example 1).
Variables that are not children of SupvervisionType or one of its subtypes provide defined True and False states in their description (see Example 2).
EXAMPLE 1 If the Value Attribute of the Variable RotorBlocked (see section 7.12) is True, this means that the rotor of a pump is blocked. If the Value Attribute of this Variable is False, it means that the rotor is not blocked.
EXAMPLE 2 The Variable ClockwiseRotation (see section 7.20) provides in the Description Attribute the Value Attributes for the mandatory Properties TrueState and FalseState. Description Attribute: Direction of rotation in which the shaft is seen to be turning in a clockwise direction when viewing the drive end of the shaft. A “True” status means that the rotation of pump is clockwise and a “False” status means that the rotation of pump is anticlockwise.