This VariableType is an abstract type. That is, no instances of this type can exist. However, it might be used in a filter when browsing or querying. The DiscreteItemType derives from the DataItemType and therefore shares all of its characteristics. It is formally defined in Table 6.

Table 6 – DiscreteItemType definition

Attribute	Value
BrowseName	DiscreteItemType
IsAbstract	True
ValueRank	−2 (−2 = ‘Any’)
DataType	BaseDataType
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DataItemType defined in 5.2; i.e. the Properties of that type are inherited.
HasSubtype	VariableType	TwoStateDiscreteType	Defined in 5.3.3.2
HasSubtype	VariableType	MultiStateDiscreteType	Defined in 5.3.3.3
HasSubtype	VariableType	MultiStateValueDiscreteType	Defined in 5.3.3.4
Conformance Units
Data Access DiscreteItemType

This VariableType defines the general characteristics of a DiscreteItem that can have two states. The TwoStateDiscreteType derives from the DiscreteItemType. It is formally defined in Table 7.

Table 7 – TwoStateDiscreteType definition

Attribute	Value
BrowseName	TwoStateDiscreteType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Boolean
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DiscreteItemType defined in 5.3.3; i.e. the Properties of that type are inherited.
HasProperty	Variable	TrueState	LocalizedText	PropertyType	Mandatory
HasProperty	Variable	FalseState	LocalizedText	PropertyType	Mandatory
Conformance Units
Data Access TwoState

TrueState contains a 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.

for example: "RUN", "CLOSE", "ENABLE", "SAFE", etc.

FalseState contains a 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.

for example: "STOP", "OPEN", "DISABLE", "UNSAFE", etc.

If the item contains an array, then the Properties will apply to all elements in the array.

The StatusCode SemanticsChanged bit shall be set if any of the FalseState or TrueState Properties are changed (see 5.2 for additional information).

This VariableType defines the general characteristics of a DiscreteItem that can have more than two states. The MultiStateDiscreteType derives from the DiscreteItemType. It is formally defined in Table 8.

Table 8 – MultiStateDiscreteType definition

Attribute	Value
BrowseName	MultiStateDiscreteType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	UInteger
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DiscreteItemType defined in 5.3.3; i.e. the Properties of that type are inherited.
HasProperty	Variable	EnumStrings	LocalizedText[]	PropertyType	Mandatory
Conformance Units
Data Access MultiState

EnumStrings is a string lookup table corresponding to sequential numeric values (0, 1, 2, etc.)

Example:

"OPEN"

"CLOSE"

"IN TRANSIT" etc.

Here the string "OPEN" corresponds to 0, "CLOSE" to 1 and "IN TRANSIT" to 2.

Clients should be prepared to handle item values outside of the range of the list; and robust servers should be prepared to handle writes of illegal values, by providing errorcode “Bad_OutOfRange”.

If the item contains an array then this lookup table shall apply to all elements in the array.

NOTE The EnumStrings property is also used for Enumeration DataTypes (for the specification of this DataType, see OPC 10000-3).

The StatusCode SemanticsChanged bit shall be set if the EnumStrings Property is changed (see 5.2 for additional information).

This VariableType defines the general characteristics of a DiscreteItem that can have more than two states and where the state values (the enumeration) do not consist of consecutive numeric values (may have gaps) or where the enumeration is not zero-based. The MultiStateValueDiscreteType derives from the DiscreteItemType. It is formally defined in Table 9.

Table 9 – MultiStateValueDiscreteType definition

Attribute	Value
BrowseName	MultiStateValueDiscreteType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Number
References	NodeClass	BrowseName	DataType TypeDefinition	ModellingRule
Subtype of the DiscreteItemType defined in 5.3.3; i.e. the Properties of that type are inherited.
HasProperty	Variable	EnumValues	EnumValueType[]PropertyType	Mandatory
HasProperty	Variable	ValueAsText	LocalizedTextPropertyType	Mandatory
Conformance Units
Data Access MultiStateValueDiscrete

EnumValues is an array of EnumValueType. Each entry of the array represents one enumeration value with its integer notation, a human-readable representation, and help information. This represents enumerations with integers that are not zero-based or have gaps (e.g. 1, 2, 4, 8, 16). See OPC 10000-3 for the definition of this type. MultiStateValueDiscrete Variables expose the current integer notation in their Value Attribute. Clients will often read the EnumValues Property in advance and cache it to lookup a name or help whenever they receive the numeric representation.

Only DataTypes that can be represented with EnumValues are allowed for Variables of MultiStateValueDiscreteType. These are Integers up to 64 Bits (signed and unsigned).

The numeric representation of the current enumeration value is provided via the Value Attribute of the MultiStateValueDiscrete Variable. The ValueAsText Property provides the localized text representation of the enumeration value. It can be used by Clients only interested in displaying the text to subscribe to the Property instead of the Value Attribute.

The StatusCode SemanticsChanged bit shall be set if the EnumValues Property value is changed (see clause 5.2 for additional information).