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The StateMachineType is the base ObjectType for all StateMachineTypes. It defines a single Variable which represents the current state of the machine. An instance of this ObjectType shall generate an Event whenever a significant state change occurs. The Server decides which state changes are significant. Servers shall use the GeneratesEvent ReferenceType to indicate which Event(s) could be produced by the StateMachine.

Subtypes may add Methods which affect the state of the machine. The Executable Attribute is used to indicate whether the Method is valid given the current state of the machine. The generation of AuditEvents for Methods is defined in OPC 10000-4. A StateMachine may not be active. In this case, the CurrentState and LastTransition Variables shall have a status equal to Bad_StateNotActive (see Table B.17).

Subtypes may add components which are instances of StateMachineTypes. These components are considered to be sub-states of the StateMachine. SubStateMachines are only active when the parent machine is in an appropriate state.

Events produced by SubStateMachines may be suppressed by the parent machine. In some cases, the parent machine will produce a single Event that reflects changes in multiple SubStateMachines.

FiniteStateMachineType is subtype of StateMachineType that provides a mechanism to explicitly define the states and transitions. A Server should use this mechanism if it knows what the possible states are and the state machine is not trivial. The FiniteStateMachineType is defined in B.4.5.

The StateMachineType is formally defined in Table B.1.

Table B.1 – StateMachineType Definition

Attribute		Value
BrowseName		StateMachineType
IsAbstract		False
References	NodeClass		BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the BaseObjectType defined in 6.2. Note that a Reference to this subtype is not shown in the definition of the BaseObjectType.
HasSubtype	ObjectType		FiniteStateMachineType	Defined in B.4.5
HasComponent	Variable		CurrentState	LocalizedText	StateVariableType	Mandatory
HasComponent	Variable		LastTransition	LocalizedText	TransitionVariableType	Optional

CurrentState stores the current state of an instance of the StateMachineType. CurrentState provides a human readable name for the current state which may not be suitable for use in application control logic. Applications should use the Id Property of CurrentState if they need a unique identifier for the state.

LastTransition stores the last transition which occurred in an instance of the StateMachineType. LastTransition provides a human readable name for the last transition which may not be suitable for use in application control logic. Applications should use the Id Property of LastTransition if they need a unique identifier for the transition.