STate transition Diagram  

Introduction to State Transition Diagrams (STD's)

An STD is a way of describing the time-dependent behaviour of a system. The basic consistency rule is: "A system's behaviour in any state must be the same, no matter by which path the state is arrived at".


  • A state is an observable mode of behaviour of the system.
  • At any time a particular STD can only be in one state.
  • .. but a system's behaviour could be described by more than one state transition diagram

Transition conditions:

  • internal events or events external to the system

Transition actions:

  • actions in response to the events
  • triggering one-shot actions
  • synchronizing between different STD's
  • producing control outputs

Drawing STD's:

  • Identify observable states of the system
  • Select the states with normal behaviour
  • Specify the conditions that mark a transition
  • Specify the actions to produce the observable behaviour in the destination state for each transition
  • If the system is complex, partition the diagram in several STD's

The classical elevator example:

  • just one lift
  • one floor can be requested at a time
  • no call buttons on any floors
  • the lift waits at the requested floor for five minutes if no-one selects a floor. Then it returns to the ground floor.
  • There is a photocell protection on the doors
  • The lift will not move with the doors open



A graphical tool is used to produce the diagrams: TCM ( )