State Transition Diagrams (STD's)

menu: home , si-daq

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".
States:

• 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

STD's: