A finite state machine remembers one of a finite set of states. Inputs and current state determine its next state; outputs follow the chosen output rule. State names summarize meaningful history, making a controller easier to describe than a collection of individual flip-flop bits.
Worked example
A simple door controller has CLOSED and OPEN states. From CLOSED, an open request moves it to OPEN. From OPEN, a close request moves it to CLOSED. Without a relevant request it holds its current state. Those transitions define the logical controller, before assigning binary state codes.
What to watch for
A state should retain only the history needed for future behavior. Physical safety, sensor faults and conflicting commands need additional requirements in a real door system.
Try it yourself
If the controller is CLOSED and receives no request, what should happen under this specification?
Answer: It remains CLOSED. The state-transition rule includes this hold case explicitly.
How to study this topic
Begin with named states and a complete transition table. Then choose encodings and derive the logic. Trace inputs over several cycles, checking outputs as well as next states, including reset and unexpected encodings.
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