Modelling a system with an automaton
Choosing the states, events and transitions of a system described in plain language, adding outputs with a Mealy machine, then checking the model: unreachable state, deadlock, forbidden scenario.
- 29 cards
- 4 topics
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The outline, topic by topic
Each topic is weighted the way the exam weighs it.
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A few cards from the deck
Finite-state machine: what is it?
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A model of a system that, at any moment, is in exactly one state among finitely many, and changes state when an event occurs.
The state sums up everything the system must remember of the past: what it does not remember cannot influence what follows.
A keypad opens on 1 2 1 3 and remembers how many correct keys in a row it has seen. It has recognised 3 keys (1 2 1). The key 2 is pressed. Which state is it in?
- 2 keys recognised (1 2)
- Back to zero, since the sequence is wrong
- Still 3 keys recognised, the key is ignored
- 1 key recognised, the last key alone
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2 keys recognised (1 2)
After 1 2 1 2, the last two keys 1 2 are the start of the code: the state is "1 2 read". It must neither return to zero nor forget everything. The 2 does not start the code, so the last key alone counts for nothing.
A … remembers what must be kept of the past; a … is a move from one state to another, triggered by an ….
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A state remembers what must be kept of the past; a transition is a move from one state to another, triggered by an event.
For a turnstile: "locked" is a state, "ticket validated" an event, and the move from one to "unlocked" a transition.
Transition: what is it?
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A move from one state to another, triggered by an event; it is drawn as an arrow labelled with that event.
An arrow that returns to its own state is a loop: the event happens without changing the state, though it may still produce an output.