6.2.2 State Transition Diagrams
We'll describe the operation of the FSM for our combination lock using a state transition diagram. Initially, the FSM has received no bits of the combination, a state we'll call SX. In the state transition diagram, states are represented as circles, each labeled for now with a symbolic name chosen to remind us of what history it represents. For this FSM, the unlock output U will be a function of the current state, so we'll indicate the value of U inside the circle. Since in state SX we know nothing about past input bits, the lock should stay locked and so U = 0. We'll indicate the initial state with a wide border on the circle. We'll use the successive states to remember what we've seen so far of the input combination. So if the FSM is in state SX and it receives a 0 input, it should transition to state S0 to remind us that we've seen the first bit of the combination of 0-1-1-0. We use arrows to indicate transitions between states and each arro...