A general block diagram of sequential circuit as shown, this is know as finite state machine (FSM) depending upon the way the external outputs as obtained from the circuit, there are two different models of sequential circuit, these are mealy and Moore model.

Mealy model. In the case of mealy model, the next state is a function of the present state and the present inputs. its also a function of the present state and the present input, following is the block diagram of mealy model.

In general, the next state and the output of a mealy model are uniquely defined by

Next state = $F_1$ (present state, inputs)

Outputs = $F_2$ (present state, inputs)

Moore model. The block diagram of a moore model of circuit is shown below, similar to the mealy model the next state in a moore model is also a function of the present state and the inputs but the outputs of moore model are functions of the present state and are independent of the inputs therefore moore model is defined as

Next state = $F_1$ (present state, I/P)

Outputs = $F_2$ (present state)