Combinational logic refers to circuits whose output is a function of the present value of the inputs only.
soon as inputs are changed, the information about the previous inputs is lost,
that is, combinational logic circuits have no memory
logic circuits are those whose outputs are also dependent upon past inputs, and
hence outputs. In other words the output of a sequential circuit may depend upon
its previous outputs and so in effect has some form of "memory"
mathematical model of a sequential circuit is usually referred to as a sequential
The general block diagram of a sequential switching circuit is shown below:
Sequential circuits are basically combinational circuits with the additional properties of storage (to remember past inputs) and feedback. (see fig. 1).
Note that the output Z is a function of inputs x and internal states y.