COMBINATIONAL LOGIC CIRCUITS

The digital system consists of two types of circuits, namely:
(i) Combinational circuits and
(ii) Sequential circuits
A combinational circuit consists of logic gates, where outputs are at any instant and
are determined only by the present combination of inputs without regard to previous
inputs or previous state of outputs. A combinational circuit performs a specific
information-processing operation assigned logically by a set of Boolean functions.
Sequential circuits contain logic gates as well as memory cells. Their outputs depend
on the present inputs and also on the states of memory elements. Since the outputs of
sequential circuits depend not only on the present inputs but also on past inputs, the
circuit behavior must be specified by a time sequence of inputs and memory states.
A combinational circuit consists of input variables, logic gates, and output variables.
The logic gates accept signals from inputs and output signals are generated according
to the logic circuits employed in it. Binary information from the given data transforms
to desired output data in this process. Both input and output are obviously the binary
signals, i.e., both the input and output signals are of two possible states, logic 1 and
logic 0. Figure 4.1 shows a block diagram of a combinational logic circuit. There are
n number of input variables coming from an electric source and m number of output
signals go to an external destination. The source and/or destination may consist of
memory elements or sequential logic circuit or shift registers, located either in the
vicinity of the combinational logic circuit or in a remote external location. But the