Combinational circuit binary adder subtractor pdf a circuit in which we combine the different gates in the circuit, for example encoder, decoder, multiplexer and demultiplexer. The output of combinational circuit at any instant of time, depends only on the levels present at input terminals. The combinational circuit do not use any memory. The previous state of input does not have any effect on the present state of the circuit.

A combinational circuit can have an n number of inputs and m number of outputs. We’re going to elaborate few important combinational circuits as follows. Half adder is a combinational logic circuit with two inputs and two outputs.

The half adder circuit is designed to add two single bit binary number A and B. It is the basic building block for addition of two single bit numbers. This circuit has two outputs carry and sum. Full adder is developed to overcome the drawback of Half Adder circuit.

It can add two one-bit numbers A and B, and carry c. The full adder is a three input and two output combinational circuit. The Full Adder is capable of adding only two single digit binary number along with a carry input. But in practical we need to add binary numbers which are much longer than just one bit.

To add two n-bit binary numbers we need to use the n-bit parallel adder. It uses a number of full adders in cascade. The carry output of the previous full adder is connected to carry input of the next full adder.

In the block diagram, A0 and B0 represent the LSB of the four bit words A and B. Hence Full Adder-0 is the lowest stage.

Hence its Cin has been permanently made 0. The rest of the connections are exactly same as those of n-bit parallel adder is shown in fig.