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ELECTRONICS MECHANIC - CITS

LESSON 9 - 29 : Combinational Logic Circuit

Objectives

At the end of this lesson you shall be able to
• state various logic circuits such as half adder, full adder, encoducer etc.
• state different types of flipflop and their application
• explain about careless & their types.

Binary Addition Circuits
The operation of adding two binary numbers is one of the fundamental tasks performed by a digital computer. The
four basic addition operations are 0 + 0 = 0, 1 + 0 = 1, 0 + 1 = 1 and 1 + 1 = 10. In the first three operations, each
binary addition gives sum as one bit, i.e., either 0 or 1.
But the fourth addition operation gives a sum that consists of two binary digits. In such result of the addition, lower
significant bit is called as the sum bit, whereas the higher significant bit is called as the carry bit. The logic circuits
which are designed to perform the addition of two binary numbers are called as binary adder circuits.
Half Adder
A logic circuit block used for adding two one bit numbers or simply two bits is called as a half adder circuit. This
circuit has two inputs which accept the two bits and two outputs, with one producing sum output and other produce
carry output.
To accomplish the binary addition with Ex-OR gate, there is need of additional circuitry to perform the carry op-
eration. Hence, a half adder is formed by connecting AND gate to the input terminals of the Ex-OR gate so as to
produce the carry as shown in below.
Half adder has Application especially multiedition must be added along.

Full Adder
A binary full adder is a multiple output combinational logic network that perform the arithmetic sum of three input bits.
As the half adder cannot respond to the three inputs and hence the full adder is used to add three digits at a time.
It consists of three inputs, in which two are input variables represent the two significant bits to be added, labeled
as A and B, whereas the third input terminal is the carry from the previous lower significant position and labeled
as Cin. The two outputs are a sum and a carry outputs which are labeled as ∑ and Cout respectively.
Full adder can be formed by combining two half adders and an OR gate as shown in above where
output and carry-in of the first adder becomes the input to the second half adder that produce the total sum output.
The total carry out is produced by ORing the two half adder carry outs as shown in figure(above).

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