Page 74 - Handout Digital Electronics

CARRY SUM B A
0 0 0 0
0 1 0 1
0 1 1 0
1 0 1 1

From the truth table, it is easy to derive the Boolean functions used to implement the Half Adder logic
circuit. For example, SUM = BA + BA = A  B and CARRY = BA

Symbol for a Half Adder

A HA Sum

B Carry

The Major disadvantage of a Half Adder is its inability to take a carry from the right.

9.3 Full Adder (FA)
The Full Adder was developed to eliminate the major disadvantage of the Half Adder that is its inability
to handle carries from the right. The Full Adder is implemented on two Half Adders. The operation of a
Full Adder is best described by a truth table.

Full Adder Truth Table

Carryout(Cout) Sum Carry in (Cin) B A
0 0 0 0 0
0 1 0 0 1
0 1 0 1 0
1 0 0 1 1
0 1 1 0 0
1 0 1 0 1
1 0 1 1 0
1 1 1 1 1

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