3-6
You will notice that the output produced is the same as the output for the Truth Table of an X-OR.
Therefore, an X-OR gate can be used as a quarter adder.
The combination of gates in figure 3-6 will also produce the desired results. When A and B are both
LOW (0), the output of each AND gate is LOW (0); therefore, the output of the OR gate is LOW (0).
When A is HIGH and B is LOW, then B is HIGH and AND gate 1 produces a HIGH output, resulting in
a sum of 1 at gate 3. With A LOW and B HIGH, gate 2 output is HIGH, and the sum is 1. When both A
and B are HIGH, neither AND gate has an output, and the output of gate 3 is LOW (0); no carry is
produced.
Figure 3-6. —Quarter adder.
HALF ADDER
A half adder is designed to combine two binary digits and produce a carry.
Figure 3-7 shows two ways of constructing a half adder. An AND gate is added in parallel to the
quarter adder to generate the carry. The SUM column of the Truth Table represents the output of the
quarter adder, and the CARRY column represents the output of the AND gate.
Figure 3-7. —Half adders and Truth Table.
