Experiment 2 - The Half Adder

How do you add two binary numbers? First consider adding two 1-bit numbers.

The addition of two 1-bit binary numbers looks like...

0 0 1 1 +  0 +  1 +  0 +  1 = 00 = 01 = 01 = 10 Binary Addition.

A B Q CO 0 0 0 0 0 1 1 0 1 0 1 0 1 1 0 1 Truth table half adder.

To let the addition of two 1-bit numbers be done by a logic circuit, you need to look at the truth table...

If you are only interested in a 1-bit response Q, you only need an XOR operation.

However, if you add up 1 and 1, the answer is more complicated. A 1 must be transferred to the next digit. This bit is called a Carry-Bit. As the output of a logic circuit, it is indicated with CO (Carry Out). The result of CO is realized with an AND operation.

Half adder circuit. (Enlarge)

Circuit diagram half adder.

This circuit is called a half adder.

But what do you need to add two 2-bit numbers...?