Full adders are obviously made of 2 half adders, as well as an OR gate. What is special about a Full adder is that is has 3 inputs and only 2 outputs. In a later lesson you will see how this makes sense. Cin stands for "carry in" and Cout stands for "carry out." Each Cin is connected to the Cout of a previous full adder. Each bit of the input, in a pair of A and B, is used for every following full adder.
The image below is the written representation of a full adder and its 3 inputs (Cin, A, B) and 2 outputs (Cout, S). The chart below shows the combinations of inputs and their resulting outputs:
| CI | A | B | S | CO |
| 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 | 0 |
| 0 | 1 | 0 | 1 | 0 |
| 0 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 0 |
| 1 | 0 | 1 | 0 | 1 |
| 1 | 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 | 1 |
Try to figure out how multiple full adders are connected.
Congratulations, you have finished Lesson 5.
This concludes our brief introduction to logic circuits. To further explore logic circuits and to build your own circuits using software simulators, read:
by Chris Meyers