The “N” letter on NAND stands for NOT, meaning that NAND logic gate is an AND gate with an inverter attached. So, its output is the opposite from AND. Its symbol is the same of AND but with a “o” on its output, meaning that the output is inverted. You can build yourself a NAND gate by connecting an AND gate to an inverter.
Figure 6: NAND logic gate.
|
A |
B |
Y |
|
0 |
0 |
1 |
|
0 |
1 |
1 |
|
1 |
0 |
1 |
|
1 |
1 |
0 |
Another way to understand NAND logic gate: its output will only be at “0” when all its inputs are also at “1”. Otherwise its output will be “1”.
If you need more inputs, just draw them on the symbol shown on Figure 6. However, if you want to create more inputs using gates with fewer inputs, you cannot connect them using the same idea shown on Figure 4. You need to use AND gates for the “extra” inputs (you can link them together to expand the number of inputs like shown on Figure 4) and a NAND gate for the “final” gate, see Figure 7.
Figure 7: How to expand the number of inputs of a NAND gate.
You can also easily transform NAND and NOR gates into inverters by shorting their inputs, like shown on Figure 8. This is a very common trick. For example, you need an inverter in your circuit and have some NAND gates available at a given integrated circuit. Instead of adding another integrated circuit to your project just to have one inverter (what would increase the final circuit size and also the cost), you may want to use one of the available NAND gates.
Figure 8: Transforming a NAND gate into an inverter.
The most famous NAND gate integrated circuit is 7400 and you can its pinout on Figure 9. Of course there are several other integrated circuits that provide NAND gates with more inputs. For example, 7411 provides three three-input NAND gates and 7430 provides one eight-input NAND gate.
click to enlarge
Figure 9: 7400 integrated circuit provides four two-inputs NAND gates.
