Introduction to Logic Gates

NOR

The “N” letter on NOR stands for NOT, meaning that NOR logic gate is an OR gate with an inverter attached. So, its output is the opposite from OR. Its symbol is the same of OR but with a “o” on its output, meaning that the output is inverted. You can build yourself a NOR gate by connecting an OR gate to an inverter.

Figure 13: NOR logic gate.

Another way to understand NOR logic gate: its output will only be at “1” when all its inputs are at “0”. Otherwise its output will be “0”.

If you need more inputs, just draw them on the symbol shown in Figure 13. However, if you want to create more inputs using gates with fewer inputs, you cannot connect them using the same idea shown in Figure 11. You need to use OR gates for the “extra” inputs (you can put them together to expand the number of inputs like shown in Figure 11) and a NOR gate for the “final” gate, see Figure 14.

Figure 14: How to expand the number of inputs of a NOR gate.

You can also easily transform NAND and NOR gates into an inverter by shorting their inputs, like shown in Figure 15. This is a very common trick. For example, you need an inverter in your circuit and have some NOR or 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 or NOR gates.

Figure 15: Transforming a NOR gate into an inverter.

The most famous NOR gate integrated circuit is 7402 and you can its pinout in Figure 16. Pay attention because the location of the inputs and outputs on this integrated circuit is different from the other integrated circuits we’ve seen before. Of course there are several other integrated circuits that provide NOR gates with more inputs. For example, 7427 provides three three-input NOR gates.

Figure 16: 7402 integrated circuit provides four two-inputs NOR gates.