Page 30 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 2: Logic Gates

Notes
Figure 2.12: 7400 Integrated Circuit Provides Four Two-Inputs NAND Gates

To avoid the short circuiting use caution when wiring chip and be sure to
check which pins are outputs and inputs.

2.1.5 The NOR Gate
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 2.13: NOR Logic Gate

Table 2.5: Truth Table

A B Y
0 0 1
0 1 0
1 0 0
1 1 0

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 2.13. However, if you
want to create more inputs using gates with fewer inputs, you cannot connect them using the
same idea shown in Figure 2.11. 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 2.11) and a NOR gate for the
“final” gate, see Figure 2.14.

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