Page 28 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 2: Logic Gates

2.1.3 The OR Gate Notes
As its name implies, an OR logic gate performs an ”OR“ logic operation, which is an addition.
It has at least two inputs. So, if A and B are its inputs, at the output we will find A + B. So, OR
logic gate can be summarized by the formula Y = A + B. You can see its symbol in Figure 2.6 and
its truth table right below it.
Figure 2.6: OR Logic Gate

Table 2.3: Truth Table

A B Y
0 0 0
0 1 1
1 0 1
1 1 1
Another way to understand OR logic gate: its output will only be at ”0“ when all its inputs are
also at ”0“. Otherwise its output will be ”1“.
If you need more than two inputs, the same idea applies. If you are projecting a circuit and need
an OR logic gate with more inputs, you can go ahead and simple draw an OR logic gate like
the one in Figure 2.6 and put more inputs on it. But if you are working with integrated circuits
with OR logic gates with fewer inputs that you need, you can expand the number of inputs by
connecting them like shown in Figure 2.7.
Figure 2.7: Expanding OR Inputs Using 2-Input Gates

The most famous OR gate integrated circuit is 7432 and you can its pinout in Figure 2.8. Of course
there are several other integrated circuits that provide OR gates with more inputs. For example,
7427 provides three three-input OR gates.
Figure 2.8: 7432 integrated Circuit Provides Four Two-Inputs OR Gates

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