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In the English language, the term "Gate" is
"a structure that can be swung, drawn, or lowered to block an
entrance or a passageway". An example is a door on a fence, which can
be opened to let people in, and closed to keep people out. In computers,
logic gates have a similar application. A logic gate has one or more
inputs (either 1 or 0), and one output. Logic gates are designed to allow
the passage of binary signals through the gate for certain combinations of
inputs.
These gates might seem simple, primitive, and useless.
Although one logic gate alone is almost useless, a combination of the
gates working together can serve many purposes. Today's computers, which
do very complex work, are made up of millions of these simple logic gates.
There are three basic logic gates. They are the AND, OR and NOT gates.
These gates are used in different combinations to make more complex gates
such as the XOR. As you will find out in a moment, the three basic logic
gates are very easy to understand.
But before we learn about logic gates, there are some
terms that we should know about:
1.Truth Table: A truth table is a
table of combinations. It shows all the input possibilities and the
outputs which result from these possibilities. For example, here
is a truth table showing what will be the products when the number 2 is
multiplied by the numbers from 1-5.
| Numbers |
Product |
| 2 * 1 |
2 |
| 2 * 2 |
4 |
| 2 * 3 |
6 |
| 2 * 4 |
8 |
| 2 * 5 |
10 |
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This is called a truth table because it is a table which
has facts about the product of two and numbers from 1-5. A truth
table for logic gates will be used to show you all the possible inputs for
the gates, and the resulting outputs for each combination of inputs.
2. Gate Symbols: Gate symbols are used in diagrams
for planning before computer circuitry is built. The gates do not
really look like the symbols, the symbols are used to make the diagrams
simpler and more comprehensive.
3. More Things to Know About Binary: From the
previous lessons, we know that 1's and 0's are the two possibilities in
binary. In the computer world, you will frequently hear
"HIGH" used instead of 1, and "LOW" being used instead
of 1.
4. Inputs and Outputs: The inputs and outputs for
logic gates are represented by letters. Commonly, the output is
represented by the letter Q or the letter Y. We will be using the
letter Y for the output. The inputs are almost always represented by
the first letters of the alphabet. A and B are used the most often, while
C, D and other letters are used when the diagrams are more complex.
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