Logic Circuits
Electronics which relies on switching between two states (on or off, voltage or no voltage, high voltage or low voltage, for example), is made up of what are called logic circuits. This name comes from the fact that the circuits deal with two possibilities. In the social science of logic two such opposing states are often verbalized as "yes or no," "true or false," and in slightly more sophisticated reasoning, "if yes(or no), then some action (or other) will be taken."
Computers operate electronically using such circuitry or "logic gates." These are built into integrated circuits that are part of the central processing unit and other computer "chips" on the mother board and most of the cards mounted to it. The most basic logic gate has one input and one output. Logic gate inputs or outputs represent one bit of information in one of two states, usually thought of as a 1 for "on," "yes," or "true," and a 0 for "off," "no," or "false." Individual gates are connected in large numbers to produce the sophisticated computing power available to computers owners today.
The most common logic gates currently in use make decisions or, more correctly, turn on or off, given one or two conditions. This type of action follows four basic logical operations for which the gates are named, NOT, AND, OR, and EXCLUSIVE OR. These form the basis for computer operations and are built upon and expanded to create sophisticated programs and computer controlled actions. The NOT gate is the simplest of the four. It is composed of one input and one output. The "logic table" for this gate is simple as well. In words, it can be written as follows: If there is no input, there is an output. If there is an input, there is no output. The name NOT makes sense if we write this in a way that is not quite as correct grammatically, but perhaps more illustrative: If NOT input, then output. If input, then NOT output.
The AND gate is a bit more complex, having two inputs and one output. It requires that both inputs be present, or on, in order for there to be an output. As its name implies, one must have input a AND input b in order for an output to exist. With the OR gate, only one input or the other need be present in order for an output to exist (If a OR b, then output). But the OR gate also allows output if BOTH inputs are present, just as with an AND gate. The EXCLUSIVE OR gate does away with this last situation because it only allows output if ONLY input a OR b are present, but not both. More complex gates can be made by adding more inputs, combining gates with others of the same type of different types.
While this may all seem confusing to read about, mathematically, and physically as well, it is really quite simple. Logic circuits that use the most basic mode of operation (when voltage is applied the input or output is on, when there is no voltage, they are off) may be though of, almost literally, as being composed of tiny switches. What is truly difficult to comprehend for most people is how a combination of millions, or perhaps billions, of tiny switches going on and off in some precise, planned patterns millions of times per second can produce the amazing things made possible by modern computers.
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