logic gate (AND, OR, XOR, NOT, NAND, NOR and XNOR)
A logic gate is an elementary building block of a digital circuit. Most logic gates have two inputs and one output. At any given moment, every terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage levels. The logic state of a terminal can, and generally does, change often, as the circuit processes data. In most logic gates, the low state is approximately zero volts (0 V), while the high state is approximately five volts positive (+5 V).
There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR.
The AND gate is so named because, if 0 is called false and 1 is called true, the gate acts in the same way as the logical and operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is true when both inputs are true. Otherwise, the output is false.
Using combinations of logic gates, complex operations can be performed. In theory, there is no limit to the number of gates that can be arrayed together in a single device. But in practice, there is a limit to the number of gates that can be packed into a given physical space. Arrays of logic gates are found in digital integrated circuits (ICs ). As IC technology advances, the required physical volume for each individual logic gate decreases and digital devices of the same or smaller size become capable of performing ever-more-complicated operations at ever-increasing speeds.
See an introductory tutorial on logic gates:
This was last updated in May 2015
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