## Logic Gates

**Logic gates:**

Binary information is represented in digital computers by physical quantities called Signals. Electrical signals such as voltages exist throughout the computer in either one of two recognizable states. The two states represent a binary variable that can be equal to 1 or 0. Consider an example of a digital computer may employ a signal of 3 volts to represent binary 1 and 0.5 volt to represent binary 0. The input terminals of digital circuits accept binary signals of 3 and 0.5 volts and the circuits respond at the output terminals with signals of 3 and 0.5 volts to represent binary input and output corresponding to 1 and 0 respectively.

Binary logic deals with binary variables and with operations that assume a logical meaning. It is used to describe, in algebraic or tabular form, the manipulation and processing of binary information. The manipulation of binary information is done by logic circuits called Gates. Gates are blocks of hardware that produce signals of binary 1 or 0 when input logic requirements are satisfied. Each gate has a distinct graphic symbol and its operation can be described by means of an algebraic expression. The input and output relationship of the binary variables for each gate can be represented in tabular form by a truth table.

- The basic logic gates are AND and inclusive OR with multiple inputs
- NOT with a single input

Each gate has one or two binary input variables designated by A and B and one binary output variable designated by x.

- Inverter circuit inverts the logic sense of a binary signal. it produces the NOT, or complement , function.
- The small circle in the output of the graphic symbol of an inverter designates a logic complement.
- NAND function is the complement of AND function, as indicated by the graphic symbol, which consists of an AND graphic symbol followed by a small circle. The designation of NAND is derived from the abbreviation of NOT-AND
- NOR gate is complement of OR gate and uses an OR graphic symbol followed by a small circle.
- Both NAND and NOR gates may have more than two inputs and the output is always the complement of the AND or OR function, respectively.
- Exclusive OR gate has a graphic symbol similar to the OR gate except for the additional curved line on the input side. The output of this gate is 1 if any input is 1 but excludes the combination when both inputs are 1. A more fitting name for the exclusive OR operation would be an Odd function.
- Exclusive OR and exclusive NOR gates are commonly available with two inputs, and only seldom are they found with three or more inputs.

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