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ExclusiveOR gateThe last six gate types are all fairly direct variations on three basic functions: AND, OR, and NOT. The ExclusiveOR gate, however, is something quite different. ExclusiveOR gates output a "high" (1) logic level if the inputs are at different logic levels, either 0 and 1 or 1 and 0. Conversely, they output a "low" (0) logic level if the inputs are at the same logic levels. The ExclusiveOR (sometimes called XOR) gate has both a symbol and a truth table pattern that is unique: There are equivalent circuits for an ExclusiveOR gate made up of AND, OR, and NOT gates, just as there were for NAND, NOR, and the negativeinput gates. A rather direct approach to simulating an ExclusiveOR gate is to start with a regular OR gate, then add additional gates to inhibit the output from going "high" (1) when both inputs are "high" (1): In this circuit, the final AND gate acts as a buffer for the output of the OR gate whenever the NAND gate's output is high, which it is for the first three input state combinations (00, 01, and 10). However, when both inputs are "high" (1), the NAND gate outputs a "low" (0) logic level, which forces the final AND gate to produce a "low" (0) output. Another equivalent circuit for the ExclusiveOR gate uses a strategy of two AND gates with inverters, set up to generate "high" (1) outputs for input conditions 01 and 10. A final OR gate then allows either of the AND gates' "high" outputs to create a final "high" output: ExclusiveOR gates are very useful for circuits where two or more binary numbers are to be compared bitforbit, and also for error detection (parity check) and code conversion (binary to Grey and vice versa).


Home Digital Logic Gates Multipleinput gates ExclusiveOR gate 