LOGIC GATES (AND, OR, XOR, NOT, NAND, NOR and XNOR)

                                   AND | OR | XOR | NOT | NAND | NOR | 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."

AND GATE

       The OR gate gets its name from the fact that it behaves after the fashion of the logical
       inclusive "or." The output is "true" if either or both of the inputs are "true." If both
       inputs are "false," then the output is "false."

OR GATE

        The XOR ( exclusive-OR ) gate acts in the same way as the logical "either/or." The
        output is "true" if either, but not both, of the inputs are "true." The output is "false" if
        both inputs are "false" or if both inputs are "true." Another way of looking at this
        circuit is to observe that the output is 1 if the inputs are different, but 0 if the inputs
        are the same.

XOR GATE

        A logical inverter , sometimes called a NOT gate to differentiate it from other types
        of electronic inverter devices, has only one input. It reverses the logic state.

NOT GATE

      The NAND gate operates as an AND gate followed by a NOT gate. It acts in the
       manner of the logical operation "and" followed by negation. The output is "false"
      if both inputs are "true." Otherwise, the output is "true."

NAND GATE

      The NOR gate is a combination OR gate followed by an inverter. Its output is
      "true" if both inputs are "false." Otherwise, the output is "false."
                                                                       
                                                                    

                                                                        NOR GATE
                                                                               

      The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an
       inverter. Its output is "true" if the inputs are the same, and"false" if the inputs
       are different.
                                                               
                                                                   
                                                 
                                                                         XNOR GATE

        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: