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XNOR gate
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INPUT  OUTPUT  
A  B  A XNOR B 
0  0  1 
0  1  0 
1  0  0 
1  1  1 
The XNOR gate (sometimes spelled "exnor" or "enor" and rarely written NXOR) is a digital logic gate whose function is the logical complement of the exclusive OR (XOR) gate. The twoinput version implements logical equality, behaving according to the truth table to the right. A HIGH output (1) results if both of the inputs to the gate are the same. If one but not both inputs are HIGH (1), a LOW output (0) results.
Symbols
There are 2 symbols for XNOR gates: the 'distinctive' symbol and the 'rectangular' symbol. For more information see Logic Gate Symbols. The ANSI symbol for the XNOR gate is a standard XOR gate with an inversion bubble connected.
The XNOR gate with inputs A and B implements the logical expression <math>A \cdot B + \overline{A} \cdot \overline{B}</math>.
Hardware description and pinout
XNOR gates are represented in most TTL and CMOS IC families. The standard 4000 series CMOS IC is the 4077 and the TTL IC is the 74266. Both include four independent, twoinput, XNOR gates. The pinout diagram is as follows:
1 Input A1 2 Input B1 3 Output Q1 4 Output Q2 5 Input B2 6 Input A2 7 V_{ss} 8 Input A3 9 Input B3 10 Output Q3 11 Output Q4 12 I > 
This device is available from most semiconductor manufacturers such as NXP. It is usually available in both throughhole DIP and SOIC format. Datasheets are readily available in most datasheet databases. DIL is a Dual In Line package, and SIL is a Single In Line package.
Alternatives
If no specific XNOR gates are available, one can be made from four NOR gates or five NAND gates in the configurations shown below. In fact, any logic gate can be made from combinations of only NAND gates or only NOR gates.
XNOR gate can be made with 3 gates (1x OR & 2x NAND gates)
See also
40x40px  Wikimedia Commons has media related to XNOR gates. 
 AND gate
 OR gate
 NOT gate
 NAND gate
 NOR gate
 XOR gate
 Kronecker delta function
 Logical biconditional
References
