The 74HC266N CMOS integrated circuit contains 4 XNOR logic gates, which is the negated form of the XOR gate, it is generally more used to perform flip flops, which is one of the main functions of this gate.
Specifications:
Wide Operating Voltage Range of 2 V to 6 V
HighCurrent Inverting Outputs Drive Up To10 LSTTL Loads
Low Power Consumption, 20µA Max ICC
Typical TDP = 10 ns
±4mA Output Drive at 5 V
Low Input Current of 1 µA Max
Logic Gate XNOR 74HC266N Pinout:
the “ExclusiveNOR” gate is a combination of the ExclusiveOR gate and the NOT gate but has a truth table similar to the standard NOR gate in that it has an output that is normally at logic level “1” and goes “LOW” to a logic level “0” when ANY of its inputs are at logic level “1”.
However, an output “1” is only obtained if BOTH of its inputs are at the same logic level, either binary “1” or “0”. For example, “00” or “11”. This input combination would then give us the Boolean expression of Q = (A ⊕ B) = A.B + A.B
Then the output of a digital logic ExclusiveNOR gate ONLY goes “HIGH” when its two input terminals, A and B are at the “SAME” logic level which can be either at a logic level “1” or at a logic level “0”. In other words, an even number of logic “1’s” on its inputs gives a logic “1” at the output, otherwise is at logic level “0”.
Then this type of gate gives an output “1” when its inputs are “logically equal” or “equivalent” to each other, which is why an ExclusiveNOR gate is sometimes called an Equivalence Gate.
The logic symbol for an ExclusiveNOR gate is simply an ExclusiveOR gate with a circle or “inversion bubble”, ( ο ) at its output to represent the NOT function. Then the Logic ExclusiveNOR Gate is the reverse or “Complementary” form of the ExclusiveOR gate, (A ⊕ B) we have seen previously.
The ExclusiveNOR Gate, also written as “ExNOR” or “XNOR”, a function is achieved by combining standard gates together to form more complex gate functions and an example of a 2input ExclusiveNOR gate is given below.
Symbol  Truth Table  
2input ExNOR Gate

B  A  Q 
0  0  1  
0  1  0  
1  0  0  
1  1  1  
Boolean Expression Q = A ⊕ B  Read if A AND B the SAME gives Q 
Giving the Boolean expression of Q = AB + AB
The logic function implemented by a 2input ExNOR gate is given as “when both A AND B are the SAME” will give an output at Q. In general, an ExclusiveNOR gate will give an output value of logic “1” ONLY when there is an EVEN number of 1’s on the inputs to the gate (the inverse of the ExOR gate) except when all its inputs are “LOW”.