XNOR gate

Datasheets are readily available in most datasheet databases and suppliers.

Both the 4077 and 74x266 devices (SN74LS266, 74HC266, 74266, etc.) have the same pinout diagram, as follows:

For the NAND constructions, the lower arrangement offers the advantage of a shorter propagation delay (the time delay between an input changing and the output changing). For the NOR constructions, the upper arrangement requires fewer gates.

From the opposite perspective, constructing other gates using only XNOR gates is possible though XNOR is not a fully universal logic gate. NOT and XOR gates can be constructed this way.

Although other gates (OR, NOR, AND, NAND) are available from manufacturers with three or more inputs per gate, this is not strictly true with XOR and XNOR gates. However, extending the concept of the binary logical operation to three inputs, the SN74S135 with two shared "C" and four independent "A" and "B" inputs for its four outputs, was a device that followed the truth table:

This is effectively Y = NOT ((A XOR B) XOR C). Another way to interpret this is that the output is true if an even number of inputs are true. It does not implement a logical "equivalence" function, unlike two-input XNOR gates.

Additionally, the XOR function seems to act as a parity function or Mod2 for the sum of all inputs. Note how *y* is equal to 1 if the sum of all inputs is even, which implies that y = 0 if the sum of all inputs is odd. We may conclude from this that x XOR y XOR z = is even (x+y+z).