The NOR operation is a combination of the NOT and the OR operation (Not-OR).
The logic diagram and the truth table for a three-input NOR operation are shown
The NOR-logic diagram above is composed of an OR gate whose output is NOTted
(inverted) by the NOT circuit. The output of the OR gate is the input to the NOT
circuit; the NOT circuit inverts this input and produces the complement at the output,
as shown in the truth table.
A NOR gate will always NOT (invert) an ORed