When does a NOR gate output a HIGH signal?

A NOR gate outputs a HIGH signal only when all of its inputs are LOW. This is a fundamental characteristic of the NOR gate, which stands for "Not OR." Essentially, the NOR gate performs the logical operation of an OR gate followed by a NOT operation. Therefore, when every input to the NOR gate is LOW (0), it confirms that the condition for a HIGH output (1) is met by the absence of any HIGH (1) inputs.

If at least one input were HIGH, the output would be LOW, which contradicts the requirement for a HIGH output. This unique behavior enables NOR gates to serve as universal gates in digital logic, meaning they can be used to create any other logic gate configuration.