NOR stands for NOT OR. An OR gate followed by a NOT circuit makes it a NOR gate. Figure shows the symbol and truth table of NOR gate. The truth table of a NOR gate is obtained from the truth table of an OR gate by complementing the output. The output of a NOR gate is a logic ‘1’ when all its inputs are logic ‘0’. For all other input combinations, the output is a logic ‘0’. The output of a two-input NOR gate is logically expressed as Y = (A+B)’
CMOS NOR Gate
The circuit below has two inputs and one output. Whenever at least one of the inputs is high, the corresponding N-type transistor will be closed while the P-type transistor will be open. Consequently, the ouput voltage will be low. Conversely, if both inputs are low, then both P-type transistors at the top will be closed circuits and the N-type transistors will be open. Hence, the output voltage is high. The function of this gate can be summarized by the table. If logical 1’s are associated with high voltages then the function of this gate is called NOR for negated OR. Again, there is never a conducting path from the supply voltage to ground.