NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0).
If one or both input is HIGH (1), a LOW output (0) results. NOR is the result of the negation of the OR operator. NOR is a functionally complete operation—combinations of NOR gates can be combined to generate any other logical function. By contrast, the OR operator is monotonic as it can only change LOW to HIGH but not vice versa.
In most, but not all, circuit implementations, the negation comes for free—including CMOS and TTL. In such logic families, the only way to implement OR is with 2 or more gates, such as a NOR followed by an inverter. A significant exception is some forms of the domino logic family.
Symbols of NOR gate
There are three symbols for NOR gates: the American (ANSI or 'military') symbol and the IEC ('European' or 'rectangular') symbol, as well as the deprecated DIN symbol.
Alternatives of NOR gate
If no specific NOR gates are available, one can be made from NAND gates in the configuration shown below. Interestingly, any logic gate can be made either from a combination of NAND gates or from a combination of NOR gates.