The NOR or “Not OR” gate is also a combination of two separate logic functions, Not and OR connected together to form a single logic function which is the same as the OR function except that the output is inverted.
To create a NOR gate, the OR function and the NOT function are connected together in series with its operation given by the Boolean expression as, A + B
The Logic NOR Function only produces and output when “ALL” of its inputs are not present and in Boolean Algebra terms the output will be TRUE only when all of its inputs are FALSE.
Switch Representation of the NOR Function
The truth table for the NOR function is the opposite of that for the previous OR function because the NOR gate performs the reverse operation of the OR gate. Then we can see that the NOR gate is the complement of the OR gate.
NOR Function Truth Table
| Switch A | Switch B | Output | Description |
| 0 | 0 | 1 | Both A and B are open, lamp ON |
| 0 | 1 | 0 | A is open and B is closed, lamp OFF |
| 1 | 0 | 0 | A is closed and B is open, lamp OFF |
| 1 | 1 | 0 | A is closed and B is closed, lamp OFF |
| Boolean Expression (A OR B) | A + B | ||
The NOR Function is sometimes known as the Pierce Function and is denoted by a downwards arrow operator as shown, A NOR B = A↓B.
Logic NOR gates are available as standard i.c. packages such as the TTL 74LS02 Quadruple 2-input NOR Gate, the TTL 74LS27 Triple 3-input NOR Gate or the 74LS260 Dual 5-input NOR Gate.
