A NOR gate is complemented OR gate. That is, the output of a NOR gate will be 1 only when all input are 0, and it will be 0 if any input is a 1.
Below figure 5.9 shows the truth table and block diagram symbol for a NOR gate. The symbol ‘ ’ is usually used to represent a NOR operation is Boolean expressions.
An OR gate followed by a NOT gate operates as a NOR gate (see the below figure). In the Figure the output of the OR gate will be A +B that in turn is fed as input to the NOT gate. Hence, the final output will be complement of A +B, which is equal to (A + B) or A.B or A B. In fact, the small circle on the output of NOR gate (see Figure 5.10) represents Complementation.
Multiple-input NOR gates can be analyzed similarly. A three-input NOR gate with inputs A,B, and C will have an output equal to A + B + C or A. B. C meaning that the output will be 1 only when all three inputs are 0, and it will be 0 if any of the three inputs is a 1.
