An OR gate performs an ORing operation on two or more than two logic variables. The OR operation on two independent logic variables A and B is written as Y = A+B and reads as Y equals A OR B and not as A plus B.
An OR gate is a logic circuit with two or more inputs and one output. The output of an OR gate is LOW only when all of its inputs are LOW. For all other possible input combinations, the output is HIGH.
This statement when interpreted for a positive logic system means the following:
The output of an OR gate is a logic ‘0’ only when all of its inputs are at logic ‘0’. For all other possible input combinations, the output is a logic ‘1’.
Figure 1 shows the circuit symbol and the truth table of a two-input OR gate. The operation of a two-input OR gate is explained by the logic expression Y = A+B
As an illustration, if we have four logic variables and we want to know the logical output of (A+ B+C +D), then it would be the output of a four-input OR gate with A, B, C and D as its inputs.
Figures 2 and 4 show the circuit symbol of three-input and four-input OR gates. Figure 3 shows the truth table of a three-input OR gate.
Logic expressions explaining the functioning of three input and four-input OR gates are Y = A+B+C and Y = A+B+C +D.