2-11 also goes HIGH and remains HIGH until both inputs are again LOW. At T₅, both X and Y go HIGH causing f to go HIGH. Figure 2-8. OR gate input and output signals. TRUTH TABLE Using the inputs X and Y, lets construct a Truth Table for the OR gate. You can see from the discussion of figure 2-8 that there are four combinations of inputs. List each of these combinations of inputs and the respective outputs and you have the Truth Table for the OR gate. X Y f 0 0 0 0 1 1 1 0 1 1 1 1 f = X + Y When writing or stating the Boolean expression for an OR gate with more than two inputs, simply place the OR sign (+) between each input and read or state the sign as OR. For example, the Boolean expression for an OR gate with the inputs of A, B, C, and D would be: f = A+B+C+D This expression is spoken "f equals A OR B OR C OR D." You can substitute the complements for the original statements as we did with the AND gate or use negative logic; but for an output from an OR gate, at least one of the inputs must be TRUE.