Figure 3-13. Negative logic circuit
Now look at Figure 3-14. Again, specifically notice the title and then on a separate paper
complete the truth table and draw a symbol to accurately represent it. Compare your results with the
answers in Figure 3-17.
Figure 3-14. Positive logic circuit
Figure 3-13 performs an AND function in negative logic, as indicated by the truth table, so it is
represented by an AND symbol with circles at the inputs and output. Figure 3-14 performs an OR
function in positive logic and is represented by an OR symbol with no circles on the inputs and output.
But, comparing the two schematics we find that the circuits are identical. A negative AND gate, then,
performs the same function as a positive OR gate. In most cases the schematics of the two gates are not
identical but it is possible. The important thing to note here is that logically, a negative AND and a
positive OR are equivalent; they have the same truth table. Conversely, a positive AND and a negative
OR are also logically equivalent.