Figure 3-13. Negative logic circuit

Logic Circuits and Devices
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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.