NAND and NOR gates can be compared the same way.
A negative NAND is logically
equivalent to a positive NOR as shown in Figure 3-15.
Figure 3-15. Negative NAND equals positive NOR
The truth table (B, Figure 3-15) is identical for the two symbols in A of the figure. Likewise, a
positive NAND and a negative NOR are logically the same. The symbols represented in A (fig 3-15)
may or may not be the same, (i.e., the symbol may have a line over it in one case, or without a line in
another), but the function is the same.
Figure 3-16. Positive NAND equals negative NOR
a. The only reason for using a different symbol is to present the most accurate picture of the
circuit function. In reality, NOR and NAND gates are not usually rigidly defined as we presented them.
b. The symbol used in a negative logic system may very well be a positive NOR or NAND.
The symbol selected is the one that best describes the way the circuit is being used. This is especially
true in control logic where binary 1 or 0 is irrelevant and only highs and lows are important.