2-16
Q18. What is the output Boolean expression for a NAND gate with inputs A, B , and C?
Q19. A NAND gate has inputs labeled as A, B , and C. If A and B are HIGH, C must be at what logic
level to produce a HIGH output?
THE NOR GATE
As you might expect, the NOR gate is an OR gate with an inverter on the output.
LOGIC SYMBOL
The standard logic symbol for this gate is shown in figure 2-14. More than just the two inputs may
be shown.
Figure 2-14. —NOR gate.
The NOR gate will have a HIGH output only when all the inputs are LOW.
When broken down, the two functions performed by the NOR gate can be represented by the
equivalent circuit depicted in figure 2-15. When both inputs to the OR gate are LOW, the output is LOW.
A LOW applied to an inverter gives a HIGH output. If either or both of the inputs to the OR gate are
HIGH, the output will be HIGH. When this HIGH output is applied to the inverter, the resulting output is
LOW. The Boolean expression for the output of this NOR gate is f = K
L
+
. The expression is spoken,
"K OR L quantity NOT."
Figure 2-15. —NOR gate equivalent circuit.
NOR GATE OPERATION
The logic level inputs and corresponding outputs for a NOR gate are shown in figure 2-16. At time
T0, both K and L are LOW; as a result, f is HIGH. At T1, K goes HIGH, L remains LOW, and f goes
LOW. At T2, K goes LOW, L goes HIGH, and the output remains LOW. The output goes HIGH again at
T3 when both inputs are LOW. At T4 when both inputs are HIGH, the output goes LOW and remains
LOW until T5 when both inputs go LOW. Remember the output is just opposite of what it would be for an
OR gate.
