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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.

Figure 2-16. - NOR gate input and output signals.

TRUTH TABLE

The Truth Table for a NOR gate with K and L as inputs is shown below.

f = K + L

Q.20 How does a NOR gate differ from an OR gate?
Q.21 What will be the output of a NOR gate when both inputs are HIGH?
Q.22 What is the output Boolean expression for a NOR gate with R and T as inputs?
Q.23 In what state must the inputs to a NOR gate be in order to produce a logic 1 output?