NOR Gate | Symbol, Truth table & Circuit

A NOR gate is a combination of an OR gate and NOT gate. If we connect the output of OR gate to the input of a NOT gate, the gate so obtained is known as NOR gate. This gate is also called as Negated OR gate.

In Boolean expression, the NOR gate is expressed as and is being read as “A or B negated” or “A or B bar”.

NOR gate symbol

The logic symbol of NOR gate is shown in figure 1(a).

NOR gate symbol
Fig. 1

Figure 1(b) shows the NOR gate as the combination of OR gate and NOT gate.

NOR gate truth table

The NOR gate truth table for figure 1(a) is shown below.

A B
0 0 1
0 1 0
1 0 0
1 1 0

The NOR gate truth table for figure 1(b) is shown below.

A B X = A+B
0 0 0 1
0 1 1 0
1 0 1 0
1 1 1 0

From NOR gate truth table, it can be concluded that the output will be logical 1 or high when all inputs are at logical 0 or low.

NOR gate as Universal gate

A universal gate is a gate which can implement any Boolean function without need to use any other gate type. The NAND and NOR gates are universal gates.

The repeated use of the NOR gate can produce all other logic gates. Thus in digital circuits, it serves as a building block.

NOT gate using NOR gate

To produce NOT gate using NOR gate, the two inputs are joined together as shown in figure 2.

NOT gate using NOR gate
Fig. 2

The truth table of the above combination is given below.

Input (A) Output (Y = )
0 1
1 0

AND gate using NOR gate

To produce AND gate using NOR gate, the two output of the NOT gates (made from NOR gate by joining the two inputs) are connected to the NOR gate as shown in fig. 3.

AND gate using NOR gate
Fig. 3

The truth table of the above combination is given below.

A B X Z Y = A.B
0 0 1 1 0
0 1 1 0 0
1 0 0 1 0
1 1 0 0 1

OR gate using NOR gate

To produce OR gate using NOR gate, the output of the NOR gate is fed as input to NOT gate (made from NOR gate by joining the two inputs) as shown in fig. 4.

OR gate using NOR gate
Fig. 4

The truth table of the above combination is given below.

A B X Y = A+B
0 0 1 0
0 1 0 1
1 0 0 1
1 1 0 1

 

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