Explain The Logic EX-OR Gate (Exclusive-OR Gate) and Its Operation With Truth Table

EX-OR Gate (Exclusive-OR Gate)

Two special logic circuits that occur quite often in digital systems are the exclusive-OR and exclusive-NOR circuits.

The circuit of an exclusive-OR gate is shown in fig 1 .

Fig 1

As obvious, there are two inputs and one output. The exclusive-OR operation is denoted by . Hence the expression for the output of the circuit is given as :

Truth Table of EX-OR Gate

The truth table for two input  EX-OR gate is shown in Table 1 .

Table 1

 The output expression for two input EX-OR gate, in short, is given as

This circuit is also called an inequality comparator or detector because it produces output only when the two inputs are different.

The main characteristic property of an EX-OR gate is that it can perform modulo-2 addition. It should be noted that the same EX-OR truth table applies when adding two binary digits (bits). A 2-input EX-OR circuit is therefore sometimes called a modulo-2 adder or a half adder. The name half adder refers to the fact that possible carry-bit, resulting from an addition of two preceding bits, has not been taken into account. The name Exclusive-OR is derived from the fact that its output is a 1, only when exclusively one f its input is a 1 (it excludes the condition when both the inputs are 1).

Ex-OR Gate Equivalent Circuit

The Ex-OR function is not a basic logic gate but a combination of different logic gates connected together. Using the 2-input truth table above, we can expand the Ex-OR function to:

which means that we can realise this new expression using the following individual gates.

Fig 2

One of the main disadvantages of implementing the Ex-OR function above is that it contains three different types logic gates OR, NAND and AND within its design.

One easier way of producing the Ex-OR function from a single gate is to use the universal gates NAND gate or NOR gates as shown below.

Ex-OR Function Realisation using NAND gates

Fig 3

Ex-OR Function Realisation using NOR gates

Fig 4

Standard Package

Fig 5 shows the pinout diagram of an IC7486 which is a quad 2-input EX-OR gate.

Fig 5

It contains four 2-input EX-OR gate in a 14-pin DIP.

Pulsed Operation

The pulsed operation of an EX-OR gate is illustrated in fig 6 .

Fig 6

During time intervals  t₁  input A is high but input B is low and during time interval t₃ input A is low and input B is high, therefore output is high during these two time intervals. During time intervals t0 and t_2, both inputs are low, hence output is low. And during time interval t₄  both inputs are high, therefore output is low.