# Difference between Multiplexer and Demultiplexer

In this article we are going to cover the difference between multiplexer and demultiplexer in detail. They are the part of combinational circuits. Combinational circuits are those, whose output depends on the present input provided and does not get influenced by the previous state of input. Combinational circuits do not have any memory. Some examples of combinational circuit are adders, subtractors, encoder, decoder, multiplexer and demultiplexer.

## What is Multiplexer ?

A multiplexer or MUX is a digital logic circuit or a combinational circuit. MUX has many inputs and only one output. Its output depends upon the input given to the select lines ( discussed below ). A multiplexer ( MUX ) acts as a digital switch.

Suppose a multiplexer has 2n number of inputs then it has n number of select lines. If a MUX has 22 number of input pins then total inputs are 4 and number of select lines are 2. It is represented as 4:1 mux ( 4 input and one output ).

Similarly, if a MUX has 23 number of input pins then the number of input pins are 8 and number of select lines must be 3. It is known as 8:1 MUX ( 8 input and 1 output ). We have 2:1, 4:1, 8:1, 16:1 MUX as examples.

### 2:1 Multiplexer

To understand better lets take an example of 2:1 MUX. The block diagram of 2:1 MUX is given below –

Here, E ( enable ) is the compulsory pin to power on the MUX, it should always be at logic 1. At E = 0 there is no output. D0 and D1 are inputs, S is the select line and Y is the output.

#### Truth Table for 2:1 MUX

 E S D0 D1 Y 0 X X X X 1 0 ( D0 ) 0 X 0 1 0 ( D0 ) 1 X 1 1 1 ( D1 ) X 0 0 1 1 ( D1 ) X 1 1

At E = 1, when D0 is selected. The output reflects D0 and D1 gets blocked. Similarly, when D1 is selected, the output reflects D1 and D0 gets blocked.

The output equation Y will be written only when logic 1 is obtained in the output ( in SOP form )

Y = ES’D0 + ESD1

### 4:1 Multiplexer

Lets take another example of 4:1 MUX

4:1 MUX has 4 input, 1 output and 2 select lines. E ( enable will always be taken 1. The figure below shows block diagram for 4:1 MUX.

Here, E is enable, D0 D1 D2 D3 are 4 inputs, S1 and S0 are select lines and Y is the output.

### Truth Table for 4:1 MUX

 E S1 S0 Y 1 0 0 D0 1 0 1 D1 1 1 0 D2 1 1 1 D3

When select lines are –

0,0 then the input D0 will be obtained in output.

0,1 then the input D1 will be obtained in output.

1,0 then the input D2 will be obtained in output.

1,1 then the input D3 will be obtained in output.

The output equation Y will be written in SOP form

Y = S1’S0’D0 + S1’S0D1 + S1S0’D2 + S1S0D3

## What is Demultiplexer ?

A Demultiplexer or DEMUX is a digital logic circuit or a combinational circuit. DEMUX has only one input and many outputs. Its output depends upon the input given to the select lines ( discussed below ). A Demultiplexer ( DEMUX ) acts as a signal allocator.

Suppose a demultiplexer has 2n number of outputs then it has n number of select lines. If a DEMUX has 22 number of output pins then total outputs are 4 and number of select lines are 2. It is represented as 1:4 DEMUX ( 1 input and 4 output ).

Similarly, if a DEMUX has 23 number of output pins then the number of output pins are 8 and number of select lines must be 3. It is known as 1:8 DEMUX ( 1 input and 8 outputs ). We have 1:2, 1:4, 1:8, 1:16 DEMUX as examples.

### 1:2 Demultiplexer

To understand better lets take an example of 1:2 DEMUX. The block diagram of 1:2 DEMUX is given below –

Here, E ( enable ) is the compulsory pin to power on the DEMUX, it should always be at logic 1. At E = 0 there is no output. Din is the input, Y0 and Y1 are outputs and S is the select line.

### Truth Table for 1:2 DEMUX

 E Din S0 Y0 Y1 1 0 0 ( Y0 ) 0 X 1 1 0 ( Y0 ) 1 X 1 0 1 ( Y1 ) X 0 1 1 1 ( Y1 ) X 1

The above table shows when Y0 is selected then Din is obtained in Y0 output and Y1 gets blocked. Similarly, when Y1 is selected then Din is obtained in Y1 output and Y0 gets blocked.

The output equation Y0 and Y1 will be written only when logic 1 is obtained in the output ( in SOP form )

Y0 = EDinS0

Y1 = EDinS0

### 1:4 Demultiplexer

Lets take another example of 1:4 DEMUX

1:4 DEMUX has 1 input, 4 output and 2 select lines. E ( enable will always be taken 1. The figure below shows block diagram for 1:4 DEMUX.

Here, E is enable, Y0 Y1 Y2 Y3 are 4 outputs, S1 and S0 are select lines and Din is the input.

### Truth Table for 4:1 DEMULTIPLEXER

 S1 S0 Y0 Y1 Y2 Y3 0 0 Din 0 0 0 0 1 0 Din 0 0 1 0 0 0 Din 0 1 1 0 0 0 Din

The above table shows, when select lines are –

0,0 Y0 gets selected and others get blocked.

0,1 Y1 gets selected and others get blocked.

1,0 Y2 gets selected and others get blocked.

1,1 Y3 gets selected and others get blocked.

### Difference Between Multiplexer and Demultiplexer in Tabular Form

Difference between multiplexer and demultiplexer.

 Multiplexer Demultiplexer A Multiplexer is a combinational circuit that chooses one output from many inputs, depending upon the select lines. A Demultiplexer is a combinational circuit that provides many outputs from one input, depending upon the select lines. It has many inputs and one output. It has one input and many outputs. If a Multiplexer has 2n number of inputs then it must have n number of select lines. If a Demultiplexer has 2n number of outputs then it must have n number of select lines. A Multiplexer is a digital switch. A Demultiplexer is used for signal allocation. In wireless transmission of signals, a multiplexer is used in transmitter side. In wireless transmission of signals, a demultiplexer is used in receiver side.

#### Conclusion

To conclude, we can say that multiplexer and demultiplexer are opposite to one another. Their properties and working are complement of other.

Author

Akash Sharma

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