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. D_in 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 D_in is obtained in Y₀ output and Y₁ gets blocked. Similarly, when Y₁ is selected then D_in 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 = ED_inS₀’

Y₁ = ED_inS₀

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 D_in 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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