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 2^{n} number of inputs then it has n number of select lines. If a MUX has 2^{2} 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 2^{3} 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. D_{0 }and D_{1} are inputs, S is the select line and Y is the output.
Truth Table for 2:1 MUX
E 
S 
D_{0} 
D_{1} 
Y 


0 
X 
X 
X 
X 
1 
0 ( D_{0 }) 
0 
X 
0 
1 
0 ( D_{0} ) 
1 
X 
1 
1 
1 ( D_{1 }) 
X 
0 
0 
1 
1 ( D_{1 }) 
X 
1 
1 
At E = 1, when D_{0} is selected. The output reflects D_{0 }and D_{1 }gets blocked. Similarly, when D_{1 }is selected, the output reflects D_{1} and D_{0} gets blocked.
The output equation Y will be written only when logic 1 is obtained in the output ( in SOP form )
Y = ES’D_{0} + ESD_{1}
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, D_{0} D_{1 }D_{2} D_{3} are 4 inputs, S_{1 }and S_{0} are select lines and Y is the output.
Truth Table for 4:1 MUX
E 
S_{1} 
S_{0} 
Y 
1 
0 
0 
D_{0} 
1 
0 
1 
D_{1} 
1 
1 
0 
D_{2} 
1 
1 
1 
D_{3} 
When select lines are –
0,0 then the input D_{0} will be obtained in output.
0,1 then the input D_{1} will be obtained in output.
1,0 then the input D_{2} will be obtained in output.
1,1 then the input D_{3} will be obtained in output.
The output equation Y will be written in SOP form
Y = S_{1}’S_{0}’D_{0} + S_{1}’S_{0}D_{1} + S_{1}S_{0}’D_{2 }+ S_{1}S_{0}D_{3 }
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 2^{n} number of outputs then it has n number of select lines. If a DEMUX has 2^{2} 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 2^{3} 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, Y_{0 }and Y_{1} are outputs and S is the select line.
Truth Table for 1:2 DEMUX
E 
D_{in} 
S_{0} 
Y_{0} 
Y_{1} 
1 
0 
0 ( Y_{0} ) 
0 
X 
1 
1 
0 ( Y_{0} ) 
1 
X 
1 
0 
1 ( Y_{1} ) 
X 
0 
1 
1 
1 ( Y_{1} ) 
X 
1 
The above table shows when Y_{0} is selected then D_{in} is obtained in Y_{0} output and Y_{1} gets blocked. Similarly, when Y_{1} is selected then D_{in} is obtained in Y_{1} output and Y_{0} gets blocked.
The output equation Y_{0} and Y_{1} will be written only when logic 1 is obtained in the output ( in SOP form )
Y_{0} = ED_{in}S_{0}’
Y_{1} = ED_{in}S_{0}
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, Y_{0} Y_{1 }Y_{2} Y_{3} are 4 outputs, S_{1 }and S_{0} are select lines and D_{in} is the input.
Truth Table for 4:1 DEMULTIPLEXER
S_{1} 
S_{0} 
Y_{0} 
Y_{1} 
Y_{2} 
Y_{3} 
0 
0 
D_{in} 
0 
0 
0 
0 
1 
0 
D_{in} 
0 
0 
1 
0 
0 
0 
D_{in} 
0 
1 
1 
0 
0 
0 
D_{in} 
The above table shows, when select lines are –
0,0 Y_{0} gets selected and others get blocked.
0,1 Y_{1} gets selected and others get blocked.
1,0 Y_{2} gets selected and others get blocked.
1,1 Y_{3 }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 2^{n} number of inputs then it must have n number of select lines. 
If a Demultiplexer has 2^{n} 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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