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  ../images/main/bullet_green_ball.gif Multiplexer

A multiplexer (MUX) is a digital switch which connects data from one of n sources to the output. A number of select inputs determine which data source is connected to the output. The block diagram of MUX with n data sources of b bits wide and s bits wide select line is shown in below figure.

   

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../images/digital/mux_nx1_block.gif
   

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MUX acts like a digitally controlled multi-position switch where the binary code applied to the select inputs controls the input source that will be switched on to the output as shown in the figure below. At any given point of time only one input gets selected and is connected to output, based on the select input signal.

   

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  ../images/main/bulllet_4dots_orange.gif Mechanical Equivalent of a Multiplexer

The operation of a multiplexer can be better explained using a mechanical switch as shown in the figure below. This rotary switch can touch any of the inputs, which is connected to the output. As you can see at any given point of time only one input gets transferred to output.

   

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../images/digital/mux_nx1_sig.gif
   

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  ../images/main/bulllet_4dots_orange.gif Example - 2x1 MUX

A 2 to 1 line multiplexer is shown in figure below, each 2 input lines A to B is applied to one input of an AND gate. Selection lines S are decoded to select a particular AND gate. The truth table for the 2:1 mux is given in the table below.

   

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Symbol

../images/digital/mux2x1_truth.gif
   

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Truth Table

S

Y

0

A

1

B

   

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  ../images/main/bullet_star_pink.gif Design of a 2:1 Mux

To derive the gate level implementation of 2:1 mux we need to have truth table as shown in figure. And once we have the truth table, we can draw the K-map as shown in figure for all the cases when Y is equal to '1'.

   

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Combining the two 1' as shown in figure, we can drive the output y as shown below

   

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Y = A.S' + B.S

   

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Truth Table

B

A

S

Y

0

0

0

0

0

0

1

0

0

1

0

1

0

1

1

0

1

0

0

0

1

0

1

1

1

1

0

1

1

1

1

1

   

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Kmap

../images/digital/mux2x1_kmap.gif
   

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Circuit

../images/digital/mux2x1_ckt.gif
   

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  ../images/main/bulllet_4dots_orange.gif Example : 4:1 MUX

A 4 to 1 line multiplexer is shown in figure below, each of 4 input lines I0 to I3 is applied to one input of an AND gate. Selection lines S0 and S1 are decoded to select a particular AND gate. The truth table for the 4:1 mux is given in the table below.

   

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Symbol

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Truth Table

S1

S0

Y

0

0

I0

0

1

I1

1

0

I2

1

1

I3

   

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Circuit

../images/digital/combo223.gif
   

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  ../images/main/bulllet_4dots_orange.gif Larger Multiplexers

Larger multiplexers can be constructed from smaller ones. An 8-to-1 multiplexer can be constructed from smaller multiplexers as shown below.

   

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  ../images/main/bullet_star_pink.gif Example - 8-to-1 multiplexer from Smaller MUX
   

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Truth Table

S2

S1

S0

F

0

0

0

I0

0

0

1

I1

0

1

0

I2

0

1

1

I3

1

0

0

I4

1

0

1

I5

1

1

0

I6

1

1

1

I7

   

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Circuit

../images/digital/mux_4_2_1.gif
   

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  ../images/main/bullet_star_pink.gif Example - 16-to-1 multiplexer from 4:1 mux
   

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Copyright 1998-2014

Deepak Kumar Tala - All rights reserved

Do you have any Comment? mail me at:deepak@asic-world.com