DIGITAL ELECTRONICS
ELTN 3113
Section 7
Multiplexers
U.A.D.N Anuradha
Department of Electronics
Wayamba University of Sri Lanka
Data Transmission
• Most data in digital systems are transmitted directly through wires
and printed circuit (PC) boards.
• Sometimes, telephone lines or cables are used to transmit data from
one place to another.
• If all the data are sent over parallel lines simultaneously, it is
expensive and need large size cables.
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Cont.…,
• To overcome that, data from several lines are collected to a point
and then it is sent in a single wire in serial form. At the receiving
end, it is collected in parallel form.
• Multiplexers and Demultiplexers are used for that purpose.
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What Is Multiplexer?
• A MULTIPLEXER is a digital circuit that has multiple inputs and a
single output.
• The selection of one of the n inputs is done by the select inputs
• It has one output selected at a time.
• It is also known as DATA SELECTOR.
• A multiplexer has
 N data inputs(multiple)
 1 output (single)
 M select inputs, with 2M =N
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Block Diagram Of Multiplexer
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Types of Multiplexers




2-to-1 (1 select line)
4-to-1 (2 select lines)
8-to-1 (3 select lines)
16-to-1 (4 select lines)
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2-to-1 (1 Select Lines) Multiplexer
• Here 2:1 means 2 inputs and 1 output
TRUTH TABLE
S
Output Y
0
D0
1
D1
BLOCK DIAGRAM
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Cont.…,
• The logical level applied to the S input determines which AND gate is
enabled, so that its data input passes through the OR gate to the
output.
• The output, Y=D0S’+D1S
When
S=0,AND gate 1 is enabled and AND gate 2 is disabled. So, Y=DO
S=1,AND gate 1 is disabled and AND gate 2 is enabled . So, Y=D1
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4-to-1 (2 select lines) Multiplexer
• 4:1 MUX has 4 inputs(D0, D1, D2, D3) & 2 select lines(S0, S1)
BLOCK DIAGRAM
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Cont.…,
• The logical level applied to the S input determines which AND
gate is enabled, so that its data input passes through the OR gate
to the output.
• The output, Y=S1’S0’D0+S1’S0D1+S1SO’D2+S1S0D3
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8-TO-1 (3 SELECT LINES) MULTIPLEXER
• 8:1 MUX has 8 inputs(D0, D1, D2, D3, D4, D5, D6, D7) & 3 select
lines(S0,S1, S2,)
TRUTH TABLE
BLOCK DIAGRAM
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Cont.…,
• The logical level applied to the S input determines which AND gate is
enabled, so that its data input passes through the OR gate to the
output.
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Uses of Multiplexers
• Used in data communications for several computers to
communicate over 1 line
• Used in radio to select one channel from many
• Used to route data within a computer
• Used for function generation
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Application Of Multiplexer
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Example Of A Combinatorial Circuit: A Multiplexer (MUX)
• Consider an integer ‘m’, which is constrained by the following relation:
m = 2n, where m and n are both integers.
• A m-to-1 Multiplexer has
m Inputs: I0, I1, I2, ................ I(m-1)
one Output: Y
n Control inputs: S0, S1, S2, ...... S(n-1)
One (or more) Enable input(s)
• such that Y may be equal to one of the inputs, depending upon the
control inputs.
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Implementing Digital Functions: by using a Multiplexer:
• In this example to design a 3 variable logical function, we try to
use a 4-to-1 MUX rather than a 8-to-1 MUX.
F(x, y, z)=Σ (m(1, 2, 4, 7)
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Cont.…,
• In a canonic form:
F = x’.y’.z+ x’.y.z’+x.y’.z’ +x.y.z …… (1)
One Possible Solution:
Assume that x = S1 , y = S0.
If F is to be obtained from the output of a 4-to-1 MUX,
F =S’1. S’0. I0 + S’1. S0. I1 + S1. S’0. I2 + S1. S0. I3 ….(2)
From (1) and (2),
I0 = I3 =Z
I1 = I2 =Z’
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Cont.…,
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Cont.…,
Another Possible Solution:
Assume that z = S1 , x = S0 .
If F is to be obtained from the output of a 4-to-1 MUX,
F = S’0 .I0 . S1 + S’0 .I1 . S’1 + S0 .I2 . S’1 + S0 .I3 . S1 ………… (3)
From (1) and (2),
I0 = y’ = I2
I1 = y = I3
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Cont.…,
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Thank you......
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