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An Encoder is a digital circuit that perform the inverse operation of a decoder. An encoder has 2 n (or fewer) input and n output lines.


Academic year: 2021

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Example :Desgin Full-adder from two (74139) decoder.



An Encoder is a digital circuit that perform the inverse operation of a decoder . An encoder has 2n (or fewer) input and n output lines.

The output lines generate the binary code corsponding to the input value. One of the most common Decoders is (4 to 2) encoder.The encoder in this system must translate the decimal input signals to binary output signals.

Inputs Outputs F0 F1 F2 F3 A B

1 0 0 0 0 0


0 0 1 0 1 0

0 0 0 1 1 1

Truth table Of (4-2) Encoder

The truth table is the origin of most logic circuit you must have al the combinations that generate a logic 1 in the truth table, therefore

A=F2 + F3 , B=F1 + F3

Block Diagram Of Encoder

Logic Diagram of Encoder



A multiplexer is a combinational circuit that selects binary information from one of many input lines and directs it to a single output line. F 0 4 to 2 Encoder F0 F1 F2 F3 A B


The selection of a particular input line is controlled by a set of selection lines. Normally , there are 2n inputs and n selection lines whose bit combination determine which input is selected and 1 output.

2 – to- 1 Line multiplexer

A 2 to 1 multiplexer connects one of two sources to a common destination as shown

in figure below:

the block diagram of 2-to-1 multiplexer can be shown fellow :

The circuit has two data input lines, one output line, and one selection line


When S=0 , the upper AND gate is enabled and I0 has a path to the output. When S=1 the lower AND gate is enabled and I1 has a path to the output. The multiplexer acts like an electronic switch that selects one of two sources.

4 – to - 1 Line multiplexer


I0 I1


S1 S2 Y 0 0 I0 0 1 I1 1 0 I2 1 1 I3

 Each of the four inputs , Io through I3 , is applied to one input of an

AND gate . selection lines S1 and S0 are decoded to select a particular AND gate.

 The outputs of the AND gates are applied to a single OR gate that provides the 1-line output.

 The function table lists the input that passed to the output for each combination of the binary selection values .

 To demonstrate the circuit operation , consider the case when s1s0=10. The AND gate associated with input I2 has two of its input equal to 1 And the third input connected to I2 has the other three AND


gates have at least one input equal to 0, which makes their outputs equal to 0.

 The OR gat output is now equal to the value of I2 , providing a path from the selected input to the output.

 A multiplexer is also called data selector, since is selects one of many inputs and steers the binary information to the output line.

 The AND gates and inverters in the multiplexer resemble a decoder circuit and indeed they decode the selection input lines.

 In general 2n-to-1 line multiplexers constructed from an 2n input lines , one to each AND gate.

 The outputs of the AND gates are applied to a single OR gate.

 The size of a multiplexer is specified by the number 2nof its data input lines and the single output line.

 The n selection lines are implied from 2ndata lines.

 As in decoders, multiplexers may have an enable input to control the operation of the unit . when the enable input is in the in active state, the outputs are disabled , and when it is in the active state , thr circuit functions as a normal multiplexer.

When (S0=0 and S1=0) the output Y will be I0

S0 Data


Data inputs output Y I0

I1 I2 I3 S0


And if (S0=1 and S1=0)  y= I1 And if (S0=1 and S1=1)  y= I2 And if (S0=1 and S1=1)  y= I3 Data inputs Data Selected Output S1 S0 Y I0 0 0 1 0 0 S S II1 0 1 1 0 1 S S II2 1 0 1 0 0 S S II3 1 1 I0S1S0

Selection lines(s1) and (so) are decoded to select a particular AND gate. The outputs of the AND gates are applied to a single OR gates that provide the (1-line) output.

8 – to - 1 Line multiplexer

8- to – 2


Z I0 I1 I2 I3 I4 I5 I6 I7 A B C


An 8-t0-1 Mux can be used to realize any 4-variable function with no added gates. Three of variable are used as control inputs to Mux and the remaining variable is used as required on the data inputs.

Data Inputs Data Selected Output A B C I0 0 0 0 I0 I1 0 0 1 I1 I2 0 1 0 I2


I3 0 1 1 I3

I4 1 0 0 I4

I5 1 0 1 I5

I6 1 1 0 I6

I7 1 1 1 I7

Example1: using (8*1) multiplexer to realize full adder

from truth table

inputs output A B Cin (s) C0 Decimal NO. 0 0 0 0 0 0 0 0 1 1 0 1 0 1 0 1 0 2 0 1 1 0 1 3 1 0 0 1 0 4 1 0 1 0 1 5 1 1 0 0 1 6 1 1 1 1 1 7


1. first we have 23 inputs =8

2. then we have n=3 selection line to each mux 3. one output

4. full adder need 2 outputs sum and carry then we must connect two mux.

Example2: Design A<>B comparator by using Multiplexers

when A, B consist of two digits(N=2) ? hint (use 74153 Mux)


A <> B XOR Gates Decimal B0 A0 y 0 0 0 0 1 0 1 1 2 1 0 1 3 1 1 0


Example3: using multiplexer to realize the switching function F(A , B , C)=

(1,4 ,5, 7) Decimal A B C Y 0 0 0 0 0 1 0 0 1 1 2 0 1 0 0 3 0 1 1 0 4 1 0 0 1


5 1 0 1 1

6 1 1 0 0

7 1 1 1 1



Is a combinational circuit that perform the inverse operation of Mux

It has one input and 2n outputs and n control lines. The uses of demux is less than

the mux circuit in electronics world .

0 1 0 0 1 1 0 1 F 8 *1 MUX A B C output Control Demultiplexer input


Example : design (1 * 4) Demux S1 S0 output 0 0 F0 0 1 F1 1 0 F2 1 1 F3 F0 F1 F2 F3 Demux 1*4 S1 S2



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• Combinational circuit that selects binary information from one of many input lines and directs it to one output line. 2