In this article I will show you how can you implement full adder in 1:8 Demultiplexer.
Truth table for full adder is mentioned below. This truth table is written using the concept of binary addition. That is
0 + 0 = 0
1 + 0 = 1
0 + 1 = 1
1 + 1 = 0, carry = 1.
A  B  C_{in}  Sum  Carry 





0  0  0  0  0 
0  0  1  1  0 
0  1  0  1  0 
0  1  1  0  1 
1  0  0  1  0 
1  0  1  0  1 
1  1  0  0  1 
1  1  1  1  1 
The table below shows the select lines A, B & C that selects the output in terms of D_{0} to D_{7}.
A  B  C_{in}  Output 




0  0  0  D_{0} 
0  0  1  D_{1} 
0  1  0  D_{2} 
0  1  1  D_{3} 
1  0  0  D_{4} 
1  0  1  D_{5} 
1  1  0  D_{6} 
1  1  1  D_{7} 
Output will be written only for logic 1 in both sum and carry.
Sum = Σm ( 1, 2, 4, 7 )
Carry = Σm ( 3, 5, 6, 7 )
Full Adder using 1:8 Demultiplexer
Now we will implement this data from table in 1:8 demultiplexer. Two OR gates will be used to get output for sum and carry. Shown in circuit below.
Frequently Asked Questions
 What is full adder ?
 It is a combinational circuit. A full adder has three inputs ( each of one bit ) termed as A, B and C_{in} that generates sum ( S ) and carry ( C ) in the output. A full adder circuit is designed using two AND gate, one OR gate & two XOR gate. The XOR gate gives the output for sum and the OR gate gives the output for carry.
 What is Demultiplexer ( DEMUX ) ?
 A DEMUX or a demultiplexer is a digital logic circuit ( combinational circuit ). A Demultiplexer has only one inputs and many outputs. The output depends upon the logic input given to the select lines ( A, B & C ).
 What is 1:8 Demultiplexer ?
 A Demultiplexer that has 1 input and 8 outputs is known as 1:8 Demultiplexer.
 What is the use of Demultiplexer ?
 Demultiplexers are used in the receiver side of data transfer system. It separates a signal that was combined by multiplexer in the transmitter side.
 What is the major difference between multiplexer ( MUX ) and demultiplexer ( DEMUX ) ?
 A multiplexer is used in the transmitter ( Tx ) side to encode data. A demultiplexer is used in the receiver ( Rx ) side to decode data.
Author
Akash Sharma
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