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 Cin 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 D0 to D7.

 A B Cin Output 0 0 0 D0 0 0 1 D1 0 1 0 D2 0 1 1 D3 1 0 0 D4 1 0 1 D5 1 1 0 D6 1 1 1 D7

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. 1. What is full adder ?
• It is a combinational circuit. A full adder has three inputs ( each of one bit ) termed as A, B and Cin 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.
1. 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 ).
1. What is 1:8 Demultiplexer ?
• A Demultiplexer that has 1 input and 8 outputs is known as 1:8 Demultiplexer.
1. 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.
1. 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