Encoders & Decoders - GATE CSE (CSE) Digital Logic Free MCQ Test with solutions

Encoder: An encoder has 2n input lines and n output lines. In the encoder, the output lines generate the binary code corresponding to the input value which is active high.

Decoder: It is a multi-input and multi-output logic circuit that converts coded inputs into coded outputs where input and output codes are different. Input code has fewer bits than output code. There is one to one mapping from input to output.

Multiplexer: It is a digital switch. It allows digital information from several sources to be routed onto a single output line. A basic multiplexer has several data input lines and a single output line. The selection of a particular input line is controlled by selection lines. It is many to one mapping and provides the digital equivalent of an analog selector switch. Therefore it is the correct answer

Demultiplexer: It is a circuit that receives information on a single line and transmits information on one of the 2n output lines. Selection of output line is controlled by values of n selection lines. 

Concept:
Decoder expansion
n1 × m1 → n2 × m2
D1             D2
Number of D2 decoder required = ∑ K

⋮
Till 0 or 1
Calculation:
Given decoder 1 is 3 × 8 and the second decoder is 6 × 64
64/8 = 8
8/8 = 1
Number of 3 × 8 decoders = 8 + 1
Number of 3 × 8 decoders = 9
Important Points

Decoder is a circuit with n input lines and 2n output lines.

Combinational circuit in which output depends only on the state of inputs.

8870 is a DTMF decoder circuit, which decodes DTMF tune and produces corresponding output.

Invalid BCD are numbers greater than 1001, and they can be made valid BCD by adding with 0110.

For n input lined decoder will have 2n output lines.

Encoders are made up of three OR gates.

BCD to seven segment code conversion can be treated as a decoding process.

DTMF is the short form of Dual Tone Multiple Frequency.