In 3 ∶ 8 line decoder one of the eight output bits is activated, depending on which of the following parameters

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RRB ALP Electronics Mechanic 23 Jan 2019 Official Paper (Shift 3)
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  1. Input Values from 0 to 7
  2. Next State output
  3. Previous output
  4. Clock input

Answer (Detailed Solution Below)

Option 1 : Input Values from 0 to 7
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Detailed Solution

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Concept:

  • A decoder is a combinational logic constructed with logic gates.
  • It is the reverse of the encoder.
  • A decoder circuit is used to transform a set of digital input signals into an equivalent decimal code of its output. For ‘n’ inputs a decoder gives ‘2n’ outputs.
  • Block diagram of the Decoder is shown below:

 

F1 T.S Madhu 21.07.20 D 5

Decoder expansion

n1 × m1 → n2 × m2

D1             D2

\(\frac{{{m_2}}}{{{m_1}}} = {K_1}\)

\(\frac{{{K_1}}}{{{m_1}}} = {K_2}\)

\(\frac{{{K_2}}}{{{m_1}}} = {K_3}\)

The number of D2 decoder required is given as:

∑ K = K1 + K2 + K3 + ------ 

Example:

Given decoder 1 is 3 × 8 and the second decoder is 5 × 32

\(\frac{{{32}}}{{{8}}} = {4}\)

\(\frac{{{4}}}{{{8}}} = {0}\)

Number of 3 × 8 decoders = 4 + 0

Number of 3 × 8 decoders = 4

Calculation:

One 2:4 decoder and four 3:8 decoders can represent 5:32 decoder in the following way:

F1 Tapesh Madhuri 10.05.2021 D7

Hence option (1) is the correct answer.

26 June 1

Given

Decoder

To be implemented

Required

2 x 4

4 x 16

4 + 1 = 5

2 x 4

3 x 8

2 + 0 = 2

3 x 8

6 x 64

8 + 1 = 9

4 x 16

6 x 64

4 + 0 = 4

4 x 16

8 x 256

16 + 1 = 17

 

 

 

 

 

 

 

 

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