In ECET 2026, Digital Electronics topics like Multiplexers and Demultiplexers are very important. Questions are mostly based on working, logic equations, and applications.
📘 Concept Notes
🔹 Multiplexer (MUX)
- A Multiplexer (MUX) is a combinational circuit that selects one input line from multiple inputs and routes it to a single output line.
- Known as a data selector.
- Requires selection lines to choose which input is sent to the output.
👉 Formula for number of selection lines:
Where:
= number of input lines
= number of selection lines
Example:
- For a 4:1 MUX → 4 inputs, 2 selection lines, 1 output.
🔹 Demultiplexer (DEMUX)
- A Demultiplexer does the reverse of MUX.
- It takes one input and distributes it to one of many outputs, depending on the selection lines.
- Known as a data distributor.
👉 Formula for number of outputs:
Where:
- = number of selection lines
- = number of output lines
Example:
- A 1:4 DEMUX → 1 input, 2 selection lines, 4 outputs.
🔹 Truth Table Example – 4:1 MUX
| S1 | S0 | Output Y |
|---|---|---|
| 0 | 0 | I0 |
| 0 | 1 | I1 |
| 1 | 0 | I2 |
| 1 | 1 | I3 |
Logic Equation:
🔹 Applications of MUX
- Data routing
- Function generator
- Parallel-to-serial conversion
- Communication systems
🔹 Applications of DEMUX
- Data distribution
- Serial-to-parallel conversion
- Memory addressing
- Communication receivers
🔟 10 Expected MCQs – ECET 2026
Q1. A multiplexer is also called:
A) Encoder
B) Data selector
C) Decoder
D) Data distributor
Q2. A 4:1 MUX requires how many selection lines?
A) 1
B) 2
C) 3
D) 4
Q3. The number of outputs in a 1:8 DEMUX is:
A) 2
B) 4
C) 6
D) 8
Q4. A DEMUX is also known as:
A) Data selector
B) Data distributor
C) Encoder
D) Multiplexer
Q5. A 16:1 MUX will have how many selection lines?
A) 2
B) 3
C) 4
D) 5
Q6. The logic equation of a 2:1 MUX output is:
A) 
B) 
C) 
D) 
Q7. Which is NOT an application of a MUX?
A) Data routing
B) Serial-to-parallel conversion
C) Function generation
D) Parallel-to-serial conversion
Q8. If input = 1, S1S0 = 10, in a 1:4 DEMUX, which output is HIGH?
A) Y0
B) Y1
C) Y2
D) Y3
Q9. Number of selection lines needed for 32:1 MUX = ?
A) 4
B) 5
C) 6
D) 7
Q10. Which is true?
A) MUX = many inputs, one output
B) DEMUX = one input, many outputs
C) Both A & B
D) None
✅ Answer Key
| Q.No | Answer |
|---|---|
| Q1 | B |
| Q2 | B |
| Q3 | D |
| Q4 | B |
| Q5 | C |
| Q6 | A |
| Q7 | B |
| Q8 | C |
| Q9 | C |
| Q10 | C |
🧠 Explanations
- Q1 → B: Multiplexer = Data Selector.
- Q2 → B: For 4 inputs →
. - Q3 → D: 1:8 DEMUX → 8 outputs.
- Q4 → B: DEMUX = Data Distributor.
- Q5 → C:
, so 4 select lines. - Q6 → A: 2:1 MUX output = .
- Q7 → B: Serial-to-parallel is application of DEMUX, not MUX.
- Q8 → C: S1S0=10 selects Y2.
- Q9 → C:
, so 5 select lines. - Q10 → C: Both MUX and DEMUX definitions are correct.
🎯 Why Practice Matters
- Multiplexers and Demultiplexers are repeated every year in ECET.
- Simple formulas, truth tables, and applications are direct scoring points.
