In ECET 2026, Digital Electronics is a crucial scoring area for CSE diploma students. Among the common topics, Combinational Circuits like Multiplexers (MUX) and Demultiplexers (DEMUX) are frequently tested. Understanding their working, truth tables, and applications not only helps solve MCQs quickly but also strengthens your foundation for real-world digital circuit design. Let’s go through the concept in detail, followed by 10 practice MCQs.
📘 Concept Notes – MUX & DEMUX
🖥️ Combinational Circuits
- Combinational circuits are digital circuits whose output depends only on the current inputs, not on previous input states.
- Examples: Adders, Subtractors, MUX, DEMUX, Encoders, Decoders.
🔹 Multiplexer (MUX)
- A MUX selects one of many inputs and forwards it to a single output line.
- Often called a data selector.
- Notation: n-to-1 MUX (n inputs, 1 output)
- Selection lines (S) determine which input is sent to output.
- Truth table depends on number of selection lines:
- 2^n inputs → n selection lines
- Example: 4-to-1 MUX
Inputs: I0, I1, I2, I3
Selection Lines: S0, S1
Output: Y = selected input based on S1S0
Applications:
- Data routing, signal selection, arithmetic circuits.
🔹 Demultiplexer (DEMUX)
- DEMUX takes single input and sends it to one of many output lines.
- Often called a data distributor.
- Notation: 1-to-n DEMUX
- Selection lines (S) decide which output line receives the input.
- Example: 1-to-4 DEMUX
Input: I
Selection Lines: S0, S1
Outputs: Y0, Y1, Y2, Y3
Applications:
- Memory addressing, data routing, parallel data transmission.
🔑 Key Points to Remember
- MUX = Many inputs, 1 output; DEMUX = 1 input, Many outputs.
- Number of selection lines:
- MUX: n selection lines → 2^n inputs
- DEMUX: n selection lines → 2^n outputs
- Both are combinational circuits → output depends only on current inputs.
🔟 10 Most Expected MCQs – ECET 2026 [MUX & DEMUX]
Q1. A multiplexer is also called:
A) Data distributor
B) Data selector
C) Flip-flop
D) Register
Q2. In a 4-to-1 MUX, the number of selection lines is:
A) 1
B) 2
C) 3
D) 4
Q3. DEMUX stands for:
A) Data Multiplexer
B) Dual Multiplexer
C) Demultiplexer
D) Distributed MUX
Q4. A 1-to-8 DEMUX has how many selection lines?
A) 2
B) 3
C) 4
D) 8
Q5. MUX output depends on:
A) Previous input
B) Selection lines only
C) Current input & selection lines
D) Clock pulse
Q6. DEMUX is mainly used for:
A) Data selection
B) Data distribution
C) Counting
D) Addition
Q7. Which of the following is a combinational circuit?
A) Flip-flop
B) Counter
C) 8-to-1 MUX
D) SR Latch
Q8. In 8-to-1 MUX, if selection lines are 101, output corresponds to:
A) I1
B) I2
C) I5
D) I7
Q9. MUX can reduce:
A) Number of input lines
B) Output lines
C) Number of gates only
D) Propagation delay
Q10. DEMUX takes input from:
A) Many inputs to many outputs
B) One input to one output
C) One input to many outputs
D) Many inputs to one output
✅ Answer Key Table
| Q.No | Answer |
|---|---|
| Q1 | B |
| Q2 | B |
| Q3 | C |
| Q4 | B |
| Q5 | C |
| Q6 | B |
| Q7 | C |
| Q8 | C |
| Q9 | A |
| Q10 | C |
🧠 Explanations of All Answers
- Q1 → B: MUX selects data from many inputs → data selector.
- Q2 → B: 4-to-1 MUX → 2 selection lines (2^2 = 4).
- Q3 → C: DEMUX = Demultiplexer, sends 1 input to multiple outputs.
- Q4 → B: 1-to-8 DEMUX → 3 selection lines (2^3 = 8).
- Q5 → C: Output depends on current input and selection lines.
- Q6 → B: DEMUX distributes input to selected output lines.
- Q7 → C: MUX is purely combinational; flip-flops & counters are sequential.
- Q8 → C: Selection lines 101 → binary 5 → output I5.
- Q9 → A: MUX reduces number of input lines by combining multiple signals.
- Q10 → C: DEMUX sends single input to many outputs.
🎯 Why This Practice Matters for ECET 2026
Combinational circuits are easy to score and often appear in direct MCQs in ECET. MUX and DEMUX are high-yield topics, helping students grab quick marks with minimal calculation. Practicing with truth tables and selection lines improves speed and accuracy for exam day.
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