BSc CSIT 1st Semester Digital Logic (CSC116) Important Questions, Past Papers & Exam Guide (TU)
Introduction
Digital Logic (CSC116) is one of the most consistent subjects in the BSc CSIT first semester — consistent in what it teaches (Boolean algebra, logic gates, K-maps, flip-flops), and consistent in what TU asks on every exam. The same question patterns appear year after year: convert a number, simplify a Boolean expression with a K-map, draw a flip-flop timing diagram, design a half adder. A student who has practiced these patterns thoroughly has already covered most of what will appear.
This guide is the exam preparation companion to the Complete Digital Logic (CSC116) Notes Guide, which covers every unit with worked examples, C programs, and full explanations. This guide does something different: it maps the TU exam — which question types appear in every paper, which topics carry the most marks, which specific questions have repeated across past papers, and how to spend your final week efficiently.
Whether you're preparing weeks in advance or doing a final sprint before the exam, use this guide to ensure your study time hits the highest-probability topics first.
Official Course Information
| Particular | Details |
|---|---|
| Course Title | Digital Logic |
| Course Code | CSC116 |
| Semester | First Semester |
| Nature of Course | Theory + Lab |
| Full Marks | 60 + 20 + 20 |
| Pass Marks | 24 + 8 + 8 |
| Credit Hours | 3 |
Text Book: M. Morris Mano — Digital Logic and Computer Design Reference Books: Brian Holdsworth — Digital Logic Design; John Patrick Hayes — Introduction to Digital Logic Design
How TU Sets the Digital Logic Theory Paper
Understanding the exam format before studying changes how you allocate time.
Short questions (2–5 marks): Definitions, truth tables, one-step conversions, brief comparisons. All seven gate types, number system definitions, flip-flop types, and memory type differences appear here regularly.
Medium questions (5–8 marks): Apply a technique — simplify a Boolean expression, build a K-map, trace a flip-flop's operation through a clock cycle. These test whether you can execute a method correctly step by step.
Long questions (8–10 marks): Design a circuit from a specification, prove a Boolean theorem, explain a full sequential circuit with timing diagram, or design and analyze a counter. These are the marks-heavy questions where diagrams and clean working matter most.
Lab/practical component: The lab tests hands-on circuit building and testing — NOT just theory. Students who only study theory often struggle here. Practice identifying gates on breadboards, testing truth tables manually, and debugging simple circuits.
Exam Weightage by Unit
| Unit | Topic | Likely Format | Priority |
|---|---|---|---|
| Unit 1 | Binary Systems and Number Codes | Short + Numerical (conversions) | ★★★★★ |
| Unit 2 | Boolean Algebra and Logic Gates | Short + Long (prove theorems) | ★★★★★ |
| Unit 3 | K-Map Simplification | Long (solve given expression) | ★★★★★ |
| Unit 4 | Combinational Logic (Adders, etc.) | Long + Diagram | ★★★★★ |
| Unit 5 | MSI/LSI (MUX, Decoder, ROM, PLA) | Long (heaviest unit) | ★★★★★ |
| Unit 6 | Sequential Logic and Flip-Flops | Long (most teaching hours) | ★★★★★ |
| Unit 7 | Registers, Counters, Memory | Long + Diagram | ★★★★☆ |
All seven units carry exam weight. Unit 6 (Sequential Logic, 10 hours) and Unit 5 (MSI/LSI, 8 hours) tend to generate the longest questions.
Unit 1: Binary Systems — Important Questions
Frequently Asked Long Questions
- Explain the four number systems (binary, decimal, octal, hexadecimal) with examples and conversion methods.
- Explain BCD, Excess-3, Gray code, and 2421 code with examples and conversion.
- Explain 1's complement and 2's complement. How are they used to represent negative numbers?
- What is an overflow? When does it occur in 2's complement arithmetic?
- Explain the difference between signed and unsigned binary numbers.
Frequently Asked Short Questions
- Convert (156)₁₀ to binary. (Answer: 10011100)
- Convert (0110 1001)BCD to decimal.
- Convert (3A7)₁₆ to decimal.
- What is Gray code? Why is it used in position encoders?
- Explain the binary storage cell concept.
Solved Conversion (Must Practice Format)
Convert (725)₁₀ to binary:
| Division | Quotient | Remainder |
|---|---|---|
| 725 ÷ 2 | 362 | 1 |
| 362 ÷ 2 | 181 | 0 |
| 181 ÷ 2 | 90 | 1 |
| 90 ÷ 2 | 45 | 0 |
| 45 ÷ 2 | 22 | 1 |
| 22 ÷ 2 | 11 | 0 |
| 11 ÷ 2 | 5 | 1 |
| 5 ÷ 2 | 2 | 1 |
| 2 ÷ 2 | 1 | 0 |
| 1 ÷ 2 | 0 | 1 |
Reading remainders bottom to top: (725)₁₀ = (1011010101)₂
Verify: 512+128+64+16+4+1 = 725 ✓
Always show this step-by-step table in the exam. Step marks are awarded.
Exam probability: ★★★★★ — Number conversions appear in every paper.
Unit 2: Boolean Algebra and Logic Gates — Important Questions
Frequently Asked Long Questions
- State and prove De Morgan's First Theorem: (A+B)' = A'B'
- State and prove De Morgan's Second Theorem: (AB)' = A'+B'
- Explain NAND and NOR as universal gates. Show how to implement AND, OR, NOT using only NAND gates.
- Simplify Boolean expression: F = AB + A(B+C) + B(B+C)
- Prove: A + A'B = A + B (absorption)
- Draw the logic diagram for F = AB' + A'B using basic gates, then convert to NAND-only implementation.
Frequently Asked Short Questions
- Draw truth tables for AND, OR, NOT, NAND, NOR, XOR, XNOR.
- State De Morgan's theorem.
- Why are NAND and NOR called universal gates?
- State the duality principle of Boolean algebra.
- What is complementation? Differentiate A' from Ā.
Boolean Laws Quick Reference
| Law | Expression |
|---|---|
| Identity | A+0=A, A·1=A |
| Null | A+1=1, A·0=0 |
| Idempotent | A+A=A, A·A=A |
| Complement | A+A'=1, A·A'=0 |
| Commutative | A+B=B+A, AB=BA |
| Associative | A+(B+C)=(A+B)+C |
| Distributive | A(B+C)=AB+AC |
| Absorption | A+AB=A, A(A+B)=A |
| De Morgan | (A+B)'=A'B', (AB)'=A'+B' |
Exam probability: ★★★★★ — DeMorgan's proof and NAND-only implementation appear almost every year.
Unit 3: K-Map Simplification — Important Questions
Frequently Asked Long Questions
- Simplify the Boolean function F(A,B,C,D) = Σm(0,1,2,5,8,9,10) using a 4-variable K-map.
- Explain don't care conditions in K-maps. Simplify F(A,B,C,D) = Σm(1,3,7,11,15) + d(0,2,5).
- Compare K-map simplification and Boolean algebraic simplification.
- Simplify F(A,B,C) = Σm(0,2,4,5,6) and implement using NAND gates.
- Explain prime implicants and essential prime implicants with an example.
Worked K-Map Example (3-variable)
Simplify F(A,B,C) = Σm(0,2,4,5,6)
K-map layout (rows: A, columns: BC):
BC
00 01 11 10
A=0: [1] [0] [0] [1]
A=1: [1] [1] [0] [1]
Groups:
- Group 1 (column BC=00, both rows): minterms 0,4 → B'C' (size 2)
- Group 2 (row A=1, BC=00 and BC=01): minterms 4,5 → AB' (size 2)
- Group 3 (column BC=10, both rows + wrap): minterms 0,2,4,6 → C' (size 4 — prefer this larger group)
Largest groups: C' (covers 0,2,4,6) and AB' (covers 4,5) Check minterm 5: covered by AB' ✓
F = C' + AB'
Verify with original: F(A=1,B=0,C=1) = minterm 5 → AB' = 1·1 = 1 ✓ F(A=0,B=1,C=1) = minterm 3 → not in list → C' = 0, AB' = 0 → F = 0 ✓
Rules for K-map grouping:
- Groups must be powers of 2: 1, 2, 4, 8, 16
- Groups can wrap around edges
- Always make the largest possible groups
- Minimize number of groups
Exam probability: ★★★★★ — K-map question is almost certain every paper.
Unit 4: Combinational Logic — Important Questions
Frequently Asked Long Questions
- Design and explain a half adder. Derive the Boolean expressions for Sum and Carry.
- Design a full adder using two half adders. Draw the logic circuit.
- Design a half subtractor and full subtractor. Compare with adders.
- Explain how a ripple carry adder is built from full adders.
- Design a 2-bit magnitude comparator.
- Explain exclusive-OR circuits. What is their significance in parity checking?
Key Formulas — Adders and Subtractors
Half Adder:
- Sum = A ⊕ B (XOR)
- Carry = AB
Full Adder:
- Sum = A ⊕ B ⊕ Cin
- Carry-out = AB + BCin + ACin
Half Subtractor:
- Difference = A ⊕ B
- Borrow = A'B
Full Subtractor:
- Difference = A ⊕ B ⊕ Bin
- Borrow-out = A'B + A'Bin + BBin
Exam probability: ★★★★★ — Half adder and full adder circuits appear in almost every paper.
Unit 5: Combinational Logic with MSI/LSI — Important Questions
Frequently Asked Long Questions
- Explain multiplexer. Design a 4:1 MUX with a logic diagram and truth table.
- Explain decoder. Design a 2:4 decoder. How can it implement any Boolean function?
- Explain encoder. What is a priority encoder? Give an example.
- Explain ROM, PLA, and PAL. Compare them in terms of programmability.
- Design a BCD to seven-segment decoder.
- What is a Binary Parallel Adder? How does carry propagation work?
MUX vs DEMUX Comparison
| Aspect | Multiplexer (MUX) | Demultiplexer (DEMUX) |
|---|---|---|
| Function | Many inputs → 1 output | 1 input → Many outputs |
| Select lines | Choose which input passes | Choose which output receives |
| Use | Data selection, routing | Data distribution |
| Example | 4:1 MUX: 4 inputs, 2 select lines | 1:4 DEMUX: 1 input, 2 select lines |
ROM vs PLA vs PAL
| ROM | PLA | PAL | |
|---|---|---|---|
| AND plane | Fixed (decoder) | Programmable | Programmable |
| OR plane | Programmable | Programmable | Fixed |
| Flexibility | Lowest | Highest | Moderate |
Exam probability: ★★★★★ — This is the heaviest unit (8 hours). At least one MSI long question expected.
Unit 6: Sequential Logic — Important Questions
Frequently Asked Long Questions
- Define sequential circuits. How do they differ from combinational circuits?
- Explain SR flip-flop with truth table, timing diagram, and state limitations.
- Explain JK flip-flop. How does it eliminate the invalid state of the SR flip-flop?
- Explain D flip-flop and T flip-flop with truth tables and timing diagrams.
- Compare all four flip-flop types (SR, JK, D, T) in a table.
- Explain master-slave JK flip-flop. Why is it needed?
- Explain clocked vs unclocked (latch) flip-flops.
- Design a sequential circuit with given state table and state diagram.
Flip-Flop Comparison Table
| Aspect | SR | JK | D | T |
|---|---|---|---|---|
| Inputs | S, R | J, K | D | T |
| Invalid state? | Yes (S=R=1) | No (toggles) | No | No |
| Toggle? | No | Yes (J=K=1) | No | Yes (T=1) |
| Primary use | Basic latch | General purpose | Data storage | Counter |
| Next state | Set/Reset | Set/Reset/Toggle/Hold | Follow D | Hold/Toggle |
Critical exam tip: Always state that the SR flip-flop has an undefined state when S=R=1, and explain how JK resolves this by toggling. This distinction earns marks even in short-answer format.
Exam probability: ★★★★★ — This is the largest unit (10 hours). At least one flip-flop question is almost certain.
Unit 7: Registers, Counters, and Memory — Important Questions
Frequently Asked Long Questions
- Explain shift registers. What are the four types? Describe SIPO and PISO.
- Explain ripple counter (asynchronous). Draw a 4-bit binary ripple counter.
- Explain synchronous counter. Compare with ripple counter.
- Design a MOD-6 counter. Show state diagram and circuit.
- Explain RAM and ROM. Differentiate between SRAM and DRAM.
- Explain EPROM, EEPROM, and Flash memory.
Memory Comparison Table
| Memory | Volatile | Writable | Speed | Use |
|---|---|---|---|---|
| SRAM | Yes | Yes | Fastest | Cache memory |
| DRAM | Yes | Yes | Fast | Main RAM |
| ROM | No | No | Moderate | BIOS, firmware |
| PROM | No | Once | Moderate | Custom firmware |
| EPROM | No | With UV light | Moderate | Older embedded |
| EEPROM | No | Electrically | Moderate | Configuration |
| Flash | No | Electrically (block) | Fast | USB drives, SSDs |
Ripple vs Synchronous Counter
| Aspect | Ripple (Async) | Synchronous |
|---|---|---|
| Clock | Each FF clocks the next | All FFs share one clock |
| Speed | Slower (propagation delay accumulates) | Faster (no accumulated delay) |
| Design | Simpler | More complex |
| Accuracy | Lower (glitches possible) | Higher |
Exam probability: ★★★★☆ — Counters and shift registers are very commonly asked.
Most Repeated TU Past Questions
Asked 4 or More Times
- K-map simplification (3 or 4 variable) with NAND implementation
- Half adder and full adder design with truth table and circuit
- JK flip-flop explanation with truth table and timing diagram
- Universal gate proof (NAND implementing AND, OR, NOT)
- Number system conversion (decimal to binary, binary to hexadecimal)
- De Morgan's theorem proof
Asked 2–3 Times
- 4:1 Multiplexer design and truth table
- Gray code conversion and applications
- 4-bit ripple counter design
- SR vs JK flip-flop comparison
- 2's complement arithmetic (addition and subtraction)
- BCD to seven-segment decoder
- Shift register types and operation
- RAM vs ROM differentiation
- Priority encoder
Occasionally Asked (Short Questions)
- Excess-3 code explanation
- Quine-McCluskey method (basic)
- Race conditions in sequential circuits
- PLA vs PAL vs ROM
Formula and Rule Quick Reference
| Topic | Formula/Rule |
|---|---|
| 1's complement | Flip all bits |
| 2's complement | Flip all bits, add 1 |
| Half adder Sum | A ⊕ B |
| Half adder Carry | AB |
| Full adder Sum | A ⊕ B ⊕ Cin |
| Full adder Cout | AB + BCin + ACin |
| De Morgan 1 | (A+B)' = A'B' |
| De Morgan 2 | (AB)' = A'+B' |
| Gray code from binary | MSB same; each next bit = XOR of current and previous binary bits |
| MUX 2ⁿ inputs | Needs n select lines |
| Decoder n inputs | Activates one of 2ⁿ outputs |
| K-map group size | Must be power of 2 (1,2,4,8,16) |
| Ripple counter n bits | Counts 0 to 2ⁿ−1, needs n flip-flops |
Seven-Day Study Plan
Day 1 — Binary Systems (Unit 1)
- Practice decimal → binary → hexadecimal → octal conversions using the step table format shown above
- Learn 2's complement and practice binary addition/subtraction
- Memorize BCD and Gray code conversion rules
Day 2 — Boolean Algebra and Gates (Unit 2)
- Write out De Morgan's proofs from memory (both theorems)
- Draw truth tables for all seven gate types
- Practice Boolean simplification using absorption and De Morgan's
- Draw NAND-only implementation of AND, OR, NOT
Day 3 — K-Maps (Unit 3)
- Work through one 3-variable and two 4-variable K-map problems
- Practice identifying prime implicants and essential prime implicants
- Attempt one problem with don't-care conditions
Day 4 — Combinational Circuits (Unit 4)
- Draw half adder and full adder circuits from memory
- Derive Sum and Carry-out expressions from scratch
- Practice converting SOP expressions to NAND-NAND implementations
Day 5 — MSI/LSI (Unit 5)
- Draw a 4:1 MUX and 2:4 decoder circuit with truth tables
- Write the ROM/PLA/PAL comparison table from memory
- Understand how a decoder can implement any Boolean function
Day 6 — Flip-Flops and Sequential Logic (Unit 6)
- Write the comparison table for SR, JK, D, T flip-flops from memory
- Draw a JK flip-flop timing diagram for a given clock sequence
- Explain master-slave operation and why it's needed
Day 7 — Registers, Counters, Memory + Full Revision
- Draw a 4-bit ripple counter
- Write the memory type comparison table
- Go through the most-repeated questions list and attempt 5 of them without notes
Common Mistakes That Cost Marks
K-map grouping errors: Grouping cells that aren't adjacent in the K-map layout, or stopping at a smaller group when a larger one was available. Remember: K-map adjacency wraps around edges (top-to-bottom and left-to-right).
SR flip-flop "undefined state" omission: Many students describe the SR flip-flop without mentioning the S=R=1 invalid state. TU examiners consistently look for this — always include it.
Wrong De Morgan's application: Applying De Morgan's inside-out instead of outside-in, or forgetting to flip the operator (+ ↔ ·) along with complementing each variable.
Half adder when full adder is needed: Designing a half adder for a multi-bit addition problem instead of chaining full adders. Check whether a carry-in input is needed.
NAND vs NOR confusion under pressure: NAND output is 0 only when ALL inputs are 1. NOR output is 1 only when ALL inputs are 0. Mixing these up on a truth table is one of the most common short-question errors.
Number conversion without showing steps: Writing only the final answer for a conversion question loses step marks. Always show the division table or place-value calculation.
Frequently Asked Questions
Is Digital Logic difficult for BSc CSIT students? The individual topics aren't conceptually hard — Boolean algebra is straightforward logic, and gates follow simple rules. The difficulty comes from the volume of techniques (conversions, K-maps, flip-flops, counters) that all need to be practiced, not just read.
Which unit is most important for the exam? Unit 6 (Sequential Logic, 10 hours) and Unit 5 (MSI/LSI, 8 hours) carry the most teaching hours and consistently generate long questions. But K-maps (Unit 3) and half/full adder circuits (Unit 4) also appear almost every year.
How should I study K-maps? Work through problems, not just read about them. Start with 2-variable maps, then 3-variable, then 4-variable. Do at least one problem with don't-care conditions. The grouping intuition only comes through repetition.
Does De Morgan's theorem always appear? Very consistently — either as a standalone "state and prove" question or embedded in a Boolean simplification question. Prepare both theorems to the point where you can write the proof without reference.
Are number system conversions worth spending time on? Yes — they appear as short questions in almost every paper, they're quick marks if practiced, and they're very easy to lose marks on if you make arithmetic errors. The step-table format shown above is the safest approach.
What's the difference between a latch and a flip-flop? A latch is level-triggered — it responds as long as its enable signal is active. A flip-flop is edge-triggered — it responds only at the exact moment the clock transitions (either rising or falling edge). This distinction is a common short-question topic.
Conclusion
Digital Logic rewards students who practice techniques rather than just read about them. K-map grouping, 2's complement arithmetic, and flip-flop timing diagrams all feel confusing the first time and become automatic by the tenth. Use this guide to identify what to practice, the complete guide for the explanations behind each technique, and the past-question analysis to prioritize your time in the final week.
For full chapter-wise explanations, worked K-map examples, C programs, and the complete syllabus breakdown, see the Complete Digital Logic (CSC116) Guide.
For related 1st semester subjects: Mathematics-I (MTH117) Important Questions, C Programming (CSC115) Important Questions, and Introduction to Information Technology (CSC114) Complete Guide.
Also read C programming important questions
Handwritten pdf notes of 1st sem Digital logic.
For more guides and resources, visit utsabojha.com.np
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