---
title: "Fault tolerance and redundancy for reliability"
subject: "Instrumentation II"
chapter: 7
key: "semester_5/instrumentation_ii:fault-tolerance-schemes"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/fault-tolerance-schemes
site: BCT Engineering
---

# Fault tolerance and redundancy for reliability

Instrumentation II · Chapter 7: Circuit Design

- Asked in: 2071 Shrawan, 6 marks; 2070 Asar, 3+3 marks; 2078 Bhadra, 6 marks; 2075 Chaitra, 6 marks (asked 4 times)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/fault-tolerance-schemes
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-7

## Question

Explain how the reliability of a circuit can be increased using fault tolerance, i.e. careful design, testable architectures and redundant architectures.

## Short answer

Reliability is increased by careful design (derating, protection, simplicity) to avoid faults, testable architectures (test points, built-in self-test, parity, watchdog) to detect them, and redundant architectures (triple modular redundancy, stanby spares) using extra hardware, information, time, or software to mask failures.

## Answer

**Reliability** is the probability that a system performs its required function for a specified time under stated conditions. Fault tolerance keeps the system working in spite of faults, using three approaches.

#### Careful design (fault avoidance)
Prevent faults from entering the design:
- Use high-quality components with derating (operate at 50-70 percent of rated stress), and design for worst-case tolerance and temperature.
- Keep the design simple with fewer components and connectors (fewer failure points).
- Add protection: fuses, TVS, reverse-polarity protection, watchdog timers and brown-out reset.
- Follow EMC and layout rules; thermal design and burn-in testing; design reviews and standards.

#### Testable architectures
A fault that cannot be detected cannot be handled, so the design must be testable:
- Provide **test points**, scan chains/boundary scan (JTAG), and built-in self test (BIST) at power-up.
- Use **parity, CRC, checksums** to detect data errors, and watchdog timers to detect software hangs.
- Provide diagnostics (LED/status codes), and partition the system into modules so that faults can be isolated.
- Monitor supply voltages and temperature.

#### Redundant architectures
Redundancy adds extra resources so that a failure does not cause system failure:
1. **Hardware redundancy:**
   - **Passive (static):** Triple Modular Redundancy (TMR): three identical modules and a majority voter mask one fault.
   - **Active (dynamic):** a standby spare is switched in on fault detection (hot or cold standby).
   - **Hybrid:** TMR plus spares.
2. **Information redundancy:** error-correcting codes (Hamming) and CRC.
3. **Time redundancy:** repeating the operation and comparing the results (catches transient faults).
4. **Software redundancy:** N-version programming and recovery blocks.

```text
 In -> [M1] --\
 In -> [M2] ---> [Voter] -> Out   (TMR)
 In -> [M3] --/
```

Redundancy costs more in size, power and complexity, so it is used where the cost of failure is high (medical, aerospace, industrial control).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is a careful-design (fault avoidance) practice?
   A. Running parts at maximum ratings
   B. Derating components to 50-70 percent of rated stress
   C. Adding more connectors
   D. Skipping thermal design
   Answer: B. Derating reduces stress and failure rate. (Key point: Careful design)

2. Why is testability needed?
   A. A fault that cannot be detected cannot be handled
   B. It reduces component cost
   C. It removes the need for reliability
   D. It is only for production lines
   Answer: A. Detection through BIST, parity, watchdog and test points enables handling. (Key point: Testable architecture)

3. How does Triple Modular Redundancy work?
   A. Three modules in series add their delays
   B. One active, two cold spares unvoted
   C. Three voters on one module
   D. Three identical modules with a majority voter mask one fault
   Answer: D. The voter outputs the majority value so a single failure is masked. (Key point: TMR)

4. What does time redundancy catch?
   A. Permanent hardware damage only
   B. Software licensing issues
   C. Transient faults, by repeating the operation and comparing results
   D. Power supply sizing
   Answer: C. A transient may not recur on the repeat, so a mismatch reveals it. (Key point: Time redundancy)

## Related in Instrumentation II

- [Software fault tolerance and recovery](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asw-fault-tolerance-rollback) — Instrumentation II, chapter 9, asked 2×
- [Factors driving circuit reliability](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Areliability-factors) — Instrumentation II, chapter 7, asked 3×
- [Establishing requirements for circuit design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Areq-to-design) — Instrumentation II, chapter 7, asked 4×
- [Technical dilemmas in circuit design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adesign-dilemmas) — Instrumentation II, chapter 7
- [High speed circuit design guidelines](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ahigh-speed-guidelines) — Instrumentation II, chapter 7, asked 3×

## Related in other subjects

- [Availability and fault tolerance in GFS](https://www.bctengineer.com/question.md?key=semester_8%2Fbig_data_technologies%3Agfs-availability-fault-tolerance) — Big Data Technologies, chapter 2
- [Replication, fault tolerance, scalability](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Areplication-scalability-ft) — Distributed System, chapter 7
- [Fault tolerance and types of faults](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Afault-types) — Distributed System, chapter 9
- [Fault tolerance techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Afault-tolerance-techniques) — Distributed System, chapter 9