02 Chapter Map - Digital Codes & Error Detection
Chapter 2 Overview & Map of Content (MOC)
Weighted digital codes (8421, 2421, 5211, 84-2-1), self-complementing Excess-3 logic, Gray (reflected) code XOR conversions, and parity generators/checkers.
📚 Study Notes Index
| # | Note | What it covers | PYQ years | Typical marks |
|---|---|---|---|---|
| 2.01 | 2.01 BCD & Weighted Digital Codes | Number system vs code, BCD addition, weighted codes (8421 / 2421 / 5211 / 84-2-1), ASCII, multi-format conversion | 2018, 2019, 2022 | 8–12 |
| 2.02 | 2.02 Excess-3 Code & Self-Complementing Logic | Excess-3 arithmetic, self-complementing proof, weights-sum-to-9 theorem, BCD→XS-3 converter design | 2017, 2018, 2021, 2023, 2025 | 8–10 |
| 2.03 | 2.03 Gray Code & Code Conversions | Reflected code, binary↔Gray XOR algorithms, shaft-encoder rationale, converter hardware | 2017, 2018, 2024 | 10 |
| 2.04 | 2.04 Error Control, Parity Generators & Checkers | Detecting vs correcting codes, even/odd parity generators and checkers, parity limitations, Hamming code | 2015, 2017, 2018, 2020, 2022, 2023, 2024, 2025 | 4–12 |
🎯 Exam Weight
Two questions in this chapter are near-certain
- 4-bit even parity checker — 2015, 2020, 2023, 2024 (up to 12 marks), wording identical every time → 2.04 Error Control, Parity Generators & Checkers §3.1
- “Excess-3 is self-complementary — justify” — 2021, 2023, 2025 (8–10 marks), a four-line proof → 2.02 Excess-3 Code & Self-Complementing Logic §3
Both are short, mechanical, and repeat verbatim. Together they are worth more than the entire rest of the chapter.
The recurring trap across this whole chapter
Digit-wise codes vs whole-number codes.
- Encode each decimal digit separately: BCD, Excess-3, 2421, 5211, 84-2-1
- Convert the entire value: Gray code, pure binary
The 2018 and 2019 papers both put these side by side in one question specifically to catch people out. Read for the word “number”.
🔗 Related Resources
- Course Teaching Plan: ECE 2103 - Digital Electronics and Logic Circuits
- Previous chapter: 01 Chapter Map - Number Systems & Arithmetic — the base-conversion machinery these codes are built on.
- Next chapter: 03 Chapter Map - Boolean Algebra & Logic Simplification — the K-map tools used to minimise the converter and parity circuits designed here.
- Lab tie-in (ECE 2104): Week 4 builds the BCD→Excess-3 converter; Week 5 builds both Gray↔Binary converters.