Safety-critical standards — DO-178C (airborne software), IEC 61508, ISO 26262

5.5.18 · Coding › Embedded Systems & Real-Time Software

YEH standards KYA hain

EK ki jagah TEEN standards KYUN? Kyunki failure ke consequences, operating environment, aur certification authorities alag hain. Ek aircraft ko FAA/EASA har flight ke liye certify karta hai; ek car public ke liye mass-produce hoti hai; ek industrial plant continuously chalta hai. IEC 61508 generic ancestor hai; ISO 26262 uska automotive child hai. DO-178C aviation mein alag evolve hua lekin wahi safety logic share karta hai.

HAR standard rigor ko KAISE grade karta hai: "levels"

Core mechanism ek risk classification level hai jo drive karta hai ki aapko kaun-se objectives/activities perform karne hain.

Figure — Safety-critical standards — DO-178C (airborne software), IEC 61508, ISO 26262

DO-178C detail mein

IEC 61508 detail mein

ISO 26262 detail mein

Comparison table (20% jo 80% deta hai)

Feature	DO-178C	IEC 61508	ISO 26262
Domain	Airborne	Generic E/E/PE	Automotive
Level ka naam	DAL A–E	SIL 1–4	ASIL A–D (+QM)
Level ka basis	System safety assessment	Quantitative $P F D$ / $P F H$	$S \times E \times C$ analysis
Sabse rigorous	DAL A	SIL 4	ASIL D
Signature concept	MC/DC coverage, 71 objectives	$P F D_{a v g}$ , SFF, HFT	ASIL decomposition, item definition
Cert authority	FAA / EASA	Notified bodies	OEM self-cert + audits

Common mistakes

Flashcards

DO-178C MC/DC coverage requirement mein "C" kya guarantee karta hai jo decision coverage nahi karta?

Ki har condition independently decision outcome ko affect kare (doosre fixed rakhe jaate hain).

DO-178C: sabse zyada safety-critical DAL kaun sa hai?

DAL A (catastrophic failure condition).

ISO 26262: sabse zyada safety-critical ASIL kaun sa hai?

ASIL D.

IEC 61508: sabse demanding SIL kaun sa hai?

SIL 4.

Simplified 1oo1 average PFD formula?

P F D_{a v g} = λ_{D U} T_{1} /2

PFD_avg mein factor 1/2 kyun?

Yeh us risk ka average hai jo proof-test interval mein 0 (abhi test hua) se poore interval tak linearly badhta hai.

ISO 26262 mein ASIL determine karne wale teen parameters kaun se hain?

Severity (S), Exposure (E), Controllability (C).

Kaun sa standard doosron ka generic "parent" hai?

IEC 61508.

DO-178C kitne objectives define karta hai?

71 objectives (jinhe independence ke saath satisfy karna hai, woh DAL ke saath badhte hain).

Decision A && B ke liye MC/DC ko kitne tests chahiye?

n+1 = 3.

DO-178C mein "with independence" ka matlab kya hai?

Verification ek alag person/team dwara ki jaaye, developer se alag.

SIL 3 low-demand PFD band?

1 0^{- 4} \leq P F D < 1 0^{- 3}

(RRF 1000–10000).

ASIL QM kya hai?

Quality Management — normal good practice se aage koi safety requirement nahi.

PFD vs PFH — kab kaunsa use hota hai?

PFD low-demand mode ke liye (rare actuation); PFH high/continuous-demand mode ke liye.

Recall Feynman: 12-saal ke bachche ko samjhao

Socho tum ek fence bana rahe ho. Apne backyard ke liye, ek wobbly fence theek hai. Zoo mein lion enclosure ke liye, tumhe prove karna hoga ki har post solid hai — kyunki agar fail ho gaya, toh koi kha liya jaayega. Yeh standards rulebooks hain jo kehte hain: pehle poocho "yeh toot gaya toh kitna bura?" Agar jawab hai "lion bahar nikal jaayega" (catastrophic), toh super carefully test karo, doosre se double-check karao, aur records rakho ki tumne kiya. Agar jawab hai "gate cheekh karega" (no effect), toh relax kar sakte ho. Labels (DAL-A, SIL-4, ASIL-D) sirf badges hain jo kehte hain "yeh lion-cage level hai — extra careful raho."

Connections

Functional Safety — woh overarching discipline jise yeh implement karte hain.
Code Coverage Metrics — statement / decision / MC/DC, DO-178C dwara test kiya jaata hai.
Fault Tolerance & Redundancy — HFT, 1oo1/1oo2 architectures PFD ke peeche.
Failure Rate & Reliability — $λ$ , exponential failure model PFD derivation mein use hota hai.
V-Model Software Lifecycle — woh process model jo yeh standards prescribe karte hain.
FMEA & Hazard Analysis — DAL/SIL/ASIL assignment ko feed karta hai.
Real-Time Operating Systems — often inhi levels par certify karne padte hain.