Field Service for Industrial Ethernet: Bypass, PRBS, Secure OTA
← Back to: Industrial Ethernet & TSN
Field Service turns “it works or it breaks” into an evidence-driven loop: isolate the impact per port, measure link health with counters/loopback/PRBS, recover safely with gated rollback, and keep a minimal black-box record so every incident is repeatable and verifiable.
The goal is simple: cut time-to-diagnose and time-to-recover without risky actions—every step leaves measurable pass/fail criteria (X) and a traceable evidence chain.
H2-1 · Definition & Service Goals
Field service is not a toolbox list. It is an auditable closed loop that turns “guess & reboot” into isolate → measure → recover → prove. The outcome must be measurable, reversible, and evidence-backed.
What “Serviceability” Means in Industrial Ethernet
- Isolatable — a single bad port must be contained (per-port bypass / isolate / service mode) without collapsing the whole node.
- Measurable — link health must be testable with repeatable procedures (loopback, PRBS/pattern, counters, snapshots).
- Recoverable — recovery actions must be reversible and gated (secure OTA, A/B, rollback, health-gate commit).
- Provable — every action must leave an evidence trail (time-stamped snapshot + version + result) for root-cause and prevention.
Service Goals as Field-Safe SLAs
TTD
Time-to-Diagnose
Reach a defensible “where to look next” answer fast: inside node vs outside link.
Minimum safe actions: read counters → capture snapshot → run light loopback-ready check.
Avoid first: firmware flash / mass parameter changes / full-network stress tests.
TTR
Time-to-Recover
Restore operation without spreading damage: isolate the bad port first, then recover with gated actions.
Minimum safe actions: per-port bypass/isolate → verify stability → optional deeper tests in a service window.
Recovery must not erase evidence; snapshots before and after are mandatory.
SAFE
No-Regret Actions
Actions that are reversible, recordable, and non-propagating.
- Reversible: bypass/off/on, service mode enter/exit, rollback-ready A/B switching.
- Recordable: every action emits an event code + timestamp + snapshot ID.
- Non-propagating: isolate before tests; keep impact inside one port.
Acceptance Metrics & Required Evidence (thresholds as X)
MTTR
Time from first alarm to stable recovery + health gate pass.
Required fields: alarm_ts, isolate_ts, recover_ts, health_score_after.
Misdiagnosis rate
% of cases where the initial layer split is overturned by later evidence.
Required fields: initial_call, initial_evidence, final_call, overturn_reason.
Rollback success
A/B rollback results in stable operation for X hours.
Required fields: slot_from, slot_to, trigger, health_gate, stability_window.
Field reproducibility
Symptom can be reproduced in a controlled service-mode test (loopback/PRBS).
Required fields: test_mode, window, result, environment (temp/power), fw_version.
Pass criteria examples: flap ≤ X/hour, CRC ≤ X/min, PRBS BER ≤ X, rollback ≥ X%, “stable after recovery” ≥ X hours.
H2-2 · Symptom Taxonomy & Triage Entry
Field triage must start with non-destructive actions. The first step is never “change everything”; the first step is “freeze evidence and split the fault domain”.
Non-Destructive First Actions (Always Safe)
Do first
- Read counters with a known time window (avoid reset/rollover confusion).
- Capture one snapshot: version + port state + counters + temp/power + event code.
- If impact spreads, isolate the suspect port (bypass/isolate/service mode entry).
Avoid first
- Firmware flash without snapshot + rollback gate.
- Large parameter sweeps that erase the “before” state.
- Full-network stress tests before containment.
Five Common Symptom Classes (Symptom → Minimum Check → Next Hop)
Link down / flapping
- Check link-state timeline (up/down bursts, not just current state).
- Verify flap counters and whether counters reset after a reboot.
- Capture snapshot at the moment of transition (event code + port state).
Next: Isolate
Then: Measure
CRC / PCS error spike
- Confirm counter window (per-second vs per-minute) to avoid “false spikes”.
- Check burst pattern (rare huge burst vs steady trickle).
- Correlate with temperature/power snapshot (same timestamp).
Next: Measure
Keep: Evidence
Throughput drop / burst loss
- Split “drop” vs “error”: check drop counters alongside CRC/PCS errors.
- Capture a snapshot during the low-throughput window (not after recovery).
- If impact propagates, isolate the port before deeper tests.
Next: Decide
Keep: Evidence
Latency / jitter anomaly
- First prove it is not link-quality: check whether CRC/PCS counters rise with the anomaly.
- Correlate with temperature/power/firmware events (same timeline window).
- Freeze evidence before any timing-parameter changes.
Next: Measure
Keep: Evidence
Post-update regression
- Confirm version + configuration migration outcome (snapshot before/after).
- Check whether the health gate was passed before commit.
- Prepare rollback with clear criteria (stability window X).
Next: Recover
Keep: Evidence
H2-3 · Observability: Counters, Snapshots & Pass Criteria
Field alignment depends on a shared measurement dictionary. Observability must define what to measure, when to capture, and what “pass” means (threshold + window + stability).
Counter Dictionary (Layered Fault-Domain Split)
PHY
Answers: electrical/link quality versus internal logic. Supports loopback/PRBS correlation and “outside link” suspicion.
Typical use: separate “bad link” from “bad software” early.
PCS
Answers: coding/sync/symbol-layer problems versus frame-layer drops. Helps interpret “CRC-looking” failures.
Typical use: decide whether to go deeper into service-mode tests.
MAC
Answers: frame accounting and local acceptance/filters. Separates “frames exist” versus “frames consumed correctly”.
Typical use: reconcile throughput drop without physical errors.
Switch-port
Answers: congestion/drop versus link errors. Flags propagation risk (storm-control triggers, drops, queue pressure).
Typical use: decide when isolation is mandatory to stop spread.
Rule: every counter must be interpreted with a known window (per-second/per-minute) and a clear reset policy (accumulating vs cleared-on-read).
Snapshot Policy (Trigger → Minimum Fields → Consistency)
Trigger points
- Link flap (up/down edges)
- CRC/PCS burst (threshold or pattern)
- Temperature rise / power anomaly
- Reset / watchdog event
- Firmware slot switch (A/B) or config change
Minimum snapshot fields
- timestamp + event code + snapshot ID
- fw version + config fingerprint
- port state (link state, mode)
- layered counters (PHY/PCS/MAC/port)
- temperature + input power + brownout flag
- action context (bypass / loopback / PRBS / rollback)
- counter window policy (W seconds, reset policy)
Consistency rules
- Capture “before” and “after” around any action.
- Bind test results to snapshot ID (evidence chain).
- Prefer small structured export first; attach full logs later.
- Never change parameters before freezing evidence.
Pass Criteria Template (Threshold X + Window W + Stability S)
Link stability
- flap ≤ X / hour
- window W = X minutes (sliding)
- stability S = X hours after recovery
Error rate
- CRC ≤ X / minute
- window W = X minutes (fixed)
- burst rule = no burst > X within W
Service-mode BER
- PRBS BER ≤ X
- window W = X seconds
- repeatability = same result across X runs
Rollback success
- rollback success ≥ X%
- stability S = X hours post-rollback
- health gate must pass before commit
The same metric must never be compared across different windows or reset policies. “Pass” always binds (X, W, S) to a snapshot ID.
H2-4 · Per-Port Bypass & Service Mode Architecture
Per-port bypass is the field “hard handle”: contain a bad link first so the system can keep operating, then measure in a controlled service window. A bypass design must be fail-safe, transient-aware, and evidence-driven.
Three Bypass Targets (Mode → When → Evidence)
Keep-alive
Bypass data path
Goal: keep the network running while the suspect path is routed around.
Evidence: snapshot before/after, global error-rate drop, stability window S.
Contain
Isolate the port
Goal: stop storms/error propagation and stabilize the rest of the node.
Evidence: isolate_ts, port-state change, system-wide stabilization proof.
Service
Service mode (loopback/PRBS)
Goal: run repeatable tests after impact is contained.
Evidence: test result bound to snapshot ID (BER, duration, run count).
Design Hooks (Fail-Safe, Transients, Protection, Loss)
Fail-safe default
Power-loss and reset must land in a predictable state (no hidden topology changes).
Switch transients
Switching can trigger short disruptions; enforce service windows and record before/after snapshots.
Protection path integrity
Bypass routing must not break ESD/surge return paths or degrade clamp effectiveness.
Insertion loss / bandwidth
The bypass path must not create a new marginal link; verify loss budget and pass criteria after switching.
Field operation evidence rule
Any switch action must produce: action event code + timestamps + pre/post snapshots + stability window confirmation.
H2-5 · Loopback Strategy (PHY / MAC / External)
Loopback is a fast boundary tool in field service. It separates “external link / peer / cable path” from “local PHY/MAC/board domain” using repeatable pass criteria (threshold X + window W + stability S).
Evidence rules (must-haves)
- Capture a snapshot before and after enabling loopback (bind results to snapshot ID).
- Use a known counter window W and reset policy; never compare mixed windows.
- Loopback proves a boundary; it is not a full end-to-end service guarantee.
Three-Layer Loopback Comparison (Goal → How → Pass)
PHY internal loopback
Goal: prove local PHY TX/RX path under a controlled closure.
How: enable PHY loopback; keep business traffic stopped or contained.
Pass (X): CRC=0, drop=0, stable link for S, test window W.
Boundary answer
Pass here but fail outside usually increases suspicion of external link/connector/board routing (details belong to Cable Diagnostics).
MAC loopback
Goal: verify local frame path, accounting, and queue behavior.
How: enable MAC loopback; measure frame counters with a fixed window W.
Pass (X): drop ≤ X/W, stable throughput ≥ X% baseline, latency within X.
Common trap
MAC loopback success does not prove external link quality; it can bypass connector/cable segments.
External loopback plug (concept)
Goal: include connector/front-end path in the closure without full end-to-end dependence.
How: attach a certified loopback plug during a service window (no mixed business traffic).
Pass (X): errors ≤ X/W and no burst > X in W; stable S after restore.
Next hop
If internal loopbacks pass but external loopback fails, move to Cable Diagnostics and physical inspection workflow.
Decision Mapping (Results → Next Action)
- PHY loopback FAIL: treat as local domain issue first (power/clock/config capture + snapshot evidence).
- PHY/MAC PASS, external FAIL: suspicion shifts to connector/front-end/cable path → hand off to Cable Diagnostics.
- All PASS but field still fails: run PRBS (H2-6) in a service window to quantify margin and burst behavior.
H2-6 · PRBS/Pattern Test & BER Estimation
PRBS/pattern testing converts “links that seem up but behave poorly” into measurable margin indicators. It is a controlled stress method (not the same as real application traffic) and must be executed inside a service window.
Test Steps (Service Window SOP)
- Freeze evidence: capture baseline snapshot (counters + env + version + window W).
- Contain impact: isolate or bypass the port if needed (avoid propagation).
- Run PRBS: set generator/checker roles, duration W, run count N; keep business traffic off.
- Collect results: BER estimate + error distribution buckets + correlated temp/voltage points.
- Restore and verify: exit service mode, capture post snapshot, confirm stability S.
How to Interpret Results (BER + Distribution + Correlation)
BER estimate
BER ≈ errors / bits during window W. Compare only within the same mode, window, and counter policy.
Pass template: BER ≤ X with N repeats and stable S after restore.
Error distribution
Bucket errors by time: uniform small errors suggest marginality; bursts suggest transient events or intermittent contacts.
Pass template: no burst > X within W (even if average BER looks “fine”).
Env correlation
Correlate BER and burst buckets with temperature, input power, and mode transitions captured in snapshots.
Pass template: BER remains within X across specified env points.
DO NOT (Misread Prevention)
- Do not compare BER across different windows W or counter reset policies.
- Do not run PRBS while business traffic is active (statistics and bandwidth become untrustworthy).
- Do not conclude “pass” without recording version/config fingerprints and snapshot IDs.
- Do not accept a single run as proof; repeat N times and validate stability S after restore.
H2-7 · Built-in Self-Test: POST / Periodic / On-Demand
Built-in self-test turns field diagnostics into a repeatable product capability: scheduled execution, explainable scoring, and snapshot-bound evidence. Three tiers map to three disturbance levels: fast boot proof, low-impact health trending, and service-window deep tests.
Self-Test Matrix (Tier × Coverage × Impact)
POST (Power-On Self-Test)
Goal: safe-to-run gating before entering business mode.
Coverage: basic register path, port baseline state, minimum datapath sanity.
Impact: boot-time only; no dependency on peer traffic.
Output: POST code + baseline snapshot ID + score baseline.
Periodic Health (In-Flight)
Goal: trend detection without disrupting traffic.
Coverage: flap rate, error counters per window W, temperature and input power events.
Impact: low; rate-limited and hysteresis-protected.
Output: time buckets + delta score + trigger-to-deep-test criteria.
On-Demand (Service Window)
Goal: evidence-grade boundary tests for acceptance decisions.
Coverage: loopback and PRBS/pattern tests with run count N and stability window S.
Impact: service-mode only; business traffic must be stopped/contained.
Output: pass/fail verdict + snapshots (pre/post) + explanation fields.
Health Score (0–100) — Explainable Composition
Link stability
Flap rate ≤ X/hour, continuous uptime ≥ S, recovery does not oscillate.
Error quality
CRC/PCS errors ≤ X per window W; no burst > X within W; stable trend across K windows.
Thermal margin
Temperature and throttling flags remain within defined margins; no runaway under periodic load.
Power integrity
Brownout events ≤ X; input ripple events are not correlated with error bursts.
Recovery behavior
Retry/reset counts remain below X; no repeated recovery loops within S.
Score guardrails
- Hysteresis and smoothing prevent score flapping.
- Every score delta must store “which signals changed” and snapshot IDs.
Pass Criteria Template (Acceptance)
- Score gate: Health Score ≥ X (defined per product class).
- Stability gate: continuous S hours with flap ≤ X/hour.
- Trend gate: error-rate trend does not increase across K consecutive windows W.
H2-8 · Secure Remote Firmware Update & Rollback
A fail-safe remote update path must tolerate power loss and network interruption, enforce authenticity and compatibility, and provide provable rollback decisions based on post-switch health gates.
Minimal Safety Set (Four Gates)
Authenticity gate
Signed payload must verify before slot write/switch is allowed. Store verification result as an event code.
Compatibility gate
Version/hardware/config matrix must match. Incompatible images must fail closed (no switch).
Atomicity gate
A/B slot switch is a single atomic decision. Prevent half-written states from becoming active.
Health gate
Post-switch self-test must pass (score ≥ X, stability S). If failed, rollback is mandatory.
A/B Update Flow (High-Level, Evidence-First)
- Pre-snapshot: store version/config fingerprint, counters, env, and baseline score.
- Download: store package ID, size, and integrity checksum.
- Verify: authenticity + compatibility gates; record event codes and outcomes.
- Switch slot: activate standby slot; record switch timestamp and slot ID.
- Health check: run POST + gated on-demand checks as needed; evaluate score and stability S.
- Commit or rollback: commit only after health gate passes; otherwise rollback and store reason code.
Disaster Scenarios → Rollback Rules
Power loss
Detection: boot reason + incomplete state event. Action: return to last-known-good slot or safe recovery mode.
Network interruption
Detection: download timeout / missing segments. Action: resume or defer; never switch without verified image.
Incompatible image
Detection: compatibility gate fail. Action: block switch, keep current slot active, store reason code.
Post-update link instability
Detection: health gate fail (score drop, flap bursts, error trend). Action: mandatory rollback + evidence bundle export.
Upgrade Evidence Bundle (What to Store)
- Pre-snapshot: version/config fingerprint + counters + env + baseline score.
- Manifest: package ID, target slot, compatibility summary, verification outcome.
- State transitions: download/verify/switch/health/commit/rollback event codes with timestamps.
- Post-snapshot: score delta and the signals that changed (window W + stability S report).
H2-9 · Field Workflow Playbook (5 min / 30 min / Maintenance Window)
This playbook converts observability, isolation, loopback, PRBS, self-test, and A/B rollback into a time-boxed SOP. Each time-box outputs an evidence bundle and a deterministic “Next” decision, preventing ad-hoc actions in the field.
Time-Box SOP Cards (Action / Evidence / Next)
5 minutes — No-regret triage
Action
- Read snapshot + counters (bind to window W).
- Check health score and recent deltas.
- Decide: isolate/bypass if expansion risk is detected.
Evidence
- snapshot_id (pre) + timestamp.
- port_id + link state + window W.
- counter summary + score value.
- bypass/isolation event code (if used).
Next
- If burst/flap crosses X → bypass/isolate first, then proceed to 30-minute boundary tests.
- If stable but degraded → proceed to 30-minute loopback and reproducible window.
- If post-update instability → freeze changes and escalate to maintenance window path.
Forbidden
No firmware/config changes. No heavy tests. No traffic mixing with service-mode actions.
30 minutes — Boundary tests
Action
- Freeze: pre-snapshot + define window W + stop/contain business traffic.
- Isolate: bypass/port isolation if the network must remain operational.
- Loopback: PHY/MAC tiers to split local vs external domain.
- PRBS/Pattern (if needed): run N repeats, observe BER and burst behavior.
- Restore: exit service mode, capture post-snapshot, verify stability window S.
Evidence
- Loopback tier pass/fail + window W.
- PRBS: BER ≤ X (placeholder) + error distribution.
- pre/post snapshots + score delta + “which signals changed”.
- service-mode start/stop event codes.
Next
- Local loops pass, external fails → classify as external link domain; escalate to cable diagnostics page.
- Local loops fail → classify as local port/domain; use evidence to drive design/firmware escalation.
- Unclear outcome → schedule maintenance window with controlled change + rollback safety.
Forbidden
No untracked parameter tweaks. No mixed traffic during PRBS. No “try random resets” without an evidence tag.
Maintenance window — Controlled change
Action
- Change sandwich: pre-snapshot → change → health gate → post-snapshot.
- Firmware update: A/B slot + verify + switch + health gate + commit/rollback.
- Configuration change: record diff, apply, verify, and define explicit revert path.
- Hardware replacement: record part/port ID, run on-demand tests, verify stability S.
Evidence
- manifest / change record + timestamps.
- state transition event codes (download/verify/switch/health/commit/rollback).
- health gate report (score ≥ X + stability S) + pre/post snapshots.
Next
- If health gate passes and stable → commit and export final evidence bundle.
- If health gate fails → mandatory rollback and export incident bundle for forensics.
- If repeated failures → lock changes and escalate with consistent evidence schema.
Forbidden
No “manual commit” without health gate. No silent rollback. No change without a recorded diff/manifest.
H2-10 · Engineering Checklist (Design → Bring-up → Production)
Field service must be designed-in. This section defines three quality gates that pre-install isolation hooks, evidence storage, reproducible test scripts, and rollback-safe update behavior—so field actions remain predictable and provable.
Three Gates (Checklists + Deliverables)
Design Gate
Must-have hooks
- Per-port bypass/isolation control + defined power-off default state.
- Readable counter categories (PHY/PCS/MAC/port) + stable window definition W.
- Snapshot engine + ring buffer capacity target (placeholder X).
- Loopback control entry points (tiered) + service-mode event codes.
- PRBS/pattern enable controls with “no mixed traffic” guard.
- POST/Periodic/On-demand scheduler policy interface (rate-limit + hysteresis).
- A/B slots with atomic switch + health gate + mandatory rollback reason codes.
- Evidence export interface (schema fields + timestamps + IDs).
Deliverables
Checklist + evidence schema + default placeholders (X/W/N/S) for acceptance templates.
Bring-up Gate
Reproducible baselines
- Loopback scripts per tier + consistent output format and event tags.
- PRBS procedures: window W, repeats N, stability S, and ban on mixed traffic.
- Counter baseline across env sweeps (temperature / input power states).
- Health score initial distribution + smoothing/hysteresis verification.
- Snapshot export validation: pre/post comparison within the same accounting window.
- Fault rehearsal: power loss / link interruption → evidence and rollback behavior observed.
Deliverables
Script pack + baseline table template + rehearsal report template (thresholds as X placeholders).
Production Gate
Factory-ready evidence
- Factory POST + light periodic health checks must generate an evidence bundle.
- Version locking and manifest traceability enforced (no silent drift).
- Rollback drills executed for key failure cases (power loss, verification fail, health gate fail).
- Evidence export validated (bundle fields complete and readable by support tooling).
- Acceptance template published: score ≥ X, flap ≤ X/hour, BER ≤ X, stability ≥ S.
Deliverables
Factory evidence bundle + rollback drill records + acceptance sheet template (X/W/N/S placeholders).
H2-11 · Applications (How field service capabilities get used)
This section turns “features” into on-site outcomes using four common industrial Ethernet scenarios. Each card stays within the Field Service boundary: isolate fast, measure with evidence, recover safely, and leave an audit trail.
Line / Production cell
Pain: downtime is the primary cost driver.
Service moves: per-port bypass / isolate the bad port, capture a snapshot for root-cause later.
Acceptance: line continues running; MTTR ≤ X; mis-isolation rate ≤ X%.
Bypass
Snapshot
Remote I/O box / distributed nodes
Pain: truck rolls and long diagnosis cycles.
Service moves: service mode (loopback/PRBS) + black-box snapshots to make remote support decisive.
Acceptance: on-site reproduction rate ≥ X%; time-to-diagnose ≤ X.
Loopback
PRBS
Evidence
Motion control / imaging triggers
Pain: intermittent drops are hard to reproduce and consume engineering time.
Service moves: periodic health checks + trend evidence to catch degradation before a stop.
Acceptance: flap rate ≤ X/hour; score stable within X over Y days.
Health
Trends
Counters
Industrial gateway / fleet deployment
Pain: a failed update can create fleet-wide incidents.
Service moves: secure remote update + A/B slots + health gate + rollback with evidence snapshots.
Acceptance: rollback success ≥ X%; post-update health ≥ X.
A/B
Rollback
Audit
Capability mapping (Scenario → Service blocks)
A fast way to keep the page “non-overlapping” is to map scenarios only to field-service blocks: Bypass, Snapshot, Loopback, PRBS, Health, A/B Rollback.
Pass criteria placeholders: MTTR ≤ X, flap ≤ X/hour, rollback ≥ X%.
H2-12 · IC Selection (Translate serviceability into MPN-ready capability lists)
This chapter avoids “catalog dumping.” It converts on-site requirements (isolate / measure / recover / evidence) into component capabilities and then lists example MPNs commonly used to implement those capabilities. Treat the MPN lists as starting points; finalize by speed grade, port count, temperature, and certification needs.
A) PHY-side service hooks
Must-have: loopback modes, PRBS/pattern support (when available), diagnostic counters, stable link state reporting, and a management interface that field tools can read reliably.
Evidence output
- Loopback pass/fail with a defined time window (X seconds).
- PRBS/pattern error count → BER estimate ≤ X (placeholder).
- CRC / PCS error bursts ≤ X/min (placeholder).
Example MPNs (PHY)
- TI DP83822I (10/100 PHY, robust family)
- Analog Devices ADIN1200CCP32Z-R7 (10/100 industrial PHY)
- TI DP83867IRPAPT (GbE industrial PHY)
Note: feature availability varies by PHY generation; validate loopback/pattern modes and counter granularity before freezing the BOM.
B) Switch / controller observability
Must-have: per-port counters readable in the field, port isolation controls, mirroring hooks, and predictable behavior after link events (no “mystery resets”).
Evidence output
- Per-port drop/CRC counters with time-stamped snapshots.
- “Isolate port” action recorded with an event code + operator ID.
- Post-change stability: flap ≤ X/hour; storm suppressed within X seconds.
Example MPNs (Switch/Controller)
- Microchip KSZ9477S (7-port managed GbE switch)
- Microchip KSZ9897RTXI (7-port managed GbE switch family)
- NXP SJA1105EL / SJA1105ELY (5-port Ethernet switch family)
- Microchip LAN7430 (PCIe-to-GbE controller bridge, gateway-class designs)
Selection bias: prefer devices that make counters and port actions deterministic and exportable (field evidence > lab intuition).
C) Security + evidence storage
Must-have: image authentication (signed firmware), protected keys, A/B metadata integrity, and non-volatile storage for ring-buffer logs and snapshots.
Evidence output
- Pre-/post-update snapshots with version IDs and monotonic counters.
- Rollback reason codes + success/failure record.
- Audit trail retained for ≥ X events (placeholder capacity).
Example MPNs (Secure + Flash)
- Microchip ATECC608B-MAHDA (secure element family)
- NXP SE050C2HQ1/Z01SDZ (secure element family)
- Winbond W25Q128JV (SPI NOR flash family)
- Macronix MX25L12835F (SPI NOR flash family)
Practical rule: treat logs/snapshots as a first-class product output—size the flash and rotation policy to retain evidence across resets.
D) Per-port bypass hardware
Must-have: fail-safe default state, controlled switching transient, and an evidence rule: every bypass action must produce an event record + counters snapshot.
Implementation options
- Logical bypass: switch port isolate + storm guards (no physical pair re-route).
- Physical bypass: relay/mux creates a hard path around a dead node (design-specific).
- Service mode: port routed to loopback/PRBS fixtures during maintenance windows.
Example MPNs (Relays for bypass fixtures)
- Panasonic TX2SA-5V (telecom relay family, DPDT)
- Omron G6K-2F-Y DC5 (signal relay family, DPDT)
Physical bypass is topology- and speed-dependent. Validate insertion loss, symmetry, and switching transient in the target link budget.
Fail-safe & evidence rule
- Default state: define power-loss behavior (bypass vs isolate) and document it as a field rule.
- Switching: enforce a cooldown window of X seconds (placeholder) to avoid flapping loops.
- Evidence: record action + counters + timestamp for every bypass event.
Selection weighting (scorecard template)
Use a scorecard to keep reviews consistent. Each line reserves a 1–5 score (or High/Med/Low) without forcing a wide table.
Service continuity
Bypass/isolation behavior: Score [ ]
Evidence quality
Counters + snapshots + export: Score [ ]
Boundary tests
Loopback/PRBS usability: Score [ ]
Safe updates
A/B + rollback + gates: Score [ ]
Security & storage
Keys + log retention: Score [ ]
Diagram: Selection funnel (Field needs → blocks → weights → candidate families)
Keep selection discussions aligned: start from field requirements, map to blocks, apply weights, then shortlist families (not random part searches).
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H2-13 · FAQs (Field troubleshooting, evidence-first)
Scope rule: these FAQs only close long-tail on-site troubleshooting within Field Service boundaries (isolate / measure / recover / evidence). Each answer is a fixed 4-line structure with measurable pass criteria.
Link flaps but counters look clean — first check snapshot window or counter reset logic?
Likely cause
Counters reset on link-down or are read after a clear; snapshots miss bursts due to wrong trigger/window.
Quick check
Confirm snapshot trigger (flap / CRC burst) and verify counter lifetime across link transitions (since-boot vs since-link-up).
Fix
Freeze counters on trigger, extend snapshot window, and store both monotonic (since-boot) and session (since-link-up) sets.
Pass criteria
Flap ≤ X/hour over Y hours; snapshot capture rate ≥ X% of flap events; counter lifetime definition matches field SOP.
Loopback passes, real traffic drops — first isolate by per-port bypass or run PRBS in service mode?
Likely cause
Internal loopback covers only internal paths; the external path or traffic-dependent stress is failing.
Quick check
Apply reversible per-port isolate/bypass to contain impact; then run PRBS/pattern in service mode for Y seconds.
Fix
If PRBS fails: treat as link margin issue and keep service mode evidence. If PRBS passes: export snapshots and compare pre/post conditions.
Pass criteria
PRBS BER ≤ X for Y seconds; throughput drop ≤ X% under defined load; containment action logged with timestamp + port ID.
PRBS BER OK at room, fails hot — thermal drift or supply ripple correlation?
Likely cause
Temperature reduces margin or increases error bursts via supply ripple; failures appear only when hot.
Quick check
Correlate BER/error bursts with snapshot temperature and supply min/max within the same timestamp window.
Fix
Add thermal/supply-aware health gates; delay heavy tests until stabilized; enforce derating thresholds for service mode.
Pass criteria
BER ≤ X across Tmin–Tmax; supply ripple within X mV during PRBS window; health gate trips before BER exceeds X.
After FW update, only one port unstable — rollback criteria too strict or config migration bug?
Likely cause
Per-port config migration mismatch or a port-specific health gate never reaches commit after update.
Quick check
Compare pre/post snapshots for that port (FW ID, config hash, counters) and confirm service mode is not left enabled.
Fix
Patch migration mapping; clarify per-port health/commit gates; allow per-port fallback to last-known-good config.
Pass criteria
Post-update flap ≤ X/hour over Y hours on that port; rollback triggers only when health score < X; config hash stable.
Remote update succeeds but device “half-alive” — A/B commit handshake or health gate missing?
Likely cause
Slot switch completes but commit confirmation is skipped, or finalize is allowed without a health gate.
Quick check
Read A/B slot state, last reboot reason, and health score right after switch; verify commit flag transition is logged.
Fix
Enforce: verify → switch → health check → commit. If health fails within X minutes, auto-rollback to prior slot.
Pass criteria
Commit completion ≥ X%; rollback success ≥ X% within Y minutes; post-update health ≥ X for Y hours.
Bypass engaged and network recovers, but root cause unknown — what evidence is mandatory to collect?
Likely cause
“Recover-first” actions occurred without a minimum evidence set; the fault signature was lost.
Quick check
Verify the incident record includes: port ID, timestamp, FW ID, counter set, temperature, supply min/max, action code, result code.
Fix
Enforce “bypass requires snapshot”; block bypass if evidence capture fails; export the incident record immediately.
Pass criteria
Evidence completeness ≥ X% of incidents; each bypass event logs ≥ X mandatory fields; export time ≤ X seconds.
CRC bursts only during maintenance window — service mode left on or mirror/export flooding?
Likely cause
Service mode not exited (loopback/PRBS interference), or mirroring/export creates overload that looks like CRC bursts.
Quick check
Confirm service-mode flag is OFF after maintenance; verify mirroring/export is rate-limited and not enabled during production traffic.
Fix
Add service-mode timeout, require explicit “exit service mode” checklist, and cap mirror/export bandwidth during operations.
Pass criteria
CRC ≤ X/min during maintenance; returns to baseline within Y minutes after exit; export bandwidth ≤ X% of link.
Self-test passes but field fails — is the test targeting the right layer (MAC vs PHY)?
Likely cause
POST checks logic/register paths but does not validate external link margin; failures appear only under real link conditions.
Quick check
Run layered boundary checks: PHY loopback → MAC loopback → PRBS service test; identify the first failing layer.
Fix
Promote the failing layer’s test into periodic/on-demand BIST and weight it into the health score.
Pass criteria
Layered results consistent across Y runs; failing layer detected within X minutes; health score drops before field outage.
Recovery works but repeats weekly — missing periodic health checks or thresholds too loose?
Likely cause
Degradation accumulates but thresholds do not trip early; only heavy recovery is used, so the pattern repeats.
Quick check
Review weekly trends: flap rate, error bursts, temperature/supply extremes, reset counts; compare to baseline.
Fix
Add periodic lightweight checks and tighten thresholds; trigger maintenance before failure and capture evidence on trend crossing.
Pass criteria
Recurrence ≤ X/month; trend alarm triggers ≥ Y hours before outage; post-fix stability holds for ≥ Y days.
“Fix by reboot” works once — what snapshot proves it’s not a latent link-quality issue?
Likely cause
Reboot clears state/counters but does not restore physical margin; latent errors will return under stress.
Quick check
Compare pre/post reboot snapshots: error trend, health score drift, flap rate; verify post-reboot counters stay flat for Y hours.
Fix
Require a post-reboot PRBS window (service mode) or sustained counter stability check before closing the incident.
Pass criteria
Post-reboot flap ≤ X/hour and CRC ≤ X/min over Y hours; PRBS BER ≤ X for Y seconds (if run).
Per-port isolate reduces impact but breaks redundancy — what is the safest minimum service action?
Likely cause
Isolation action is too aggressive for the topology; service continuity is impacted more than necessary.
Quick check
Prefer staged actions: apply guard/rate-limit first, then timed isolate with automatic revert; confirm evidence capture before acting.
Fix
Implement staged service actions (guard → timed isolate → bypass if available) with a cooldown and mandatory logging.
Pass criteria
Containment within X seconds; required path availability ≥ X%; action reverted automatically within X minutes if stability returns.
Logs are huge and export is painful — what is the minimum black-box record set?
Likely cause
Logs are verbose without a compact, bounded incident record; export becomes the bottleneck and evidence is lost.
Quick check
Confirm a fixed-field incident record exists (ring buffer): port ID, timestamp, FW ID, counters, temp, supply, action/result codes.
Fix
Define a minimum incident schema, rotate/compress, and export summaries by default; keep raw logs as optional deep-dive.
Pass criteria
Per-incident record ≤ X KB; export completes ≤ X seconds for Y incidents; evidence completeness ≥ X% across incidents.
