docs: save BYD Seal longitudinal control (Case A harness re-wire) plan for later
On hold. Captures the measured bus topology, the ADAS-pair-through-CAN1 re-wire approach, research gates, panda forwarding work, opendbc fingerprint-based auto-detection design, validation ladder, and fallbacks.
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# BYD Seal — openpilot Longitudinal Control via Case A Harness Re-wire
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> **Status: ON HOLD** — saved for later use. Current state: MADS (lateral-only)
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> is fully working; stock ACC handles longitudinal.
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>
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> Car: BYD Seal Performance 2025, comma 3X, 26-pin harness
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> Repo: `github.com/Quintine/sunnypilot-byd`, branch `staging`
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---
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## 1. Goal
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Enable openpilot longitudinal control (replacing stock ACC) by re-routing the
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CAN pair that carries `ACC_HUD_ADAS` / `ACC_CMD` / `ACC_AEB` (813 / 814 / 815)
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through the panda, so it can **block stock ACC_CMD and inject openpilot's**,
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while keeping the ADAS ECU alive (AEB, FCW, HUD) and restoring full stock
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behavior when the device is off.
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## 2. Why it's needed (measured bus topology)
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From a live dump on the Seal (`tools/byd_can_dump.py`, route
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`00000022--3a1ad65541--1`):
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| Message | Han topology | Seal topology (measured) |
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|---|---|---|
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| ACC_HUD_ADAS (813, cruise state) | bus 2 (camera) | **bus 0 (vehicle CAN)** |
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| ACC_CMD (814, stock ACC echo) | bus 2 | **bus 0** |
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| ACC_AEB (815) | bus 2 | **bus 0** |
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| Steering feedback | 792 STEERING_TORQUE | **508 STEERING_TORQUE_ANGLE** |
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| PCM_BUTTONS (944, LKAS btn) | ? | **bus 0 @ 20 Hz** |
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| MPC_LKAS_CMD (790, torque cmd) | bus 2 | bus 2 |
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| MPC_LKAS_CMD_ANGLE (482, angle cmd) | — | bus 2 @ 50 Hz |
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| panda CAN1 | — | **silent / unused** |
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Because stock `ACC_CMD` (814) is **born on bus 0** — the same bus the ESP
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listens to — the panda cannot intercept it with the current wiring. The only
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fix is to physically move that pair through the panda.
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## 3. Target end state
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```
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┌──────────────────────────────────────────┐
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LKAS pair ════╡ CAN2 (camera) panda (in comma 3X) │
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(790/482) │ CAN0 (vehicle CAN) ╞════ vehicle CAN
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ADAS pair ════╡ CAN1 (ADAS ECU) │
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(813/14/15) └──────────────────────────────────────────┘
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relay restores BOTH pairs when device is off/unpowered
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```
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- **CAN1 → CAN0**: forward 813, 815 (AEB/FCW + HUD stay alive); **block 814**;
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panda substitutes its own 814 on CAN0 (code already exists in `bydcan.py`)
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- **CAN0 → CAN1**: forward vehicle traffic so the ADAS ECU does not fault
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- **CAN2 ↔ CAN0**: unchanged (LKAS interception, working today)
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---
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## 4. Phase 0 — Research & feasibility (no car modification)
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Four unknowns decide everything:
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- [ ] **0.1 Harness origin** — where did the 26-pin harness come from
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(comma BYD kit? HotIce0's design? self-made?) and is there a wiring diagram?
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Determines available relay contacts / free channels.
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- [ ] **0.2 Is 813/814/815 on a dedicated ADAS pair or the shared vehicle CAN?**
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*Non-destructive experiment:* T-tap panda CAN1 onto the candidate pair
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(do NOT cut it), run `tools/byd_can_dump.py --live 15` (panda noOutput =
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listen-only). If 813/814/815 appear on **bus 1**, the pair is found.
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- [ ] **0.3 Does the ADAS ECU need inbound traffic on that pair?**
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(does traffic flow vehicle→ADAS on it today?) If yes, CAN0→CAN1 forwarding
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is mandatory (panda firmware work in Phase 2).
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- [ ] **0.4 Stock restore on device-off** — the stock comma harness box switches
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**one** pair through a relay. A second intercepted pair needs a multi-pole
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relay on the same panda-driven relay line (standard community dual-tap
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approach), otherwise unplugging the device leaves ADAS permanently dead.
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Verify relay contacts with a multimeter (continuity per pair, device
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powered vs unpowered).
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**GATE:** dedicated pair exists (or ADAS separable at a connector) AND relay
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solution exists → proceed. Otherwise stay MADS-only (already working).
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## 5. Phase 1 — Wiring (hardware)
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1. Break the ADAS pair at the harness: **ADAS-ECU side → panda CAN1**,
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**vehicle side → CAN0** (joins the vehicle CAN net).
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2. Multi-pole relay (or second coil paralleled on the existing relay drive
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line) so **both** the LKAS pair and ADAS pair restore on device-off.
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3. **Termination check** (multimeter, car off): each broken segment should
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read ~60 Ω across CAN-H/L (two 120 Ω terminators). If a segment reads
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~120 Ω or open, add a 120 Ω resistor at the harness on that side.
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4. Bench-verify before driving: device on, `byd_can_dump.py --live` →
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813/815 forwarded to bus 0, 814 absent on bus 0 (until openpilot sends),
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no ADAS DTCs on dash.
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## 6. Phase 2 — Panda firmware
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1. **Forwarding architecture**: panda's `get_fwd_bus` only supports the fixed
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`{0→2, 2→0}` route — bus 1 forwarding does not exist. Add a small
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**per-mode forwarding route table** (default `{0→2, 2→0}` = zero change for
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all other cars; BYD long gets `{0→2, 2→0, 1→0, 0→1}`), with the BYD fwd
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hook blocking 814 on the 1→0 route.
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2. BYD long safety config: rx checks for 813 on bus 1; keep TX `{814, 0}`;
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extend fwd-hook test infra for multi-route tables; full libsafety suite +
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MISRA; rebuild `panda_h7.bin.signed`.
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3. **Contingency (likely needed)**: if the ADAS ECU notices its 814 vanished
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and faults (AEB disabled = unacceptable), add a **keep-alive echo** — panda
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sends the ADAS ECU's own 814 back to it on bus 1 so it thinks the bus is
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alive. Implement behind a flag; enable if ADAS faults appear.
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(Recommendation: build it from the start — safer first try.)
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## 7. Phase 3 — opendbc
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1. **Automatic wiring detection, no config**: `_get_params` receives the live
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fingerprint. If 813 appears on **bus 1** → car is long-wired →
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`alphaLongitudinalAvailable = True` + ADAS parser moves to bus 1.
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If on bus 0 (today) → stays MADS-only. One codebase serves both.
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2. carstate: third parser on bus 1 (ADAS messages); `cp_adas` accessor points
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there when long-wired; ACC_CMD echo for `bydcan` read from bus 1.
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3. Full test suite (interface sim with 3 parsers, lateral limits, docs).
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## 8. Phase 4 — On-car validation ladder
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1. Stationary, ignition on: dump shows correct routing, zero DTCs, stock AEB
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light off
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2. MADS regression (lateral must be untouched)
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3. Enable *Experimental Longitudinal* toggle → **closed lot / empty road**:
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ACC engage, gentle accel, gentle brake, standstill resume
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4. Normal roads → highway. First ~100 km = test pilot mode, hands/feet ready.
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## 9. Fallbacks
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- Device-off relay restores both pairs → full stock, always.
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- If ADAS faults despite keep-alive: revert wiring to passthrough → back to
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MADS-only. No software rollback needed (fingerprint detection handles it).
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## 10. Open questions (to answer when resuming)
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1. Harness origin + wiring diagram availability?
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2. Second CAN interface for probing (PCAN / spare panda / SavvyCAN), or probe
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via comma CAN1 T-tap?
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3. Crimp/relay work DIY or commission a modified harness from a builder?
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4. Risk acceptance: ADAS-might-fault window vs building keep-alive echo from
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the start?
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---
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### Reference: current software state (already merged, working)
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- BYD car port in `opendbc_repo/opendbc/car/byd/` (HAN EV 2023 + SEAL
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PERFORMANCE 2025, Seal fingerprint = 91 messages)
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- Platform-correct buses: 508 steering (Seal), 792 (Han); ADAS on bus 0 (Seal)
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/ bus 2 (Han); PCM_BUTTONS bus-flexible
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- MADS: `allow_always` for byd, LKAS button via PCM_BUTTONS.LKAS_ON_BTN
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- panda `SAFETY_BYD` (35) with real TX checks, MISRA clean, 120 safety tests
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- `alphaLongitudinalAvailable = False` for SEAL (bus 0 ACC_CMD can't be
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intercepted) — HAN keeps openpilot-long option
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- panda firmware `panda/board/obj/panda_h7.bin.signed` (TIZI) built and in repo
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- Diagnostic tool: `tools/byd_can_dump.py` (route analyzer + `--live` sniff)
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