init: F3 unlock project — ESP32-C3 bridge + procedure for CLAR200 Exos X18

2026-06-14 21:12:56 -04:00
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+# DECISIONS.md — clar200-unlock Decision Log
+
+Project-specific decisions. For global/cross-cutting decisions, see `~/bin/DECISIONS.md`.
+
+Format: `YYYY-MM-DD: <decision> — <why>`
+
+## Architecture
+
+- 2026-06-14: Use the drive's **F3 service UART**, not any SCSI path — every SCSI
+  command (incl. READ CAPACITY) returns Hardware Error on these EMC-locked drives;
+  F3 bypasses the SCSI layer. (See EMC_CLAR200_DRIVE_FIX_NOTES.md for the
+  exhaustive SCSI-side failure list.)
+- 2026-06-14: **ESP32-C3 as the USB-serial bridge** rather than buying a USB-TTL
+  module — one is already attached to steel141 (`/dev/ttyACM0`) and its 3.3 V
+  logic is a native match for the F3 header (many FTDI cables are 5 V = unsafe).
+- 2026-06-14: Bridge uses the C3's **native USB-CDC** as host side + **UART1
+  (GPIO4 TX / GPIO5 RX)** to the drive, with a live `~N` baud menu so baud can be
+  swept without reflashing.
+
+## Implementation
+
+- 2026-06-14: Build flag `CDCOnBoot=cdc` is **required** so `Serial` maps to
+  USB-CDC, not UART0; encoded in `firmware/flash.sh` FQBN.
+- 2026-06-14: F3 format commands are **discovered via `?`**, not copied from
+  7200.11 forums — Exos X18 dialect differs and wrong `m0,...` params can worsen
+  a drive.
+
+## Deferred / Rejected
+<!-- Decisions NOT to do something are just as valuable -- prevents re-proposing rejected ideas -->
+
+- 2026-06-14: **Rejected** further SCSI-level attempts (sg_format flag variants,
+  firmware download, sanitize) — prior session exhausted them; all return
+  Hardware Error because the EMC firmware refuses below the command layer.
+- 2026-06-14: **Rejected** doing F3 work with the drive in the pve173 JBOD —
+  too risky next to live tank; bench-only.
+- 2026-06-14: **Deferred** the BSY isolation/tape trick — the drive boots and
+  answers INQUIRY (not BSY-hung), so it's unlikely to be needed; contingency only.
+- 2026-06-14: **Not yet decided** — whether to RMA to ServerPartDeals first vs.
+  attempt F3 first. Leaning: attempt F3 (cable is free + already here), RMA as
+  fallback if F3 refuses.