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document display architecture research and settled design decisions

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- eDP direct connection ruled out: undocumented pinouts, voltage mismatch risk,
  community consensus favors controller boards
- MIPI DSI ruled out: 2-lane 15-pin, can't dual-display with HDMI on Linux,
  and the salvaged panel is eDP
- Chosen path: HDMI -> eDP-to-HDMI controller board -> salvaged panel
- Mechanical HDMI A/B switch retained as fallback for external display
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BarnacleBoy 2026-05-27 01:12:00 +00:00
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docs/port-layout.md
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@ -49,14 +49,13 @@ User-facing. Clean for typing.
|-----------|------------|
| Powered USB hub (10-port) | Decased, wired internally. Feeds all USB-A and USB-C data ports. |
| HDMI A/B switch (mechanical) | Passive toggle. OPi3B HDMI in, two outputs (internal display + starboard panel jack). |
| eDP-to-HDMI controller | Connected to A/B switch output A. Drives internal display — **may be unnecessary if native OPi3B eDP works with panel** |
|| eDP-to-HDMI controller | Connected to A/B switch output A. Drives internal display. |
## Constraints
- OPi3B has **one HDMI output**. The A/B switch selects internal or external — not both simultaneously.
- OPi3B has a **native eDP port** — if the Innolux N156HGE-EAB panel pinout matches, the external controller board is not needed.
- The internal display uses an eDP-to-HDMI controller board (matched to the salvaged panel). The native eDP port on the OPi3B is unused — direct connection was investigated but ruled out due to undocumented pinouts and voltage mismatch risk.
- DisplayLink USB graphics adapters **do not work on ARM Linux** (no drivers). Not a viable path for dual display.
- Dual simultaneous display would require a dual-HDMI SBC.
- USB-C on the OPi3B is **power only** — no DisplayPort alt mode. All video goes through HDMI or native eDP.
- Built-in WiFi 5 + BT 5.0 on the OPi3B — no USB dongles needed for wireless connectivity.
- Two native USB 3.0 ports available for direct connections (keyboard, mouse) without hub contention.