08 — Power & Chassis Integration

1. Power tree

The 19 V/90 W tree below describes the USB/HDMI bench prototype only. The N100DC-ITX accepts 19 V, while the Tang Mega 138K Pro dock lists a 12 V DC input.¹² A production common-inlet supply and its rail converters are therefore not yet specified.

flowchart TB
    BRICK["External 19 V DC brick<br/>≥ 65–90 W"] --> INLET["Panel DC inlet<br/>+ switch + fuse"]
    INLET --> N100["N100DC-ITX<br/>(19 V DC-in)"]
    INLET --> B1["Buck → 5 V<br/>(panel, USB hub, faders)"]
    INLET --> B2["Buck → 3.3 V<br/>(RP2350 nodes, I²C)"]
    B1 --> DISP["Touch panel (5 V)"]
    B1 --> HUB["Internal USB hub"]
    B1 --> FAD["Motor-fader drivers"]
    B2 --> NODES["RP2350 control nodes"]
    B2 --> SCAN["RP2350 keybed scanner"]
    N100 --> AIF["Audio interface<br/>(USB-powered or own supply)"]

Budget (indicative)

Rail	Loads	Rough draw
19 V	N100 board (CPU+RAM+NVMe)	6–25 W (idle→load)
5 V	panel, hub, fader motors (peak)	5–15 W
3.3 V	RP2350 nodes, I²C, LEDs	1–3 W
—	audio interface	1–5 W (USB bus power)

A 90 W brick is a bench estimate, not the integrated-card power budget. Size the 5 V buck for fader stall current (motors briefly draw far more than nominal).

Integrated card power gates

Measure Tang-board power at idle and with PCIe, DDR framebuffer/display, I²S converters and Ethernet active. Specify a protected 12 V rail for the board, the N100’s 19 V rail, clean converter analog rails, panel/backlight power and fader-motor peaks. Verify sequencing, brownout, back-feed, EMI, grounding and safe mute on partial power loss before choosing a single external supply or final enclosure wiring. Neither the 19 V bench tree nor its 90 W brick can be copied unchanged into the integrated instrument.

Power-path notes

Sequence: bring up 3.3 V/5 V rails before or with the N100 so the MCUs are ready when USB enumerates; nothing depends on strict ordering, but avoid back-feeding the bridge from an unpowered node.
Protection: main fuse + reverse-polarity protection at the inlet; per-buck local fusing for the motor rail.
Quiet rails for audio: keep the audio interface on clean USB/own supply; physically separate switching converters from analog audio wiring.
Soft power: a front-panel momentary button → board power button header for clean shutdown of the immutable OS (or just hard-cut, since the rootfs is read-only and power-safe — see 03).

2. Grounding & noise

Star ground from the inlet; avoid ground loops between the N100, audio interface, and panel.
Route the 19 V switching, the digital harness (USB-MIDI, I²C), and the analog audio as three separated bundles; cross at right angles where unavoidable.
Balanced outputs (04) already reject common-mode hum — terminate shields at one end only.

3. Mechanical integration into the S90

Mainboard tray: mount the Mini-ITX board on standoffs to a metal sub-tray inside the S90, away from the keybed travel.
Thermals: the N100 is fanless but needs airflow. Add passive vents in the chassis bottom/rear over the heatsink, or one quiet thermostatic fan placed away from the audio path. Verify temps under sustained DSP load.
Display: cut the top panel for the chosen panel; back it with a rigid bezel/sub-panel to carry the ~0.3–0.5 kg display and resist key-strike vibration.
Connectors at the rear: balanced XLR and unbalanced jack for line input and output, power inlet/switch, and optional service connectors. Define separate analog drive/receive and protection for each connection.
Keybed ribbon: keep the original keybed ribbon path; add the RP2350 scanner board close to the ribbon exit to keep matrix wiring short (05).
Service access: plan for getting the board and SSD out without removing the keybed.

4. Open questions ([verify])

Internal volume and standoff pattern of the specific S90 revision vs the Mini-ITX footprint.
Worst-case fader motor inrush to size the 5 V buck and fuse.
Thermal headroom of the fanless N100 inside the closed chassis under sustained load.
EMI from the 5 V switching buck into the audio interface — measure noise floor before finalising converter placement.
Tang board’s 12 V consumption, analog converter rails and thermal load with PCIe audio, display refresh and all selected network ports active.

ASRock, “N100DC-ITX” specification (19 V DC-in, fanless). https://www.asrock.com/mb/Intel/N100DC-ITX/index.asp ↩
Sipeed, “Tang Mega 138K Pro Dock” (12 V DC input). https://wiki.sipeed.com/hardware/en/tang/tang-mega-138k/mega-138k-pro.html ↩