feat(pcb): central PSU power-entry / distribution board (24-pin ATX -> 3x console feeds)

[mottihoresh]

Motivation

Optional future convenience layer between the PSU and the per-console PDUs. The per-console PDUs (#202) are being designed around a universal Molex 4-pin peripheral input, so they have no dependency on this board — this is a clean-cabling adapter, not a prerequisite. It replaces "3 loose PSU cables fanning out" with 1 PSU cable → 3 clean per-console feeds, and confines all ATX / PSU-brand specifics to this one board.

Deferred idea captured during PDU #202 work (2026-07-03).

Topology

PSU 24-pin  ->  Entry/Distribution board  ->  per-console feed  ->  PDU (#202: monitor/fuse)  ->  J_BUS

Scope

INPUT — one 24-pin ATX socket

• Motherboard-style female 24-pin ATX socket. Device-side is ATX-standard = universal, works with any PSU brand → avoids the brand-specific modular-socket problem (the reason we never reuse another brand's PSU-side cables).
• Carries all rails: multiple +12V, +5V, +3.3V, +5VSB, GND, plus PS_ON# and PWR_OK.

Current sizing — 24-pin native limit, NOT full PSU

• Size to the 24-pin native limit ≈ 18A on +12V ≈ ~216W (the limiting rail; 5V/3.3V plentiful).
• NOT the full 600W. Cockpit draws only ~50–100W; full 600W/50A would need extra PCIe/EPS inputs + heavy copper — pointless. 216W is 2–4× the real load.

OUTPUT — ~3× per-console feeds

• One feed per console PDU over house connectors: Molex 4-pin peripheral or Mini-Fit Jr — 12V / 5V / GND each.
• Optionally start the CAN trunk here.

Soft-power centralization (this is where PS_ON# belongs)

• PS_ON# was deliberately dropped from the PDUs → it belongs on this board.
  • Now: jumper-to-GND (always-on).
  • Later: MOSFET / switch soft-on.
• PWR_OK monitor.
• +5VSB available for always-on logic if a monitor MCU is added.

Optional — monitor MCU (CAN node)

Decide during design:

• Smart: small monitor MCU as a CAN node → aggregate PSU health (PWR_OK, total current via a main shunt, PS_ON control over CAN, +5VSB-powered so it's alive before soft-on).
• Dumb: pure distribution + PS_ON jumper, no MCU.

Board stack

• 2-layer, 2oz copper, wide via-stitched pours.
• ≤100×100mm (JLC cheap tier, ~$30 shipped for 5).
• 4-layer NOT needed — low signal complexity; copper weight matters more than layer count for current.

Open items

• Dumb vs smart (monitor MCU) — decide during design.
• Output connector: Molex 4-pin peripheral vs Mini-Fit Jr.
• Whether the CAN trunk originates here.
• PSU identity reconcile: docs say GOLDEN FIELD NX650; a Thermaltake Toughpower GF1 diagram surfaced during this discussion. Resolve which PSU is the reference before finalizing pinout.

Relates to

• feat(pcb): per-console power distribution + monitoring CAN node (PDU) — INA180 telemetry #202 — per-console PDU (universal Molex 4-pin input → no dependency on this board)
• feat(pcb): bus power distribution + power spec (base boards) #203 — bus power spec / docs/_source/base-boards.md