Design Log DEC 038 controller centric dock architecture replaces legacy link alpha link beta backplane - thepinkmile/Enigma-NG GitHub Wiki

DEC-038 - Controller-Centric Dock Architecture Replaces Legacy Link-Alpha / Link-Beta Backplane

  • Status: Decided
  • Date: 2026-04-19
  • Category: Mechanical / Electrical partitioning
  • Area: Controller Board, Power Module, Stator, system interconnect architecture
  • Author: Izzyonstage & GitHub Copilot

Summary

Retire the legacy Samtec LINK-ALPHA and LINK-BETA board-to-board model for the Controller ↔ Power Module and Controller ↔ Stator interfaces. The Controller becomes the fixed motherboard and enclosure-edge I/O carrier; the Power Module and Stator become removable daughtercards docked into the Controller.

Problem

The former dual-blind-mate Samtec architecture assumed the Power Module and Stator both attached directly to the Controller through high-pin-count mezzanine connectors. That arrangement no longer matches the enclosure-driven mechanical direction:

  • the Controller now owns all external I/O placement
  • the Power Module is a removable power cartridge rather than the I/O edge board
  • the Stator is a removable vertical daughterboard that must be supported against rotor mass

The old Link-Alpha / Link-Beta definitions also forced low-current Samtec contacts to carry large grouped power allocations and kept obsolete direct PM status GPIOs alive even though a shared I2C-1 bus already reaches the PM.

Decision

1. Controller owns RJ45 / Ethernet / PoE entry

The Controller Board now owns:

  • RJ45 connector
  • Ethernet ESD and magnetics handling
  • PoE PD / ACF front-end
  • the local cable-entry shield / chassis island for those connectors

The Power Module no longer carries RJ45, Ethernet LED return lines, or GbE MDI routing.

2. Power Module becomes a power-conditioning / UPS cartridge

The Power Module now receives a regulated PoE-derived auxiliary feed from the Controller instead of raw Ethernet-adjacent PoE circuitry. It continues to host:

  • USB-C PD input
  • smart-battery input
  • OR-ing / filtering / eFuse
  • supercap hold-up
  • 5V_MAIN and 3V3_ENIG generation

3. Controller ↔ Power Module dock is split into three TE connectors

Use three copies of the TE 10-position 2.5 mm board-to-board pair:

  • Controller side receptacle: 1-1674231-1
  • Power Module side plug: 1123684-7

Active partition:

Link Function Allocation
J1A main regulated rails 3 x 5V_MAIN, 2 x 3V3_ENIG, 5 x GND
J1B PoE auxiliary handoff 3 x VIN_POE_12V, 7 x GND
J1C low-speed control / telemetry I2C1_SDA, I2C1_SCL, PM_IO_INT_N, PWR_GD, ROTOR_EN, PWR_BUT, 4 x GND

4. Controller ↔ Stator dock uses two Molex hybrid connectors

Use the Molex EXTreme Guardian HD pair:

  • Controller receptacle: 2195630015
  • Stator plug: 2195620015

Active partition:

  • 5V dock (J2A): 4 x 5V_MAIN power blades, 1 x GND power blade, signal field used as extra GND returns / guards
  • 3V3 / logic dock (J2B): 4 x 3V3_ENIG power blades, 1 x GND power blade, guarded signal field carrying TCK, TMS, TDI, TTD_RETURN, I2C1_SDA, and I2C1_SCL; all remaining signal contacts tied to GND

This retains the JTAG cluster on the 3V3_ENIG / logic-biased connector and gives the Stator two mechanical support points instead of one fine-pitch mezzanine connector.

4a. Controller owns the local servo actuation interface

Because the Stator is now a removable vertical daughterboard, the servo motor and its home switch must remain close to the mechanical depression-bar linkage rather than hanging off the Stator PCB. The Controller therefore owns the full local servo electrical interface:

  • J11 on the Controller carries 5V_MAIN, GND, and SERVO_PWM
  • SW3 / SERVO_HOME is mounted on the Controller with a local pull-up and debounce network
  • CM5 GPIO 12 provides direct 50 Hz SERVO_PWM
  • CM5 GPIO 17 reads the active-low SERVO_HOME input

No I²C expander-owned servo GPIO and no standalone PCA9685 PWM driver are used in the active design.

5. PM-local I2C expander replaces legacy direct PM status GPIOs

Add PCA9534APWR on the Power Module at I2C address 0x3F. Use it for:

  • inputs: POE_STAT, SYS_FAULT, BATT_PRES_N, USB_STAT
  • outputs: SW_LED_R, SW_LED_G, SW_LED_B, SW_LED_CTRL

Retain only these direct PM lines:

  • PWR_BUT
  • PWR_GD
  • ROTOR_EN

6. PM RGB sink stage follows the Settings Board pattern

The PM runtime RGB path uses three low-side BSS138 stages with 1kΩ gate resistors and gate pull-down resistors. The pre-boot hardware flash path remains red + green only via Q4 and D6/D7; blue is runtime-only.

7. Grounding rule is unchanged

The only intentional DC bond between GND and GND_CHASSIS remains on the Power Module before the eFuse. The Controller may host local shield/chassis handling for cable-entry hardware, but it must not create a second galvanic bond to system ground.

Rationale

  • Matches the enclosure-driven mechanical architecture.
  • Moves high-speed / external I/O ownership to the board that already carries the CM5 and user-facing ports.
  • Replaces over-loaded fine-pitch Samtec power transport with dock connectors better suited to grouped power.
  • Keeps PM-local housekeeping close to the PM circuits instead of wasting direct CM5 GPIOs on status lines that already terminate on the PM.
  • Keeps PM devices clustered in i2cdetect by placing the expander at 0x3F, adjacent to the PM INA219 at 0x40.

Supersession / Obsolescence

  • DEC-014 is obsolete as an active definition for Controller blind-mate BtB connectors.
  • DEC-015 and DEC-037 are obsolete as active Controller ↔ Stator interface definitions.
  • The names Link-Alpha and Link-Beta are retained only for historical traceability in older documents; the active architecture uses J1A/J1B/J1C and J2A/J2B.
  • Rotor / Extension / Reflector Samtec Edge-Rate connectors are not affected by this decision.

Impact

  • Controller, Power Module, and Stator design specs must be updated to the new dock model.
  • Controller PoE / Ethernet ownership moves out of the Power Module docs and BOM ownership.
  • Servo motor electrical ownership moves from the Stator to the Controller so the actuation hardware remains co-located with the rotor depression bar in the vertical-Stator enclosure layout. The following Stator requirements were retired as a result (IDs at time of retirement; the active spec has since been renumbered to close the gaps):
    • FR-STA-11 (servo PWM output, original numbering) - superseded by Controller-local direct CM5 GPIO PWM.
    • FR-STA-12 (SERVO_HOME sensing, original numbering) - superseded by Controller-local home-switch input.
    • DR-STA-13 (I²C PWM driver, original numbering) - servo PWM generation moved to direct CM5 GPIO.
    • DR-STA-14 (servo connector, original numbering) - connector ownership moved to the Controller.
    • DR-STA-15 (SERVO_HOME switch, original numbering) - home-switch ownership moved to the Controller.
  • PM GPIO / SW1 RGB runtime control moves to PCA9534APWR @ 0x3F.
  • Power-budget and overview documents must stop referencing Samtec Link-Alpha / Link-Beta as active PM/Stator bottlenecks.

Cross-ref

DEC-014, DEC-015, DEC-031, DEC-034, DEC-037.