Design Log DEC 043 shared actuation module replaces direct controller servo gpio - thepinkmile/Enigma-NG GitHub Wiki

DEC-043 - Shared Actuation Module Replaces Direct Controller Servo GPIO

  • Status: Decided
  • Date: 2026-04-26
  • Category: Mechanical / Electrical partitioning
  • Area: Controller, Extension, rotor-group turnover, servo actuation
  • Author: Izzyonstage & GitHub Copilot

Summary

Replace the former direct Controller-local CM5 servo path with a reusable Actuation Module (AM) that is hosted both by the Controller and by each Extension. The AM accepts power and a single active-low ACTUATE_REQUEST line, then performs local homing, one-shot request capture, and servo PWM generation autonomously.

To support Extension-local actuation power without inventing a second cable family, the Reflector / Extension harness is widened from 16-pin BHR-16-VUA to 20-pin BHR-20-VUA while preserving the existing reflector-boundary service bus on pins 1-16 and adding grouped 5V_MAIN plus returns on pins 17-20.

Problem

The previously active design had two incompatible actuation assumptions:

  • the Controller used a direct CM5 SERVO_PWM + SERVO_HOME interface
  • the newly agreed Extension turnover architecture needed a completely local request-driven actuator with no live Controller PWM dependency

That mismatch would have created two different servo-control circuits, two software models, and no clean way to propagate actuation across Extension boundaries.

Decision

  1. Adopt one shared Actuation Module PCB everywhere servo stepping is required.

    • Controller: one AM for the main depression-bar actuation path.
    • Each Extension: one AM for group-boundary carry regeneration.

  2. The host/AM electrical contract is reduced to:

    • grouped 5V_MAIN, 3V3_ENIG, GND
    • one active-low ACTUATE_REQUEST
    • no host-visible BUSY, HOMED, or fault feedback pins

  3. The AM owns local servo behaviour.

    • power-up homing
    • home-switch reading
    • request latching / one-shot behaviour
    • servo PWM generation
    • local LED diagnostics

  4. Controller direct servo ownership is retired.

    • CM5 GPIO 8 becomes ACTUATE_REQUEST
    • the former direct SERVO_PWM / SERVO_HOME Controller path is no longer active

  5. Extension boundaries regenerate carry locally.

    • carry remains mechanical within each contiguous 5-rotor group
    • at an Extension boundary, the mechanical carry event becomes a local ACTUATE_REQUEST
    • the Extension-hosted AM then actuates the next rotor group's first-step mechanism

  6. Promote the Reflector / Extension link from 16-pin to 20-pin.

    • pins 1-16: unchanged legacy service bus
    • pin 17: 5V_MAIN
    • pin 18: GND
    • pin 19: 5V_MAIN
    • pin 20: GND

Rationale

  • Keeps the Controller and Extension actuation hardware on the same electrical architecture.
  • Makes the actuation subsystem field-replaceable and easier to maintain.
  • Preserves the mechanical carry principle while still crossing the electrically separated Extension boundary cleanly.
  • Avoids overloading the AM host interface with software handshakes the user does not want.
  • A reviewed standalone PWM driver such as the PCA9685 is not sufficient by itself because the AM also requires autonomous homing and request sequencing; a small local controller with native PWM is therefore preferred.

Impact

  • Add a new design/Electronics/Actuation_Module/ design-doc set.
  • Update Controller/Design_Spec.md and Software/Linux_OS/Power_Management.md to remove the direct CM5 servo model.
  • Update Extension/Design_Spec.md to host the AM and define the local group-boundary actuation role.
  • Update Stator, Reflector, and System_Architecture docs for the widened 20-pin Reflector / Extension harness.
  • Update the mechanical actuation docs so carry is described as mechanical-within-group and Extension-regenerated across group boundaries.

Cross-ref

DEC-038, design/Electronics/Actuation_Module/Design_Spec.md, design/Electronics/Controller/Design_Spec.md, design/Electronics/Extension/Design_Spec.md, design/Mechanical/Rotor_Actuation_Assembly/Design_Spec.md.

QUE-002 supersession note: DEC-043 supersedes the pin count stated in QUE-002 Question 3. QUE-002 Q3 referenced "Stator J10 (16-pin 2x8)" - that was the pre-DEC-043 connector width. J10 is now a 20-pin 2x10 BHR-20-VUA shrouded box header. The mating connectors on Extension (J7/J8) and Reflector (J4) are identical 20-pin 2x10 BHR-20-VUA headers. Standard 20-wire 2.54mm IDC ribbon cable assemblies are suitable for all three; no break-off PCB coupons are required for these links (QUE-002 coupons apply only to Samtec ERF8/ERM8 0.8mm-pitch connectors).