Design Log DEC 042 encoder port pin 8 carries hid local enc active n - thepinkmile/Enigma-NG GitHub Wiki

DEC-042 - Encoder Port Pin 8 Carries HID-Local ENC_ACTIVE_N

  • Status: Decided
  • Date: 2026-04-26
  • Category: Electrical / Interface architecture
  • Area: Stator ↔ Encoder connector contract, HID source selection, lightboard blanking
  • Author: Izzyonstage & GitHub Copilot

Summary

Revise the 20-pin Stator ↔ Encoder connector so pin 8 carries a generic active-low ENC_ACTIVE_N sideband. The signal is used only on the HID bank:

  • J4 / KBD_ENC drives keyboard activity into the Stator
  • the Stator source-select path chooses between physical ENC_ACTIVE_KBD_N and CM5_KEY_ACTIVE_N
  • J5 / LBD_DEC receives the selected ENC_ACTIVE_LBD_N so the lightboard can blank when idle

The signal is not propagated through the plugboard, rotor, reflector, or extension interfaces.

Problem

The 6-bit ENC_DATA[5:0] path uses the entire 64-code space, so there is no spare "idle" code that cleanly distinguishes "no key is currently active" from "a valid key code is present." Without a separate activity indication:

  • LBD_DEC cannot reliably blank the lightboard when the keyboard is idle
  • CM5 GUI / telemetry cannot observe the selected keyboard-source activity state cleanly
  • physical-keyboard and CM5-virtual key injection risk diverging in behaviour if only the data bus is muxed

Decision

  1. Pin 8 on the generic 20-pin Encoder connector is reassigned to ENC_ACTIVE_N.

    • Pins 2-7 remain ENC_DATA[5:0].
    • Pins 8-18 shift by one position relative to the previous shield/JTAG allocation so SYS_RESET_N moves to pin 18.

  2. ENC_ACTIVE_N is active-low and defaults HIGH / inactive.

    • HIGH = idle / no active HID event
    • LOW = active HID event present
    • Unused roles shall leave the signal HIGH via the MAX II weak pull-up behaviour or an equivalent schematic bias method.

  3. The signal is HID-local, not cipher-path-global.

    • KBD_ENC drives ENC_ACTIVE_N to indicate a debounced keypress is active.
    • LBD_DEC consumes ENC_ACTIVE_N and must blank all outputs when the signal is HIGH.
    • Plugboard encoder / decoder roles do not use the signal in the active design and should hold or treat it as inactive.

  4. Physical and CM5 virtual keyboard sources must stay aligned.

    • The Stator keyboard-source mux path must switch both the 6-bit source bus and the active-low activity sideband together.
    • The selected activity state must be monitored on the Stator for GUI / telemetry use.

Rationale

  • Preserves the full 64-code space without sacrificing any character value to represent idle.
  • Gives the lightboard an explicit blanking control tied to real key activity.
  • Keeps the extra signal local to the HID bank instead of widening the larger rotor / reflector / plugboard interface contract.
  • Reuses spare mux and GPIO capacity already present on the Stator rather than introducing new active parts.

Impact

  • Update design/Electronics/Stator/Board_Layout.md and design/Electronics/Stator/Design_Spec.md to revise the 20-pin port pin table and document the HID-local source-select / monitoring flow.
  • Update design/Electronics/Encoder/Design_Spec.md and design/Electronics/Encoder/Board_Layout.md to add ENC_ACTIVE_N to the generic module interface.
  • Update design/Software/CPLD_Logic/Encoder_Logic.md so KBD_ENC and LBD_DEC define the required ENC_ACTIVE_N behaviour.

Cross-ref

DEC-041, design/Electronics/Stator/Design_Spec.md, design/Electronics/Stator/Board_Layout.md, design/Electronics/Encoder/Design_Spec.md, design/Electronics/Encoder/Board_Layout.md, design/Software/CPLD_Logic/Encoder_Logic.md.