Cypher-Input Board - 64-Character Variant Design Specification

Status: Draft Project: Enigma-NG Version: v.0.1.0 Associated Hardware Revision: Rev A Last Updated: 2026-09-21 Parent Document: design/Electronics/Cypher-Input/Design_Spec.md

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1. Overview

This document specifies the 64-Character variant of the Enigma-NG Cypher-Input Board. It supports the extended Enigma-NG cipher alphabet: 26 letters + 10 digits + 2 base64-extra symbols (+//), realised via Shift for uppercase (RFC 4648 base64 alphabet: A-Z, a-z, 0-9, +, /). Space and Enter are present for CM5 UI input clarity but are not part of the cipher alphabet.

All three Cypher-Input variants (26-Char Classic, 64-Character, 10-Numeric) share an identical circuit topology (ENC module mount, LED indicator bank, Cypher Board interconnect, USM interconnect, board-identification strap plus a shared non-cipher-key I2C expander) - see design/Electronics/Cypher-Input/Design_Spec.md. Only key count/layout, LED/resistor/socket quantities, plain-bits allocation, and BOARD_ROLE_ID value differ between variants.

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2. Key Layout and Character Set

• Layout: QWERTY-style. 26 letters + 10 digits + 2 base64-extra symbols (+, /) + 2 Shift (Left/Right) + Space + Enter.
• Key count: 42 total (40 cipher-path + 2 non-cipher: Space, Enter).
• Character set composition: the 64-character (base64) cipher alphabet is realised as 26 physical letter keys (doubling as uppercase via Shift = 52 letter values) + 10 physical digit keys (case-invariant) + 2 physical base64-extra symbol keys (case-invariant) = 64 cipher values, driven by 40 cipher-path signals on the ENC module plain-bits bus. Space and Enter are read via the on-board I2C GPIO expander (U4), not the plain-bits bus, so they never enter the cipher pipeline.

Placeholder layout (provisional): the arrangement below is an initial placeholder only, to be superseded once the user's own mock layout and renders (produced with an external keyboard-layout tool) are added to the repository. Key positions, row groupings, and Shift/Space/ Enter placement are all subject to change.

  1   2   3   4   5   6   7   8   9   0   +   /
    Q   W   E   R   T   Y   U   I   O   P
      A   S   D   F   G   H   J   K   L
  SHIFT   Z   X   C   V   B   N   M   SHIFT
              [ SPACE ]         [ENTER]

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3. plain-bits Allocation

Range	Assignment
PB[0:25]	26 letter keys (case realised via Shift - see Shift rows below)
PB[26:35]	10 digit keys (case-invariant)
PB[36:37]	2 base64-extra symbol keys: + and / (case-invariant; RFC 4648 base64 alphabet)
PB[38:39]	2 Shift keys (Left/Right)
PB[40:63]	Unused - spare plain-bit positions

Provisional pending Quartus pin-planning and PCB layout on the ENC module side. See Design_Spec.md §3 for the common ENC module interface and full J4 zig-zag pin map (Board_Layout.md §1). Space and Enter are not part of this bus - see §4 below. LED colour selection never uses any plain-bits position - colour is sourced entirely from the User Settings Module, see §5.

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4. Board Identification and Non-Cipher Key I/O

• BOARD_ROLE_ID[3:0] strap value: 0b0111 (Characters + Numbers + Special; bit3 Custom not populated on this board - see Cypher/Board_Layout.md §4 encoding table and Cypher/Design_Spec.md §3a).
• U4 (PCA9534A) I2C address: 0x38, the single fixed address shared by all Cypher-Input variants (see Design_Spec.md §3a). Used on this variant for Space/Enter UI-only key readback only - see Design_Spec.md §3a.

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5. LED Indicator Behaviour

Colour and illumination values (four independent styles) are stored and configured entirely on the User Settings Module and delivered to this board on the common J3 connector (RED_DRIVE_{1,2,3,4}_N/GREEN_DRIVE_{1,2,3,4}_N/BLUE_DRIVE_{1,2,3,4}_N/ ILLUMINATION_DRIVE_{1,2,3,4}_N - see Design_Spec.md §5/§6 and User_Settings_Module/Design_Spec.md). This variant carries no dedicated local colour-selection circuit of its own.

LED count matches total physical keyswitches (42: 40 cipher-path + Space + Enter), so Space and Enter also carry a colour-indicator LED even though they are not part of the cipher pipeline.

This is the only Cypher-Input variant with Shift keys. Shift is a global state, not a per-key one: while either Shift key is held, all keys switch their baseline illumination from Colour1 to Colour3 (Shift changes the [a-z] keys' cipher meaning to [A-Z], so Colour3 indicates the board is in uppercase mode). The currently-pressed key still shows Colour2 on top of whichever baseline (Colour1 or Colour3) is currently active. Which local circuit/CPLD implementation realises this behaviour is not yet determined - see .copilot/todos/cypher-input-led-independent-rgb-pwm-review.md.

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6. Bill of Materials (64-Char Variant-Specific Components)

Variant-specific components for the 64-Character variant. Common components shared across all Cypher-Input variants are listed in design/Electronics/Cypher-Input/Design_Spec.md §11.

RefDes	Specification	MPN	Manufacturer	DigiKey PN	Mouser PN	JLCPCB PN	Alt Supplier + PN	Notes	Footprint Available	Footprint Downloaded	Qty
D1-D42	RGB SMD LED (placeholder - MPN TBD, pending user confirmation of a part that fits under Cherry MX2A-71NB)	TBD	TBD	-	-	-	-	One per key; colour source is the User Settings Module (see Design_Spec.md §5); top face - not populated in PCBA, hand-soldered by the user after delivery (see Design_Spec.md §2 Architecture)	-	-	42
R1-R42 (Red)	0402, value TBD pending LED part confirmation	TBD	TBD	-	-	-	-	Red channel current-limit	-	-	42
R1-R42 (Green)	0402, value TBD pending LED part confirmation	TBD	TBD	-	-	-	-	Green channel current-limit	-	-	42
R1-R42 (Blue)	0402, value TBD pending LED part confirmation	TBD	TBD	-	-	-	-	Blue channel current-limit	-	-	42
SW1-SW42	Mechanical keyswitch hot-swap socket, THT (rear-mount)	PG151101S11	Kailh	-	-	C41430893 (consignment)	-	Hot-swap socket for Cherry MX2A-71NB (not populated - see below)	✔	✔	42

Not part of the PCBA (sourced and installed separately):

Item	MPN	Manufacturer	DigiKey PN	Mouser PN	Notes	Qty
Mechanical keyswitch	MX2A-71NB	Cherry	1644-MX2A-71NB-ND	540-MX2A-71NB	Snap-fit into hot-swap sockets; JLCPCB global sourcing/consignment or Amazon (prototyping)	42