Cypher-Input Board - 26-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

---

1. Overview

This document specifies the 26-Char Classic variant of the Enigma-NG Cypher-Input Board. It mimics the original German Enigma machine keyboard: a QWERTZ layout of 26 letter keys only, with no Shift, digit, symbol, Space, or Enter keys. This is the simplest Cypher-Input variant.

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.

---

2. Key Layout and Character Set

• Layout: QWERTZ, 26 letter keys (A-Z), single case only (no Shift key).
• Key count: 26.
• Non-cipher keys: None (no Space or Enter on this variant).

The physical key arrangement mirrors the original German Enigma machine's three-row QWERTZ layout (9 + 8 + 9 keys): the middle row is staggered half a key-width to the right of the top row, and the bottom row realigns back under the top row (P sits under Q):

  Q   W   E   R   T   Z   U   I   O
    A   S   D   F   G   H   J   K
  P   Y   X   C   V   B   N   M   L

---

3. plain-bits Allocation

Range	Assignment
PB[0:25]	26 letter keys, QWERTZ layout (no Shift - single case only, mimics the original Enigma)
PB[26:63]	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). LED colour selection never uses any plain-bits position - colour is sourced entirely from the User Settings Module, see §5.

---

4. Board Identification

• BOARD_ROLE_ID[3:0] strap value: 0b0001 (Characters only; see Cypher/Board_Layout.md §4 encoding table).
• U4 (PCA9534A) I2C address: 0x38, the single fixed address shared by all Cypher-Input variants (see Design_Spec.md §3a). No Space/Enter keys exist on this variant; all 8 GPIO spare.

---

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 has no Shift key and carries no dedicated local colour-selection circuit of its own.

---

6. Bill of Materials (26-Char Variant-Specific Components)

Variant-specific components for the 26-Char Classic 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-D26	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)	-	-	26
R1-R26 (Red)	0402, value TBD pending LED part confirmation	TBD	TBD	-	-	-	-	Red channel current-limit	-	-	26
R1-R26 (Green)	0402, value TBD pending LED part confirmation	TBD	TBD	-	-	-	-	Green channel current-limit	-	-	26
R1-R26 (Blue)	0402, value TBD pending LED part confirmation	TBD	TBD	-	-	-	-	Blue channel current-limit	-	-	26
SW1-SW26	Mechanical keyswitch hot-swap socket, THT (rear-mount)	PG151101S11	Kailh	-	-	C41430893 (consignment)	-	Hot-swap socket for Cherry MX2A-71NB (not populated - see below)	✔	✔	26

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)	26