Hydra 83

Hydra 83

One board. Two protocols. More to come.

What Is It?

The Hydra 83 is a custom 83-key mechanical keyboard that can switch between USB HID and PS/2 via a hardware toggle. The name comes from two things:

• 83 — the total key count
• Hydra — the multi-headed creature of myth, representing the board's ability to switch between protocols. In PS/2 mode it's a standard PS/2 keyboard; in USB mode it's a USB keyboard that also accepts a PS/2 mouse on the PS/2 connector

The Hydra is intended to be a series. Future versions may add an integrated USB hub, additional protocol modes, or other switching features.

Features

• Dual-protocol keyboard output — USB HID or PS/2 Scan Code Set 2, selected with a hardware toggle
• PS/2 mouse input in USB mode — plug a PS/2 mouse into the PS/2 connector while in USB mode; QMK's built-in PS/2 mouse driver reads it and forwards it to the host over USB
• Full PS/2 keyboard implementation — make/break codes, typematic repeat, E0-prefixed extended keys, F13–F24, multimedia, browser, and power keys
• On-board BSS138 level shifters — two BSS138 MOSFETs handle bidirectional 3.3 V ↔ 5 V translation on the PS/2 clock and data lines; no external components needed
• OLED display — SSD1306 128×32 connected via I2C
• Rotary encoder — Alps EC11E with push-to-click
• USB-C connector — with SRV05-4 ESD protection
• Hardware mode switch — debounced in firmware (50 ms)
• QMK-based firmware — fully remappable, built on QMK best practices, using QMK built-ins wherever possible

How the Modes Work

PS/2 Mode

The board operates as a standard PS/2 keyboard. Connect it to any PS/2 port — Scan Code Set 2, typematic repeat, full extended key support, all modifiers. The PS/2 clock and data lines are driven by the RP2040 through the BSS138 level shifters.

USB Mode

The board enumerates as a USB HID keyboard. The PS/2 connector switches role: it becomes a PS/2 mouse host. Plug a PS/2 mouse in and QMK reads it and forwards mouse movement, buttons, and scroll to the host over USB alongside keyboard input — one USB-C cable, both devices.

Layout

The Hydra 83 has 83 keys with an integrated numpad cluster, F1–F12 doubling as F13–F24, and a rotary encoder in the top-right corner.

Repository Layout

├── Firmware/
│   ├── config.h
│   ├── halconf.h
│   ├── kb.c
│   ├── kb.h
│   ├── keyboard.json
│   ├── keymaps/
│   │   └── default/
│   │       └── keymap.c
│   ├── Makefile
│   ├── mcuconf.h
│   ├── ps2_keyboard.c
│   ├── ps2_keyboard.h
│   ├── ps2_scancodes.h
│   ├── readme.md
│   └── rules.mk
└── PCB/
    ├── gerbers/
    │   ├── bom.csv
    │   ├── designators.csv
    │   ├── Hydra_83_1.0.zip
    │   ├── netlist.ipc
    │   └── positions.csv
    ├── Hydra83_Schamatics.pdf

PS/2 mouse host support uses QMK's built-in PS2_MOUSE feature, enabled in rules.mk.

Hardware

Key Components

GPIO Pin Map

All 30 RP2040 GPIOs are used:

PS/2 Connector Pinout (Mini-DIN 6)

PCB

KiCad source files, schematic PDF, PCB plot PDF, interactive HTML BOM (ibom.html), and JLCPCB fabrication files are all in the PCB/ directory. The design is still being refined.

To view the interactive BOM, open ibom.html in any browser — it shows component placement on the board alongside the BOM table, useful for hand assembly.

Firmware

Prerequisites

• QMK Firmware set up locally
• QMK CLI configured
• ARM GCC toolchain (installed by qmk setup)

Build & Flash

# Copy the Firmware folder into your QMK keyboards directory
cd ~/qmk_firmware/keyboards
mkdir -p hydra83
cp -r /path/to/Hydra_83/Firmware hydra83/hydra83

# Compile
qmk compile -kb hydra83/hydra83 -km default

# Flash — double-tap reset on RP2040 to enter bootloader, then:
qmk flash -kb hydra83/hydra83 -km default

Or copy the pre-compiled .uf2 from the Releases page onto the RPI-RP2 drive.

Keymap Customization

Edit Firmware/keymaps/default/keymap.c. Full PS/2 Scan Code Set 2 support is already in the firmware — just change the layout array.

Debugging

qmk console

================================
Mode switch: PS/2
================================
[PS2] PS/2 driver activated
[PS2] Key pressed: keycode=0x0004, scancode=0x1C
[PS2] Typematic repeat: keycode=0x0004, scancode=0x1C
[PS2] Key released: keycode=0x0004, scancode=0x1C
================================
Mode switch: USB
================================
[USB] USB driver restored

PS/2 Protocol Details

The keyboard side operates in PS/2 device mode — the RP2040 acts as a keyboard. Each byte is 11 bits (start, 8 data bits LSB-first, odd parity, stop) at ~10 kHz. The firmware handles:

• Make codes (key down) and break codes (0xF0 + scancode, key up)
• E0-prefixed extended codes for arrows, navigation, multimedia, browser, and power keys
• Multi-byte sequences for Print Screen and Pause/Break
• Typematic repeat — 500 ms initial delay, ~30 repeats/second

In USB mode the same PS/2 lines (GP24/GP25) switch to input, and QMK's PS2_MOUSE driver reads standard PS/2 mouse packets and forwards them over USB HID.

Roadmap

• Finalize PCB layout
• Software mode toggle via keypress (in addition to hardware switch)

Contributing

Pull requests are welcome. Open an issue first for significant changes.

License

GPL-2.0 — see LICENSE. Matches the QMK Firmware license for full compatibility.

Author

Betzalel J. Lewis — GitHub

PS/2 device mode proof-of-concept: QMK PS/2 USB Dual-Mode Keyboard