ArduinoADB

ADB to USB keyboard converter using an Arduino Uno or compatible clone. Clone boards must have an ATmega16U2; boards with other USB-Serial chips will not work.

Connections

The ADB connection should be set up with the 5V pin to 5V, ground to ground, data to pin 8, and a 1-10K resistor between 5V and pin 8. Here's a visual representation for reference:

Female Mini-DIN 4
    /-----\
|====*4 3*=====|
| | *2   1*==| |
|  \  ===  / | |
|   \-----/  | |
|            | |
|        |===| |
|        |     |
|        |=[R]=|
|        |     |
|        |     |
GND     Pin8  5V

Note: If you don't have a female 4-pin Mini-DIN port, you can substitute with three female jumper wires connecting to the pins on the cable, or if using an Extended Keyboard, you can use three male jumper wires connecting to the pins. If connecting to the wire, make sure to swap the pin layout horizontally.

Setup

1. Upload the sketch to your Arduino.
2. Put your Arduino into DFU mode by shorting the two pins nearest to the reset button (they will be on the 6-pin header nearest to the reset button).
3. Open Atmel FLIP or dfu-programmer and flash the Arduino-keyboard-0.3.hex firmware to the Arduino.
4. Disconnect and reconnect the Arduino to the computer.

If you want to switch the Arduino back to normal (not a keyboard), just do steps 2-4 with Arduino-usbserial-uno.hex instead.

Usage

The Arduino itself is Plug & Play, but do not unplug the keyboard from the Arduino. I cannot confirm whether the ADB port here is hot-pluggable, but assume it isn't.

Note on Caps Lock

Some keyboards have locking Caps Lock keys, including the Apple Extended Keyboard I/II and AppleDesign keyboard. For the Caps Lock key to work properly, you must define LOCKING_CAPS in the main sketch file. This is already done for you, but if you're using a keyboard that doesn't have a locking Caps Lock key, you must comment out that line.

Original source

The code for communicating over ADB was borrowed from tmk/tmk_keyboard. If using an Arduino with an ATmega32U2/4 processor (Leonardo, Pro Micro) it may be better to use that instead, since these boards natively support the converter.