You can use TMK Converter, PJRC Teensy2.0 and other USB AVR MCU(ATMega32U4, AT90USB64/128 or etc) for this. But binary size is probably more than 10KB and it won't fit into 8K flash.
Connect ADB pins to controller just by 3 lines(Vcc, GND, Data). By default Data line uses port PD0.
ADB female socket from the front:
,--_--.
/ o4 3o \ 1: DATA
| o2 1o | 2: Power SW
- === - 3: VCC
`-___-' 4: GND
This converter uses AVR's internal pull-up, but it seems to be too weak, in particular when you want to use a long or coiled cable. The external pull-up resistor(1K-10K Ohm) on Data is strongly recommended.(It is almost must!)
Use **Makefile.rev1** for TMK Converter rev.1 and **Makefile.teensy** for Teensy2.0 instead of **Makefile**. For TMK Converter rev.2 just use **Makefile**.
How to define the keymap is probably obvious. You can find key symbols in common/keycode.h. And see [doc/keymap.md](../../tmk_core/doc/keymap.md) for more detail.
Many of old ADB keyboards have mechanical push-lock switch for Capslock key and this converter supports the locking Capslock key by default. See README in top directory for more detail about this feature.