keycap wrote: incompatible keyboard to USB? This simply involves rewiring the entire keyboard to match the membrane pinout for the microcontroller.
There is nothing "simply" about it. You would have to first reverse-engineer the rubber dome keyboard's matrix, then manually wire the old keyboard to fit that matrix. On keyboards with 2KRO, the logical matrix may be much more complex than for a "regular" hand-wired matrix with diodes, so the logical layout may not necessarily match the physical layout very much and you could get a quite complex mess of wires. And woe if you had made a mistake...
If the keyboard speaks the PS/2 or AT protocol, just buy a good converter to USB. If it speaks XT or terminal protocol, use Soarer's converter firmware with an Arduino Pro Micro or Teensy 2.0. Apple's ADB or M0110 keyboard? - Hasu has made converters.
There are schematics and source code for other converters from various protocols to PS/2 over at
kbdbabel.org.
(A Arduino Pro Micro has the same controller as a Teensy 2.0, only with a voltage regulator, fewer pins and a different bootloader. There is an old long thread about using it.)
If any of the above fails, maybe you could desolder the old keyboard's controller and solder in ribbon cables to a Teensy or similar with a modified TMK firmware. If the keyboard is not tiny then you might need a Teensy++ with more pins.
