Schematics are nice, working circuits are much better ;-)

I found old keyboard keys that I unsoldered in the early 90s, when my ambition was to design a 6809-based computer. It needed keys and I found them in a strange module I bought at a surplus. Then the switches and caps remained in their little boxes for more than 20 years...

Those are nice Siemens-made, not Cherry top-of-the-line but better for typing purpose than the usual KST. You don't need to press the key completely to close the contacts and the caps have enough markings to be useful (though the control/function keys will need stickers).

Those switches have an integrated diode, ready to be wired as you like, but they will only be used for mechanical purposes. I instead use SMB Schottky diodes to increase the voltage margin (this was discussed in the "Redneck" log).

I pulled a Europe-sized board from my stock and layed the keys out: 30 fit so it's enough for this purpose. 16 Hex keys and 14 functions/commands should be enough. The only annoying thing is the keys are not "flush", the PCB needs 3 holes. I managed to reduce this by cutting stuff on the back but a 3mm drill is still required on the PCB.

After this is done, the rest is very easy.

Switches and SMB diodes are then soldered:
(fancy wiring, huh ?)

I tested all the keys' contacts and had to change one.
I can now choose where I'll put each key for optimal convenience.

That's a nice "retro" look ;-) But more important it's functional and cheap.

The questions that remain are:

• How can I reuse the remaining caps to display the functions I want ? (ASND, ASI4, ARES, DRES, Start, Step...)The caps can be rotated 180° but not 90°.
  
• Should I solder some logic ICs on this board ? I'd like to make a connector to plug on the other boards but I don't have a 32-pins cable. OK, I could find a diskette ribbon cable (34 pins IIRC). However I could solder 4× 74HC138 and the cable needs to provide
  * GND
  * Vcc
  * 5 address data
  * 1 return signal
  A classic 10 pins ribbon cable works, reusing standard JTAG parts.
  OTOH I can't put much logic there, in part because the PCB is not well etched (which explains why I bought it for less than the standard price).

I haven't checked yet my stock of HC parts in DIP package. I can't afford buying new things at this moment so I have to be patient and use what I already have...