These last days, I have solved almost all the design issues for the user interface board.

• Serial Send : ok
• Serial Receive : hmmm ok
• Keypad scan : ok
• LED Array : ok !
• Input FIFIO : yup
• 3.6864MHz clock : yay.

The remaining issues are

• 7 segments decoder on the cheap ?
• Power : most of the current is drawn by the LEDs. The TIL311 must be replaced by a flatter, modern display technology for the next design iteration.
• Bus arbiter : the strobes should not be sent when the bus is active (avoid contentions)
• Strobe select : generates a write pulse to the right register (when the bus is available). It's mostly several 74HC138 but it's not clear yet.

So far the remaining unknown is the exact definition of the main bus. I believe it's only a matter of time and it will probably evolve so I keep some headroom.

Anyway, so far, except for the USB-serial dongle, the required integrated circuits are only MSI (or less) and the quantity is reasonable. Hopefully, I'll find a similarly smart solution for the 7 segments drivers...