PS/2 has first attention, because such keyboards are still easy to find and are relatively simple compared to USB. But in reality the PS/2 protocol is a bulky protocol already. The PC/XT protocol is perfect, even simpler than matrix keyboards, but good luck finding those. PS/2 doesn't play nice with software-defined signal processing, and it doesn't play nice with the type of input/output lines the Gigatron has now. From the top of my head, here are the pain points:
- Fast clock required, same order of magnitude as VGA scanlines. If the CPU has to sample this once per scanline, it might need to drop audio support, drop vCPU speed a lot, and with that possibly lose support for at67's nice sprite function. I'm not ready to accept any such a sacrifice yet...
- Clock signal is not under control of the host, so you don't know when you get data
- Bi-directional protocol (hopefully that can be ignored)
- Two open collector transmission lines (hmm..: the data line into the Gigatron's shift-register already has a pull-up resistor...)
- 11 bits sent per byte, so you can't just let one shift-register catch bytes and sample it at low frequency.
- Data on falling edge (7400-series is happiest with rising edges)