Updated: Feb 24, 2022
In my 6502 system, I use VIAs, pin headers, and ribbon cables to connect expansion cards (e.g., my sound card). While this has worked fine, it is a little bit of a hassle to manage the wiring and power distribution. Also, the system takes up a fair amount of space as all devices are laid flat. As I look into the future with my 65816 build, I'm brainstorming how to implement a more robust way of connecting expansion cards.
I'm considering a 65816 motherboard layout in an ATX form factor. I would use the overall board dimensions from the ATX standard, along with placement of expansion slots, standoffs, and general I/O ports. I could then use any PC case to hold the motherboard and expansion cards. I would use 16-bit ISA slot connectors for the expansion cards, but I would deviate slightly from the ISA standard for pinouts, as I don't need -5V, +/-12V, etc. (maybe I would use those later?).
I started laying out the motherboard and expansion card template to get a feel for what it might look like. I think it might work well for me. Below is a standard ATX layout. I could shrink or enlarge the board, but I think this is where I would start for my first 65816 motherboard.
The design of expansion cards would start with something like the following. I can increase the size of the card. My first two expansion cards would probably be for video and audio (with joystick support). I would probably keep a pair of VIAs on the motherboard for PS/2, SPI, and onboard LCD support. Any expansion cards that require a VIA, such as my sound card, would have the VIA on the sound card itself.
Lots more to be done, but some progress...