Hello Vogons / Everybody - this seems to be my first post here and it is about to be a long one...
So for some time I have also been considering building a motherboard for sticking 486-class CPUs into it. My approach would make it expensive but this kind of project is doomed to be against the economy and reason anyways.
My design considerations:
1. Target format: Mini-ITX - I want a nice small portable PC, I want to use standard cases and power supplies
2. I want to allow experimenting with multiple ISA and PCI cards. Maybe overclock a good 486 CPU and beat the fastest 486-class system of all times. 😀 I would also like to integrate my 320x200->1920x1200 aspect-correction scaller and HDMI video output (VGA monitors with analog input also seem to be on the path to extinction).
3. Lengthy prototyping and designing everything from scratch to make it cheaper to manufacture in large quantities doesn't seem to make any sense to me. And this project must be rewarding - so months of PCB design just to get started are out of the question.
4. I would also like to be able to build some more units in the foreseeable future for people who would really want them - that rules out a one-time hack run. It has to be repeatable.
The only viable way I see is to extend the MiSTer by replacing the under-performing ao486 with a real CPU and add bus interfaces. Quite a lot of pins are needed to connect one so DE10-Nano won't make it in terms of hardware. The simplest way would be to port MiSTer to different hardware with a simillar/same FPGA. Enclustra Mercury+ SA2 Altera® Cyclone® V SoC Module + a Baseboard with FMC connectors seems to be the best choice I can think of: 234 available FPGA IO pins. The baseboard is lacking video output so for starters an FMC card with a HDMI serializer will be needed. Maybe something more. Port MiSTer to this new hardware. That would be Stage I.
Designing a custom FMC card with a CPU / CPU socket and optionally a SDRAM/SDRAM module socket (all 3.3V-only) and maybe some-other-stuff to make it fully MiSTer-compatible would make Stage II. Add anything you can think of: PCI/ISA sockets, voltage converters (maybe 5V-tolerant CPLDs as voltage converters) for Stage IIa. Fix and repeat. Stage IIb.
If everything more or less worked out - it's time do design a motherboard housing the SoC module and the CPU (and memory or whatever). I would go for Mini-ITX format with an additional PISA-style edge connector. So - if you want to have a big system with external PCI/ISA cards all you need to do is buy an industrial PISA backplane (and housing or build custom housing or whatever you like). These are still available cheap and new - like IEI IP-10S-RS-R40 if you want to go wild. That would be Stage III. The PISA physical socket is actually an EISA socket with different pin arrangement (ISA part is true ISA, additional pins are PCI so ISA backplanes work with PISA CPU cards). For EISA or VLB people maybe one could design a custom PISA to VLB or EISA backplane - perhaps this would be possible with rearranging the FPGA configuration.
The PISA connector could be used to extend the system if to be used with different architectures in mind. If we were to include all the hardware stuff MiSTer community desires - maybe a "486" board would reach audience beyond PC fans.