I just tripped over this thread, and I have to say it's really interesting that you're getting to the stage of integrating modifications into the board. I guess maybe I didn't read closely enough to see if you've already done so, but in the process of putting this together have you created a full schematic in an accessible CAD format (Kicad/Eagle/whatever) for the Plus? (And if so, is it "open sourced" somewhere?)
Are there any Apple proprietary chips other than the IWM on the Plus motherboard? GALs which can emulate legacy PALs are still readily available, so in principle it should be pretty possible to make "new-build" Macs based on this work, especially if you're willing to ditch floppy support. (If something like BlueSCSI were integrated onboard with an accessible SD card maybe doing without wouldn't be a huge deal.) The 6522 VIA is still manufactured, and I *think* (but won't swear to it) compatible Zilog SIO chips still exist, are there any other deal-breakers?
... Digging around a bit now, and I guess I assumed that by now someone would have reverse engineered the contents of the Mac PALs by now, but I'm kind of coming up blank. Does anyone have the equations for these? I guess what I'm really thinking here is that it might also be possible to "evolve" the design in the process so it could be morphed into a full scratch-build without requiring any parts from an original Plus. Sort of thinking of a design progression something like this:
- Start by redesigning the memory addressing circuitry so instead of SIMM slots the board just uses 4MB of modern SRAM. This ditches the need to worry about refresh, CAS/RAS, and address multiplexing. This combined with the high speed of modern SRAM means it might be possible to eliminate some contention for the 68000 and allow it to run faster. (The SE runs slighter faster than the Plus because it has better cycle sharing, basically the same deal.)
- Integrate some kind of mass storage. Early on this could be something like BlueSCSI directly wired to a 5380 SCSI chip, but I wonder what the prospects might be to eliminate the 5380 and program an MCU to emulate it plus a SCSI drive directly.
- Ditch floppy support, or at least make it optional. I assume this would either require patching the Plus ROM to not look for it, or adding some stub hardware to fake it out. (Another idea of course would be to add an MCU that emulates the IWM and attached storage devices directly.)
- As mentioned, add a dingus to convert the keyboard and mouse ports to PS/2 or similar.
After that next big goal would probably be to start playing with the video timing and see if it's practical to tweak things so you can use a "standard" monitor instead of the Apple monitor. This would mean figuring out some way of presenting the Mac's 512x342 pixel grid letterboxed into a VESA compatible video mode, which means changing the horizontal frequency and the pixel clock while ideally trying to keep the CPU and sound timing at least close to the original so game programs don't obviously run at the wrong speed. VGA's 640x480 pixel clock of 25mhz isn't that far off from being a 1.5 multiple of the Mac's 15.667mhz clock. Dividing a VGA pixel clock by three instead of two would probably be close enough for the CPU, but fixing sound would be tricky.
(But maybe not that hard? The Mac sound system works by DMA-ing a byte of RAM inside each video horizontal blanking area to use for sound, which results in the roughly 22khz sample rate. If we're using the VGA 31.5khz horizontal refresh you could make a state machine that grabs that byte every line and a half... or grabs two bytes every third horizontal interval, latches one, and plays the latched one on the half cycle.)
Granted I'm not sure there's a heck of a lot of point to a "parts-build" machine like this when you could just grab an FPGA board and do the whole enchilada that way instead of soldering chips and burning GALs, but I suppose ultimately there's not that much point to anything we do.