Hi Daryl,
Thank you for sharing your projects with us. I am a complete electronics newbie and I have learned a lot from studying your pages. I did have an Apple ][ long ago and at that time I knew 6502 programming pretty well.
I found your SBC1 project really cool, and if I ever have some spare time, I would like to give it a try. Not very likely at the moment though.
Anyway, I have a few questions that maybe you can answer:
In your SBC2, you changed the memory read-write logic decoding, in such a way that MRD does not depend on PHI2O. Is this ok? Because, by doing that on the SBC1 as well, you could save an entire 74LS00 that could be removed from the board.
Also, I was wondering why you use RAM space for I/O. Any specific reason for that? Because if we did the decoding logic as follow, couldn't we assign it to ROM space instead:
RAMSEL=A15
IOSEL=NAND(A8..A15)
ROMSEL=NAND(RAMSEL,IOSEL)
We would further save a NAND module on the first 74LS00 and have some very slight extra RAM
Anyway, congratulation on all this really cool stuff.
Nelson