Well my 1st attempt at an old-school microprocessor system was to dive right in and clone a Commodore PET-2001 using current-production/equivalent 6502 and VIA/PIA peripherals:
https://www.eevblog.com/forum/projects/otp-eprom-programmer/msg1246318/#msg1246318I am now finalising the technical details of that design (just a little more perfboard prototyping to do) and the next step will be to draw it all up in Protel and layout a (mostly SMD) PCB for it before building/presenting/publishing it as a completed project with professionally manufactured PCBs.
While plodding away at that I'm making a start at working out the preliminary requirements for cloning a different computer - the Z80-based "Tandy-Radioshack" TRS-80 ("Trash 80").
The current plan is to use a current production Z84 in LQFP from Zilog. Once upon a time I learnt the basics of MPUs on a Z80 system but I have never designed with one. Is there anything important that I need to know about the compatibility of these Z80 variants with the original that ran the TRS-80 operating system? There were a number of important issues that I had to get down pat in order to properly and successfully implement the PET clone with the modern 65XX-family variants from WDC.
Like the PET clone I want to do this entirely in current production logic and again like the PET clone the circuitry can be simplified hugely because much of the original circuitry was just decoding and buffering logic for a gazillion small capacity RAM/ROM chips. The only exotica inside the TRS-80 was a (now long obsolete) "character generator" IC by Motorola which was little more than a pre-programmed ROM containing the character set. The video generation circuitry of the TRS-80 won't be much more difficult to functionally replicate in current production CMOS logic than was the video generation circuity of the PET.
Thanks.