Lately I’ve had the urge to scratch my vintage computer itches with something beyond just booting up my Commodore 128 and playing Dig Dug from cassette. There are loads of people on YouTube and elsewhere showing off their Z80 and MOS 6502 homebrew builds that run BASIC and CP/M. Following suit with these enthusiasts seems to be a fun and challenging endeavor. Resources and software for these two MPU families are plentiful, and as such when one goes to design a computer around then, their work is pretty much cut out for them.
Myself, being susceptible to both convenience and a challenge, found out that one of my spare HP 7475A desktop plotters sports a Motorola 6802 MPU (a 6800 with internal ram and a low-power mode) and a handful of support chips. An hour of desoldering later, and I have the processor itself, a few SRAMs, and a MC6850 serial communications adapter ready to live an entirely new life.
On paper, the 6800 is a pretty nice processor to design around compared to the competition in this area. It runs on a single 5V rail, has it’s own clock generator with an option for an external clock, and provides separate address and data buses, which eschews the need for excessive multiplexing and decoding. The wiki page has a pretty concise overview of these, so I won’t go into a ton of detail here except to say that the parts list can be much shorter than going with a 6502 or an Intel 8080.
The flip side of this is that there’s less out there in terms of online resources and program sources that don’t require revisions to assemble and load for these MPUs. The SWTPC 6800 computer, which was kin to the Altair 8800, was fairly popular, and a bunch of information about it is still floating around, so it may prove to be an appropriate base design for my project.
At this point I’m still working out the details of the memory map, and if I will try to reproduce an existing design for software compatibility, or go it alone and design something from scratch with the parts that I have on hand.
I’ll do my best to keep this blog updated with my progress as the project continues.