A 6502-Based Expansion Board for a Z80-Based Homebrew Computer

When I started out computing was mostly hidden away inside universities and large corporations, and consisted of expensive mini-computers and mainframes. However most of the computers you would come across outside those environments were based either on the Zilog Z80, or the MOS Technology 6502 processors.

We’ve recently seen a number of self-build and kit computers based around the Z80. I think those of us that were there have reached a certain age, and as a result, retro-computing is now cool.

Up till now I haven’t seen a 6502-based board, and now I’m seeing both at once—a 6502-based expansion board for a Z80-based homebrew computer.

Ben Chong, over at Ancient Computing, has done a lot of work around Spencer Owen’s RC2014 Z80 microcomputer kit, including designing an improved Z80 processor card and a 16550 UART board.

For the uninitiated, PCB design is pretty addictive. It’s like knitting and solving puzzles at the same time.

However the 6502-based CPU board he’s put together is by far the most impressive. On his site Ben outlines his design process, and deep dives into how he got a 6502-based card to talk to a Z80-based system. This isn’t a trivial feat as the two processors have very different design philosophies—with the 6502 using a strict synchronous bus, and the Z80 using a loosely asynchronous bus. He then talks about software before doing some crude performance testing.

If you’re interested in putting this together your own 6502-based board the folks at Ancient Computing have uploaded the software, and hardware design files, for the board—as well as their other RC2014 expansion boards—to their GitHub repo, and PCBs can be ordered directly from OSH Park.

While if you’re interested in the RC2014, there’s extensive information on the official site and an active community of people playing with the hardware and developing compatible expansion boards for the kit.