The BBC Microcomputer System (or BBC Micro) was an innovative machine back in the early 1980’s. One feature that impressed reviewers was a “tube” interface that allowed the machine to become an I/O processor for an additional CPU. When the onboard 6502 became too slow, it could become a slave to a Z-80 or even an ARM processor. The bus was actually useful for any high-speed device, but its purpose was to add new processors, a feature Byte magazine called “innovative.”
[Hoglet67] has released a very interesting set of FPGA designs that allows a small board sporting a Xilinx Spartan 3 to add a 6502, a Z80, a 6809, or a PDP/11 to a BBC Micro via the tube interface. There’s something satisfying about a classic computer acting as an I/O slave to a fairly modern FPGA that implements an even older PDP/11.
There is a set of lengthy threads on this project, but the easiest place to start is probably this pathfinder thread. You may also want to read more about the tube interface. There’s also an interview with one of the BBC Micro’s designers in the video below.
In our recent roundup of classic computer emulators, we neglected the BBC Micro but [PKM] provided a link. If you don’t have a BBC Micro lying around, maybe just doing a whole 6502 computer on an FPGA will do. Maybe you can even add your own version of the tube interface to it.