This was a funny little project during first COVID Christmas in 2020. I realized that designing an FPU card for Classic II doesn’t seem particularly complicated, as it’s mostly about correctly hooking it up to the CPU bus.
After a few days I had the design ready and after a few weeks of wait for the PCBs it turned out to work very well.
The project was published on the 68kmla forum:
Classic II FPU card design
Hi everyone, I spent some hours of my lockdown time during Christmas and worked on a simple FPU card for Macintosh Classic II. I've bought this little Macintosh some years ago - it just needed a recap to bring it back to life :) Yesterday, I have assembled the FPU board and I'm happy to...
My GitHub repo contains a KiCAD design of an expansion card that adds an MC68882 FPU to Macintosh Classic II. The board has two layers to keep the PCB cost down. It offers a switchable clock source between the system clock (16 MHz, synchronous operation) and a crystal oscillator on the board (40 MHz, asynchronous operation; tested with MC68882FN40A, the rumours claim 50 MHz will work too).
dymczykm • Updated May 2, 2023
After plugging in the card, it should be immediately visible in some tools (here System Information of MacBench):
The board dramatically improves the Floating Point performance of the computer. MacBench 1.0 with an FPU@16 MHz (system clock) reports over a 13x improvement. When using an external 40 MHz oscillator, the Floating Point result goes up even a bit more - to 15.7x (it's not proportional to the clock frequency as at some point the slow 16-bit bus becomes a limiting factor):
(look at the Floating Point row)
If you want to order a PCB directly, I have created a project on PCBWay: https://www.pcbway.com/project/shareproject/Macintosh_Classic_II_FPU_card.html
Pin Header 2.54mm
(sorry, I had most components at hand, so I only have Digikey references for the rather specific ones)
The MC68000 KiCAD library from https://dev.sigpipe.me/DashieElectronics/Jackalope
Thanks to pfuentes69 for providing the missing Digikey PNs.