Macintosh Classic II / Performa 200 FPU card

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...

https://68kmla.org/bb/index.php?threads/classic-ii-fpu-card-design.38107/

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).

classic_ii_fpu

dymczykm • Updated May 2, 2023

Results

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)

Order a PCB

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

Bill of Materials

Qty	Value	Device	Package	Parts	Digikey reference
1	ㅤ	MC68882FN40A	PLCC68	IC1	ㅤ
1	ㅤ	PLCC68 socket	PLCC68	hosts IC1	2057-PLCC-68-AT-ND
1	ㅤ	TE 5-532955-8	ㅤ	J2	A34313-ND
1	ㅤ	Pin Header 2.54mm	3 pins	J1	ㅤ
6	100nF X7R	SMD capacitor	0805	C1-C5, C7	399-17169-6-ND
1	10uF 10V	Tantalum Cap	A (3216)	C6	399-4788-1-ND
1	51 ohm	SMD resistor	0805	R1	A126374CT-ND
1	40 MHz	Crystal oscillator	DIP-8	X1	e.g. MXO45HS-2C-40M000000
1	ㅤ	Shunt jumper	ㅤ	over J1	881545-1

(sorry, I had most components at hand, so I only have Digikey references for the rather specific ones)

Pictures

References and Credits

The MC68000 KiCAD library from https://dev.sigpipe.me/DashieElectronics/Jackalope

Thanks to pfuentes69 for providing the missing Digikey PNs.