2021-03-04 14:17:14 +01:00

3.4 KiB

Macintosh Classic II FPU card

This repo contains a KiCAD design of an expansion card that adds an MC68882 FPU to your 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).


After plugging in the card, it should be immediately visible in some tools (here System Information of MacBench): FPU card present in System Information

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):

FPU benchmark showing Floating Point improvements

(look at the Floating Point row)

Order a PCB

If you want to order a PCB directly, I have created a project on PCBWay:

Bill of Materials

Qty Value Device Package Parts Digikey reference
1 MC68882FN40A PLCC68 IC1
1 PLCC68 socket PLCC68 hosts IC1 2057-PLCC-68-AT-ND
1 TE 5-532955-8 J2 A34313-ND
1 Pin Header 2.54mm 3 pins J1
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 Shunt jumper over J1 881545-1

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


FPU top

FPU bottom

FPU installed

References and Credits

The MC68000 KiCAD library from

Thanks to pfuentes69 for providing the missing Digikey PNs.