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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).

Results

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: https://www.pcbway.com/project/shareproject/Macintosh_Classic_II_FPU_card.html

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)

Pictures

FPU top

FPU bottom

FPU installed

References and Credits

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

Thanks to pfuentes69 for providing the missing Digikey PNs.

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FPU card for Macintosh Classic II.

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