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

The board dramatically improves the Floating Point performance of the computer.

MacBench 1.0 results running @16 MHz (system clock):

When using an external 40 MHz oscillator, the Floating Point results improves a bit more (it's not proportional to the clock frequency as at some point the slow 16-bit bus becomes a limiting factor):

Order a PCB

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

Bill of Materials

Qty	Value	Device	Package	Parts	Digikey reference
1		MC68882FN40A	PLCC68	IC1
1		PLCC68 socket	PLCC68	IC1
1		TE 5-532955-8		J1	A34313-ND
1		Pin Header 2.54mm	3 pins	J2
5	100nF X7R	SMD capacitor	0805	C1-C5
1	10uF 10V	Tantalum Cap	A (3216)	C6	399-4788-1-ND
1	51 ohm	SMD resistor	0805	R1
1	40 MHz	Crystal oscillator	DIP-8	X1

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

References

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