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Macintosh Classic II / Performa 200 FPU card

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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:
 
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
dymczykmUpdated May 2, 2023

Results

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

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References and Credits

Thanks to pfuentes69 for providing the missing Digikey PNs.