![notion image](https://www.notion.so/image/https%3A%2F%2Fs3-us-west-2.amazonaws.com%2Fsecure.notion-static.com%2Ffa91e75b-03e3-44bc-9659-d9720b3544b6%2FUntitled.jpeg?table=block&id=298b96f2-8d04-44d0-a45f-138dde0c8118&cache=v2)
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:
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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):
![notion image](https://www.notion.so/image/https%3A%2F%2Fgithub.com%2Fdymczykm%2Fclassic_ii_fpu%2Fraw%2Fmain%2Fimg%2Fclassic_ii_system_info.png?table=block&id=1241e39b-a812-4c60-bd09-c3031df4ac2d&cache=v2)
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):
![notion image](https://www.notion.so/image/https%3A%2F%2Fgithub.com%2Fdymczykm%2Fclassic_ii_fpu%2Fraw%2Fmain%2Fimg%2Fclassic_ii_benchmark.png?table=block&id=7ba6563a-9311-4b3c-93ea-2c1de9ee09cb&cache=v2)
(look at the Floating Point row)
Order a PCB
If you want to order a PCB directly, I have created a project on PCBWay:Â https://www.pcbway.com/project/shareproject/Macintosh_Classic_II_FPU_card.html
Bill of Materials
Qty | Value | Device | Package | Parts | Digikey reference |
1 | ã…¤ | MC68882FN40A | PLCC68 | IC1 | ã…¤ |
1 | ã…¤ | PLCC68 socket | PLCC68 | hosts IC1 | 2057-PLCC-68-AT-ND |
1 | ã…¤ | TE 5-532955-8 | ã…¤ | J2 | A34313-ND |
1 | ã…¤ | Pin Header 2.54mm | 3 pins | J1 | ã…¤ |
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 | ã…¤ | Shunt jumper | ã…¤ | over J1 | 881545-1 |
(sorry, I had most components at hand, so I only have Digikey references for the rather specific ones)
Pictures
![notion image](https://www.notion.so/image/https%3A%2F%2Fgithub.com%2Fdymczykm%2Fclassic_ii_fpu%2Fraw%2Fmain%2Fimg%2FIMG_3241.jpg?table=block&id=9ca99f23-7375-44a7-91d7-f6aae842442d&cache=v2)
![notion image](https://www.notion.so/image/https%3A%2F%2Fgithub.com%2Fdymczykm%2Fclassic_ii_fpu%2Fraw%2Fmain%2Fimg%2FIMG_3249.jpg?table=block&id=c30f2025-b43a-4db6-9fca-d9257202a411&cache=v2)
![notion image](https://www.notion.so/image/https%3A%2F%2Fgithub.com%2Fdymczykm%2Fclassic_ii_fpu%2Fraw%2Fmain%2Fimg%2FIMG_3247.jpg?table=block&id=65a5f01d-5de8-4380-b08f-daf34374288b&cache=v2)
References and Credits
The MC68000 KiCAD library from https://dev.sigpipe.me/DashieElectronics/Jackalope
Thanks to pfuentes69 for providing the missing Digikey PNs.