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):
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):
(look at the Floating Point row)
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
| 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)
The MC68000 KiCAD library from https://dev.sigpipe.me/DashieElectronics/Jackalope
Thanks to pfuentes69 for providing the missing Digikey PNs.