A couple of months ago, I posted a design sketch for an SD-card floppy disk drive emulator for classic Macintosh computers. Recently I finally got motivated enough to start building it. Using one of the oh-so-rare DB-19 connectors that I purchased a few weeks back, I rigged up a cable to connect the Mac’s external floppy port to a breadboard. Then I dug out an old CPLD board I built for an early Tiny CPU test, and wired all the floppy lines to the board’s I/O connector. The board is powered by the +5V from the floppy port.

This particular board has an Altera EPM7128S CPLD, which is so old that it actually runs at 5V (newer CPLDs run at 3.3V or below). That’s great for this purpose, because 5V is what the Mac provides and is also the voltage level on the Mac floppy data lines.

So far, I’ve built a successful blank disk emulator. It identifies itself as an 800K drive to the Mac, initially with no disk. Reading and writing of the interval drive registers works, as well as stepping the drive head. The PWM signal that tells the Mac the current drive motor RPM speed also works. By pushing a button on the board, you can “insert” a disk into the drive, but the disk is blank (it contains all 1’s).

If you push the button while the Mac is waiting for a boot disk, it thinks about it for a minute, then “ejects” the disk and shows the X-disk icon on the screen. If you boot from a regular floppy disk in the internal drive, then push the button to insert an external disk, the Mac asks “This disk is unreadable. Do you want to initialize it?” If you say yes, the Mac responds “Initialization failed! The disk is write-protected!”

It’s not very exciting yet, but it’s a good start. The next step will be to add a microcontroller and an SD card reader. With those pieces in place, I can start passing actual disk data to the CPLD, so it can pass the data on to the Mac.