For those interested my notes so far. I think I am going to go buy a printer, because I hate reading on the computer. Can't scribble notes on the pages that way. :-D
I don't know if you saw/skipped over the link I threw in about
The Semi-Virtual-Diskette device, but if you missed it, well, just thought I'd point it out again. The page has complete construction details for a device that's capable of emulating the Apple Disk ][, including the source code for the PIC microcontroller. Unless you're intent on starting completely from scratch this device is already halfway there.
The thing that's "unique" about the Mac drive relative to the Disk ][ is the Macintosh 400/800k drives have a variable speed motor that turns at a different speed. (That's mentioned in the Apple knowledge base article you found.) This allows the drive to cram more sectors into the outer tracks of the disk than the inner ones without having to vary the data rate. What follows is a couple of completely pointless paragraphs I dreamed up which might help you visualize the situation in your head, assuming you're having any trouble. (Which you probably aren't.)
Floppy disks are like record players, in the sense that they read "from the outside in", unlike CDs and DVDs which read from the "inside out", in terms of what's considered "track 0". Imagine, say, a disk spinning slowly on a record player at a constant speed, say, one revolution every ten seconds, or 6 RPM. Pick up a piece of chalk and hold it against the record for one second, near the edge. (the edge is "track 0") You'll get an arc-shaped mark 1/10th of the diameter of the record. That's a "sector". Now take the chalk and hold it on the record for one second near to the hub. (In Mac terms this is "track 79") You'll also get a mark 1/10th of the diameter of the record *at that distance from the hub*, which means this sector is physically a lot shorter than the sector near the edge, as measured with a ruler. With "normal" disk formatting that's how sectors are written to a disk, same number of them per track from outside to inside. Since each sector contains the same amount of information in a sense you're "wasting" space on the outside of the disk, since in theory the magnetic medium should be able to retain data at the same density everywhere on its surface.
Now, imagine the same experiment with the record player, except instead of your hand with the chalk floating free you have a lever attached to the chalk which is connected to a potentiometer which controls the rotational speed of the record player. With this arrangement, the record player runs slower the farther you are from the hub. The record is still turning at 6 RPM when you're making chalk marks near the hub, but when you make a mark near the edge the record player is turning at only 4 RPM. Holding the chalk there for one second will make a mark only 1/15th the diameter of the record, meaning that near the edge you can fit 15 sectors instead of the 10 sectors you got with the constant speed record. The number of sectors per track diminishes as you approach the center, but the overall the sectors are closer to a uniform size then they are on the constant speed disk.)
Anyway. this is why, according to the Apple KB article, a 400/800K floppy disk has 12 512 byte sectors in tracks 0-15, while it only fits 8 in tracks 64-79. And thus the big difference between an Apple ][ disk, which has a constant 13 or 16 256 byte sectors on all 35 tracks, and a Mac disk. Looking at the Mac drive pinouts I just Googled there's a pin labeled "PWM" which says "Motor speed control". If you can find documentation as exactly what sort of signal is present on that port that's going to be the key to adapting a "constant speed" disk emulator design to working on a Mac. If the Mac directly drives the motor via
Pulse-width modulation then I imagine you'll have to monitor that line and deduce how many pulses per second the Mac outputs at different disk locations. (And of course incorporate pulse counting into the finished unit so the microcontroller driver software behaves appropriately and provides the correct number of sectors to the Mac at at any given time.)
Blawblawblawyackityshmackity.
