FDHD internal registers — #10

Post #10 by onlyonemac

Source Forum	68kMLA
Category	Development
Post Date	Tue, 18 Aug 2015 - 21:54
Original URL	https://68kmla.org/bb/threads/fdhd-internal-registers.2527/

Post

bigmessowires said:
If you've looked at my CPLD source code, the internal floppy disk registers and their meanings are described in a big comment table, right at the top of the file: no reverse-engineering necessary. It's basically the same set of registers, whether it's the Sony 400K, 800K, or 1440K mechanism. The registers are used to do things like turn the drive motor on, step one track in or out, measure the drive's rotation speed, and read data from a track. And yes, these are not the same thing as the IWM or SWIM's registers.

I wasn't sure which was "the source code" file for the CPLD, otherwise I would have probably found what you are referring to. But I did find the information that I needed in the documents provided by one of the previous posters.

bigmessowires said:
Yup, for sure you'd have fundamental problems if you wanted to use 400K or 800K disks, due to the variable rotation speed used with those disks. For 1440K disks I think it should be possible to interface a PC floppy drive, at least in theory, but it would be pretty complicated. Like you said, it's a radically different interface. You'd need some kind of microcontroller/CPLD/FPGA/something that handled all the communication with the Mac, and simulated those internal floppy disk registers. Then it would need to translate to equivalent but completely different communication with the PC floppy drive. For some stuff like sensing track 0, this would be relatively straightforward. Other stuff would be harder, because there's no PC floppy equivalent. For example, what should you do when the Mac tries to eject the floppy? Or when the Mac tries to check the track stepping progress state, which has no equivalent on a PC floppy drive? But if you're determined enough, I think those issues could be overcome. It would be an interesting project. Good luck.

I was using a microcontroller to implement the floppy drive functionality and I was going to use a system involving a way to signal to the user to eject the disk and then have the microcontroller wait for the disk to be removed before it will raise the "disk inserted" signal again on the Mac side. For the head stepping thing I was going to implement a "safe" pre-programmed delay. But I've shelved the project for now, as my microcontroller was not fast enough for the project. I might some time either get a faster microcontroller or re-implement it using discrete logic devices (although that will be difficult for some of the more complex issues such as the eject signal and the head stepping).