As it happens, I've put a fair amount of work into exactly this problem, albeit in the context of a hard disk AND floppy emulator, as well as one capable of intefacing to a physical drive.

If you're both controlling and reading/writing the the floppy disk, as directed by a standard controller, then I'd recommend approaching this from the viewpoint of a standard floppy disk controller.

First of all, however, it would be interesting to know whether you'll be dealing with hard-sectored or soft-sectored format on the diskette, and whether you'll be working in NRZI, FM, M^2FM, MFM, or what other modulation.

If you're dealing with just the "standard" sort of floppy, e.g. the PC_compatible variety, then you'll probably have no trouble at all emulating an FDC chip, i.e. modulating/demodulating the data in firmware, as well as synchronizing your demodulation loop with the data if you use an 805x variant that operates at 8 MIPS or faster, i.e. 16x the bit rate. If you're running at 16x the data rate, you have some chance of executing a loop containing as many as 10 instructions for each bit. It's probably not necessary, but simple external synchronization hardware might be helpful in "getting in sync" with the data, though that's not necessary if you are dealing ONLY with a floppy disk with data rate of 500k bps or less.

I've presently got a similar project, i.e. a hard disk/floppy disk emulator, sitting on the bench. What you have to do when you emulate a disk drive is to extract data not present on the cable from the modulated bit stream on the write-data line, and extract the sector information from it, when the controller is writing, and when the controller is "reading," you must acquire the cylinder/head information from the cable, fetch or generate the raw data for a track from that cylinder/head, modulate it appropriately, and send it to the cable. When the controller wants to write, he'll "see" the sector ID field of the sector he wants to write and assert write-gate when the write-splice is under the heads.

If this appears to be the sort of thing you're wanting to do, then let me know, and I'll share what I can with you.

RE