but the HD20 emu… pop on your floppy emu, and BOOM access to 2gb plus of data on sd card… no need to reboot or anything.

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Unfortunately I don't think the HD20 can be hot swapped or plugged in after booting, but you've got one so you can try it. I thought it acted pretty much like a SCSI hard drive, and needs to be present when the Mac first boots, or else it doesn't work? So really "HD20 Emu" would provide the exact same functionality as SCSI2SD, no more and no less, except it could also work on the SCSI-less fat Macs, but couldn't work on newer Macs that lack the HD20 Init.

My best interpretation off-the-cuff is that perhaps /HSHK signals are differentiated from "normal" data signals by the line being pulled either up or down for a duration longer than a single byte window? (IE, the IWM raises an error condition if it doesn't get at least X many 0-1 transitions within a certain window, the /HSHK transmission is accomplished by leveraging that? IE, if what the Mac is getting from the HD-20 looks like "data" it is "data", but if the line's held up or down solid for more than x many microseconds, thereby triggering an "error", it's a handshake?)

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Yeah, after mulling it over, I reached the same conclusion. Seems kind of strange, but I guess it could work.

I thought of another issue last night. The main advantage of a HD20 Emu over SCSI2SD is that it would work on the Mac 512k and 512ke, right? But the Mac 512k lacks the HD20 Init in ROM, and the only way to load it is from a floppy, and that's probably going to require a real floppy in the internal drive, or two Floppy Emus. I don't think the hardware can act as an HD20 and a floppy at the same time, and if it can, it could only act as a chain with the emulated floppy at the end, and the ROM routines in the Mac 512k don't know how to talk to drives other than the first one in the chain.

All this talk about why it's not going to work has got me thinking more about it, so when I've got some free time maybe I'll try putting together a simple test using my best guess about how this stuff is supposed to function. If I run into major problems I probably won't pursue it further, but maybe I'll get lucky.