One reply to myself, above... ^
Thinking about it, an IDE port decoded as simply as the John Bass ROM-SCSI controller would probably conflict with the Mac Plus' SCSI controller. Most of the I/O in an "original" Mac is located above 0x800000, but for some crackerjack reason Apple stuck the Plus' SCSI into the ROM area as well. (Assuming the info in this regarding its memory mapping is accurate, that is. I only have "Inside Macintosh 1-3 handy, not the supplement that covers the Plus additions.) It's not a deal breaker per-se... one workaround would be to also use A14 or A15 in addition to the others for the IDE select. That would put its I/O well clear of the addresses documented for SCSI on the Plus... although whether that's actually true depends on how fully decoded the Plus' hardware is.
Thinking about it, an IDE port decoded as simply as the John Bass ROM-SCSI controller would probably conflict with the Mac Plus' SCSI controller. Most of the I/O in an "original" Mac is located above 0x800000, but for some crackerjack reason Apple stuck the Plus' SCSI into the ROM area as well. (Assuming the info in this regarding its memory mapping is accurate, that is. I only have "Inside Macintosh 1-3 handy, not the supplement that covers the Plus additions.) It's not a deal breaker per-se... one workaround would be to also use A14 or A15 in addition to the others for the IDE select. That would put its I/O well clear of the addresses documented for SCSI on the Plus... although whether that's actually true depends on how fully decoded the Plus' hardware is.