Re: Mac Plus floppy interface

[dempson@actrix.gen.nz (David Empson)]

Simon Biber <simon@basilisk2.cjb.net> wrote:

> I tried plugging my Apple 5.25" drive which works fine on a platinum IIe
> into the drive expansion connector on the Mac Plus.
> 
> With it connected, the Mac sits at the flashing question mark while checking
> for a disk in both the internal 3.5" and external 5.25" drives. Yes, the
> light on the 5.25" drive flashes on and off, and it is looking for a disk to
> start up off.

This may have nothing to do with the 5.25" drive.  You can see a brief
flash on the 5.25" drive light whenever an Apple IIgs accesses an Apple
3.5 Drive on the same bus.

It is a side effect of the way the 3.5" drive handles the bus signals.
Both the 3.5" drive and 5.25" drive use the same drive select signal.
The 3.5" drive passes it through, but recognises that it has been
selected and quickly disables the passed-through copy.  This results in
the 5.25" drive getting a brief pulse on its select line, which causes
it to blink its LED.

On the other hand, it might be trying to select the drive, but not
recognising it.  For example, the Mac might be detecting that there is
another drive connected, but expects it to be an HD20 and the drive
isn't reacting correctly.

> It's interesting that it doesn't try to boot off the hard disk when I have
> the 5.25" drive connected, but just sits there checking each drive.

This might be explained by the Mac thinking the 5.25" drive is an HD20,
which it may use in preference to a SCSI hard drive.

> Then I put the drive back in and started off a 3.5" floppy since it wouldn't
> start from the hard disk with it in. The Mac desktop came up, then it tried
> to read off the 5.25" drive. I get a "This disk is unreadable: Do you want
> to initialize it? (Eject) (Initialize)" box. I attempted to initialise a
> disk, it tried to do something on the drive, then said Initialise failed. I
> say Eject, but the drive can't eject! So it just finds the disk again and
> repeats the "This disk is unreadable" message, in an infinite loop.
> 
> It seems quite promising; is writing a driver to support this drive
> feasible?

I doubt it.  As I mentioned in an earlier post, the 5.25" drive requires
precise cycle-level timing between the host CPU and the IWM.  I suspect
that the 68000 can't handle this level of precision (unlike the 6502).

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand