Re: ProDos Disk Images

[Roger Johnstone <rojaws@es.co.nz>]

Eric Smith wrote:

> mjmahon@aol.com (Michael J. Mahon) writes:
>
>> I expect that the Power Mac controller does not provide the
>> variable speed control voltage that the Superdrive floppy needs.
>> When you format a ProDOS (800K) disk, do you hear the drive
>> changing speeds?
>
>
> The Superdrive/FDHD does its own speed control.  The controller in
> recent Macs isn't capable of GCR.

That's not right. All the Macs ever made with floppy drives can do GCR
disks. You may be thinking of the recent Macs which don't have any
floppy controller and rely on external USB drives. All the USB floppy
drives available are only capable of MFM disks i.e. 720KB and 1.4MB.

The issue is that Macs that have manual inject drives can make 800KB
disks which work fine on another Mac, even an old ones with the auto
inject drives, but the disks won't work in an Apple II.

I think Michael is correct and that it is the drive controller Apple
used. The original drives they used were variable speed so that they
could get more data on the longer outside tracks by slowing down the
disk's rotation. From what I understand, the manual inject drives are
fixed speed drives like those used in most other computers. To vary the
data rate they just changed the rate at which it was sent to the drive
instead of changing the disk speed. This would have required a new
controller chip, so the new chip coincided with the arrival of the new
drive.

The following is some more info I posted here a couple of years ago:

--------
There seem to be two problems here, one with Disk Copy and the other
with the drives used in later Macs. Apple has a tech note at
http://til.info.apple.com/techinfo.nsf/artnum/n30959 which mentions
problems with the manual inject drives used in later Macs, when
formatting Apple II disks.

There is also a problem with Disk Copy, at least on my iMac running Mac
OS 8.1. The iMacs don't have a floppy drive, but they do have all the
hardware required to add one. I installed a D19 socket on the side of my
iMac so I can plug in my IIgs 800KB drive, it works perfectly for Apple
II or Mac 800KB disks.

I did some experimenting today and found disks formatted from the Mac
Finder and with the files copied onto it boot on the IIgs, but the same
disk created with Disk Copy failed to boot past the *** UNABLE TO LOAD
PRODOS *** stage, neither could the disk be cataloged (I/O error).
Checking the disk with a verify program on the IIgs, it seems that
every block gets an I/O error _except_ the boot block!

(n.b.: remember that if you get the 'unable to load ProDOS' message it
means that the disk did start to boot, since this message is an error
reported by the program stored in the boot block itself, but the boot
code could not find the ProDOS file on the disk, which will be the case
if the rest of the disk can't be read)

I put the drive back on the iMac and found that the Mac Finder could
read the disk fine. I could even get Bernie to boot from it. The disk
was also checked on a PowerBook 5300 and worked fine. The same disk
created with Disk Copy on the PowerBook booted and ran on the IIgs.

The problem with Disk Copy on my iMac is not the disk drive (real Apple
II drive!) or the OS version (8.1, same as the PowerBook ) or the
version of Disk Copy (latest 6.3.3 or older 4.2 produce same results).
The only other thing I can think of is an incompatibilty between the
iMac hardware and Disk Copy which causes the disks to be written in some
funny format which a Mac can still read but an Apple II can't.

--
Roger Johnstone, Invercargill, New Zealand

Apple II - FutureCop:LAPD - iMac Game Wizard
http://homepages.ihug.co.nz/~rojaws/
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