Re: Reading apple disks on PC

[mjmahon@aol.com (Michael J. Mahon)]

Michael Pender wrote:

>Charlie <charlied@NOSPAMbboard.com> wrote in message
>bevseq01g3@enews2.newsguy.com">news:bevseq01g3@enews2.newsguy.com...
>>
>> An Apple 3.5" (800K) drive is capable of reading/writing PC-format disks
>(720K)
>> but the Apple controller is not.  I use an Applied Engineering PC
>Transporter
>> card with an Apple 3.5" drive attached to it.  It works perfectly with
>720K
>> MS-DOS disks as long as they are formatted on a real PC.  When using the
>Apple
>> 3.5" drive for ProDOS the speed is slower than if it was attached to the
>Apple
>> (SmartPort) controller.
>>
>> It is my understanding that Apple 5.25" drives cannot read/write MFM at
>all.
>>
>> Charlie
>
>I don't see why not - the Apple 5.25" drive is controlled entirely by
>software.  With a change to the software it should be able to read MFM disks
>without difficulty.
>
>Of course, saying that it is *possible* is very different than saying its
>worth the time to do.

If the transition frequencies are compatible (360K FM PC disks) then
the drive hardware is able to read the data.  The Disk ][ Controller
is not set up to sync properly, though.

Perhaps if one were willing to recode the 2MHz state machine in
the disk controller ROM (not the slot ROM), then it could be done
reliably.  As it is, it is possible to read raw bit streams off the disk,
but multiple reads are required to figure out what FM nibbles were
actually written, then they can be decoded in software.

FM disks have the same bit rate as Apple disks, but there is a
clock pulse (a "1") written between each pair of data bits, and the
sync sequences are different.

Since 360K PC 5.25" drives are getting hard to find, too, the exercise
may be moot as a way to make data interchange convenient. ;-)

-michael

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