Re: Why can't Gs 800k drives read 720k IBM disks?

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <LwZ32c2w165w@railnet.nshore.org> barr@railnet.nshore.org (Adam
Barr) writes:
>DOes anyone know why this can't be done? Whats the differance? It is a 
>disk drive right?

Yes, it IS a disk drive, but not all disk drives are created equal, nor are
all disk drive controllers created equal.

The first problem is that Apple 3.5-inch drives (prior to the Superdrive)
spin at variable rates--slower on the outer tracks, and faster on the inner
tracks.  This allows more data to be squeezed onto the longer outer tracks.
IBM drives, on the other hand, spin at a constant rate.  To compensate for
the IBM drive's constant spin, an Apple drive would have to vary its data
clock speed.  The Apple disk drive controller isn't capable of this.  (The
PC Transporter card includes a disk drive controller that can vary its data
clock rate to access IBM disks in an Apple drive, but apparently it's not
totally reliable--IMB disks formatted by a PC Transporter may not be
readable in a real IBM drive.)

The second problem is the data recording format.  Writing data onto a disk
isn't as easy as just collecting the data into a 512-byte block and
transmitting the block to the disk.  Disk drives generally have rather
severe restrictions on what kind of data are physically recordable on the
disk surface, so the data has to be encoded into some form that meets the
restrictions.  Also, each block has to be wrapped up with a header and a
checksum.  There are several different, mutually incompatible ways of
accomplishing these tasks, and the method used by IBM drives makes no sense
to the Apple disk drive controller.  (In particular, the MFM recording
technique used by IBM drives requires that the controller be capable of
distinguishing "clock bits" a half-cycle out of phase with the data bits.
The Apple controller can't do this.)

               - Neil Parker
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