Re: PCT, low-density disks, and Apple 3.5 drives

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

In article <3324bd88.207353843@netnews.worldnet.att.net>
themaples@worldnet.att.net (Bruce Maples) writes:
>If PC disks are MFM, and Apple disks are GCR, then how does the PC
>Transporter use an Apple 3.5 drive to read and write low-density
>(720k) disks?

It works because the drive itself doesn't really care what the disk format
is--the job of creating and interpreting the MFM or GCR data stream
belongs to the disk controller.  The PC Transporter contains a
PC-compatible (MFM) disk controller chip (along with special support
circuitry that adjusts the data rate to compensate for the Apple drive's
variable rotation speed), so as long as the drive is accessed though the
PC Transporter's chip, it can read PC disks just fine.

>And, if the Apple 3.5 can be used to read PC 720k disks, would it be
>possible to write a utility on the PC to write 800k ProDOS disks?  Why
>or why not?

There are three reasons:

1.  The standard PC disk controller chip has the MFM algorithm hard-wired
into its circuitry.  There's no way (short of adding special hardware) to
override it and use a GCR algorithm instead.

2.  The standard PC disk controller also has the low-level sector layout
(i.e. address and data field headers, chesksums, etc.) hard-wired into its
circuitry.  There is some flexibility to it, but not enough to recognize
the Apple II sector layout, even if reason 1 above could be overcome.

3.  Apple 3.5-inch drives have a variable spin rate--they spin more slowly
when the head is near the out edge of the disk, so that more data can be
fit onto the outer tracks.  PC 3.5-inch disk drives spin at the same rate
no matter what track the head is on.  The standard PC controller is totally
unprepared for the variable bit rate that would come in from an Apple
disk.

All of these reasons can be overcome, but not with software alone.  Special
hardware (such as that found on the PC Transporter) is necessary.

               - Neil Parker
-- 
Neil Parker                       | Unsolicited commercial e-mail to my
nparker@cie-2.uoregon.edu         | address is not welcome, and will be
nparker@cie.uoregon.edu           | discarded unread.
http://cie-2.uoregon.edu/~nparker |