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From: kadickey@phoenix.Princeton.EDU (Kent Andrew Dickey)
Newsgroups: comp.sys.apple2
Subject: Re: PCT 3.5" drives
Message-ID: <5844@idunno.Princeton.EDU>
Date: 1 Feb 91 00:44:04 GMT
References: <9101290630.AA10515@apple.com>
Sender: news@idunno.Princeton.EDU
Organization: Princeton University, Princeton, New Jersey
Lines: 46

In article <9101290630.AA10515@apple.com> THROOP@GRIN1.BITNET ("Throop,Henry B") writes:
>[People discussing why Apple 3.5" doesn't format MS-DOS readable floppies]
>
>I think I remember hearing that this stems from how the start of the track is
>found.  IBM drives have some physical way to detect the angular location of
>the disk itself, and maybe start the track at 0 degrees from this angle.
>Apple drives don't have this capability built in, but the PCT can read the
>disks anyways, as if they were softsector.  However, when you format a disk
>on the PCT, it just puts the start of track wherever is most convenient,
>which may not be the right place for a real IBM drive.

No, this is not the problem.

The main problem is what we can control in software about the 3.5"
drive.  The Apple 3.5" disk changes its drive speed depending on how far
out from the center the drive arm is.  This is done completely in
HARDWARE by the IWM chip.  Programmers have no way of changing this.
IBM disks do not change drive speed.  This is insurmountable hurdle #1.

Problem #2: The way the actualy bits are written are different.  Again,
the IWM chip does the conversion from what is senses from the disk into
a stream of 0's and 1's to send to the software.  The IWM's algorithm
doesn't work with anything but GCR coding.  The IBM disks are MFM.

The PCTransporter fixes these problems by replacing the IWM chip with
some hardware of its own which knows about IBM formats too.  This drive
can then read and write IBM 720K disks.  Installing a SWIM (Apple's
version of the same idea, which is used in all the new Macs) in a //gs
would also solve the problem.  However, a SWIM and IWM are probably not
pin compatible, so either a new board for slot 5 would be needed, or a
special picky-back board for the actual IWM chip socket (which is not
really feasible due to its placement on the motherboard).

The lack of an SWIM option for the //gs shows Apple's lack of commitment
to the // family.

I can guarantee you--you CANNOT read/write MS-DOS 3.5" disks on an
unmodified //gs.  Period.  The IWM chip prevents you.  Adding the PCT
gives you the capability since extra hardware on the PCT board allows
it.

The PCT board actually gives you quite a lot of power for you money.
It really is a good deal if you need it.

Kent Dickey
kadickey@phoenix.Princeton.EDU