Re: Disk Drive Tech Info GS<->Mac

[dempson@actrix.gen.nz (David Empson)]

Dennis <djc@altair.csustan.edu> via Paulo Figueiredo
<figueirp@ecs.ecs.csus.edu> wrote:

> Hello.  I need the help of the technically gifted:
> 
> Why won't a macintosh 800K External drive (beige) work on an Apple IIgs?

At least one reason: it doesn't implement the signal which tells the
IIgs that a 3.5" drive is connected.

> I have looked at the diagrams for the ports and MOST of the pins
> are the same on both machines.  The main catch (I think) is the PWM
> output on the mac, which I don't see on the GS.

I'm not sure if the Macintosh 800K drive uses the PWM signal.  The 400K
drive definitely does.  As you noted, the IIgs doesn't provide this
signal, so the 400K drive will not operate correctly.  The PWM signal
controls the speed at which the disk rotates, and the Mac varies the
frequency according to the track band.  The same would apply if the 800K
drive uses it.

This is done automatically inside the Apple 3.5 Drive, which is the
successor to the Macintosh 800K drive, and is the standard drive used on
the IIgs.

> More importantly, I have an Apple HD 20 (connects though the Mac's drive
> port) that I'd like to use on the IIgs.  Would a HD need the PWM signal?

Unlikely.  The question is exactly how the HD20 talks to the computer,
and how its presence is detected.

It probably implements a simple packet handshaking protocol (similar to
the SmartPort protocol used with intelligent drives on the Apple II,
such as the UniDisk 3.5).  It should be possible to use it on the IIgs
given enough information about the protocol.  The driver would have to
access the disk hardware directly, and this might not fit in well with
GS/OS (and it wouldn't work at all in ProDOS-8).

> Finally, if it is just a matter of driver software, what's required?
> Does a driver already exist?  How difficult is it to write one?

I don't think it would be useful to get this drive working on the IIgs,
considering how slow it is (compared to SCSI drives).  Of course, if
someone wanted to do the necessary research and write the driver as an
academic exercise, it may be possible.

The complications I can see are:

- No documentation on the interface or protocol used by the HD20.

- Limited documentation on the IWM (though probably sufficient).

- Very limited documentation on writing GS/OS drivers which make use
  of the IWM.  All the existing ones are written by Apple (AppleDisk3.5,
  UniDisk3.5, AppleDisk5.25).  There may be special problems that need
  to be dealt with, since the IWM is shared between at least three
  different mechanisms for accessing drives (as implemented by each of
  the aforementioned drivers).

- The task of writing a GS/OS driver, which is well documented, but not
  trivial.

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand