Subject: Re: Can IIGS share drive with Mac?
From: [email-masked] (David Empson)
Date: Fri, 7 Nov 1997 21:01:05 +1300
Newsgroups: comp.sys.apple2

Bill Robbins <[email-masked]> wrote:

> My question may have been answered in this thread already, but if so I'd
> like confirmation.  I think it IS just a bit different.
> 
> I have been planning on connecting an Iomega Zip drive between my IIgs
> (System 6.01) and my PowerBook 1400c (System OS 8) on a single SCSI bus.

Bad idea.

> I have a RAMFast SCSI in the IIgs with a hard disk connected to it.

Even worse idea, now that you've mentioned a RAMFast.

> The idea would be primarily to use the Zip as a backup for both machines
> and secondarily POSSIBLY to share data between the two.  What I would like
> to do is to set it up so that I could leave the IIgs connected ALL the
> time. The PowerBook gets plugged and unplugged all the time anyway so this
> would not be an issue, although I understand that the Zip would have to be
> terminated when the PowerBook is unplugged.

Yes.  It would also have to be unterminated while both machines were
connected.

> I assume the RAMFast is internally terminated.

Correct.
 
> - Is this settup possible?

Maybe, but it would be dangerous.  It may be possible to damage data on
the PowerBook's hard drive, and it would certainly cause problems if you
ever tried to have both computers accessing the bus at the same time.

I would only recommend doing this while one computer or the other was
powered off.  It is also important to note that you must power down
everything while making changes to the SCSI connections.

> - Can I have both computers powered up at the same time?

Not safely.  The RamFast doesn't support the SCSI Arbitration phase,
which means that it thinks it can transmit whenever it feels like it.
It might decide to do this while the Mac is in the middle of
transferring data to or from its internal hard drive (or the ZIP).

The Mac might be able to clobber communications with the RamFast,
depending on what it sees when it tries to use the arbitration phase.

Note, however that it WOULD be safe to set this up if the PowerBook is
currently configured to provide access to its hard drive from another
machine (as long as the RamFast and GS/OS can do this without corrupting
the HFS file system, which I wouldn't recommend relying on).

> - Is there any chance of both computers accessing the Zip drive, not
> necessarily at the same time?

No.  The problem is at a lower level than just worrying about
simultaneous access to a device.

Even if the arbitration problem wasn't there (e.g. if you used an Apple
SCSI card instead of the RamFast), each computer thinks that it has
exclusive ownership of the drive, and may do things like leave the file
system in an incomplete state until files are closed and the disk is
ejected.  Any access by the other computer in the meantime would see an
inconsistent state.  It is very easy to corrupt the file system in cases
like this.  (The Mac may modify the disk contents even if you aren't
writing to it - creating or updating Desktop files, etc.)

The most amusing case I've seen of this involved a SCSI "network"
consisting of three IIgses using CMS SCSI cards, and one CMS SCSI hard
drive.  The person using the system had set it up so that more than one
computer was able to write to the same partition.  Some time after he
set this up, he starting complaining that he could see filenames
appearing in his AppleWorks spreadsheets!  (This means that a directory
and file intersected each other, which would rapidly result in major
file system corruption and loss of data.)

The CMS card does support arbitration, and setting up a network like
this is explicitly supported, provided the rules are obeyed.  Each
computer is supposed to be configured to have read only access to
partitions that are written to by any other computer.  (This is all
configured using jumpers on each card.)  Thus there may be a private
read/write partition for each computer, plus one shared read-only
"server" partition that is only written to from one machine.

> - Will I fry everything to cinders if I try?

You probably won't damage anything physically (I wouldn't count on it),
but you are running a risk of data loss.

-- 
David Empson
[email-masked]
Snail mail: P.O. Box 27-103, Wellington, New Zealand