Re: (no subject)

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

In article <4546rb$llt@st-james.comp.vuw.ac.nz>,
Rob Thomson  <Rob.Thomson@vuw.ac.nz> wrote:
>
> If I got a SCSI card for my Apple IIe, could I connect it to my Mac
> Plus with a SCSI hard drive?

Do you mean an _internal_ SCSI hard drive?  Probably not.  It is not
safe to have two computers trying to access the same SCSI drive at the
same time.

The Mac Plus was the first SCSI implementation on the Mac, and it is
rather more limited than later versions.  It might not support "SCSI
arbitration", which is a fundamental requirement for having two
computers on the bus at once.

There is also the problem of getting the computers to avoid stepping
on each other while accessing the drive.  This is not solvable in
general without specially written software on both machines (including
the operating system).  The only safe solution with a shared SCSI bus
is to ensure that only one computer is turned on at a time.

As an internal hard drive is powered from the computer it is contained
in, this is not practical.

On the other hand, an external hard drive might be a different matter.
As long as only one computer is turned on at a time, they should both
be able to access the drive.


Another solution would be if each computer was restricted to accessing
a different partition on the drive (for reading and writing), or if
one partition was accessed in read-only fashion from both computers.

If both computers are switched on at once, the SCSI card in the Apple
II would have to be set to an ID other than 7 (all Macs have a fixed
SCSI ID of 7, so you cannot connect two Macs together in this way,
except in special cases like some PowerBook models, where the
PowerBook itself gets off the bus and lets the desktop Mac access its
internal drive directly).

Note: this can only be done with an Apple SCSI card.  The RamFast SCSI
card (the only one that is commercially available now) does not
support SCSI arbitration, and cannot be used on the bus at the same
time as another computer.


The next problem is that an Apple IIe cannot access a Mac-formatted
(HFS) hard drive directly.  You would have to partition the drive so
that it had a ProDOS partition for the Apple II and an HFS partition
for the Mac.  This will probably solve the simultaneous access
problem, as neither computer will be able to see the partition owned
by the other one.

I see you are in Wellington.  You may like to give me a telephone call
(475-9755) to discuss this further, and possibly to do some
experiments.  I work just down the hill from V.U.W.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand