Re: Apple SCSI

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

william strutts <wrstrutts1@home.com> wrote:

> Does this have anything to do with SCSI 1 and SCSI 2?  I
> had the same problem with getting the Mac Plus in seeing
> a SCSI 2 drive.  It would format it and I could load the
> system onto it but when I rebooted, it didn't find the drive
> anymore.  I took the drive back and got a SCSI 1 drive
> and it worked fine.

The Mac Plus has a rather clunky SCSI implementation which results in
poor performance, and it has some compatibility problems.

The terms "SCSI-1" and "SCSI-2" are somewhat misleading, as they can
refer to several different things.

1. The type of connector used.  Some external "SCSI-2" devices use the
miniature high density connector.  The Amphenol-50 connector is
generally called the "SCSI-1" connector.

2. Bus speed.  Sometimes "SCSI-2" is used to imply support for "Fast
SCSI", which is a maximum of 10 MB/second instead of the 5 MB/second
limit of the original SCSI standard.

3. Command set, and in particular standardisation.  The original SCSI
standard defined the command set for hard drives and specified a lot of
other device types but without full information on how they should
operate.  Revision 2 of the SCSI standard added features for hard drives
and also standardised a lot of the details for other devices (notably
CD-ROM drives).

In general, "SCSI-2" devices are supposed to be fully compatible with
older computers which only support "SCSI-1".  In many cases, newer
software may be needed to support all features of the new device.  Some
device types might not work at all.

The original Apple SCSI card dates back to around the time of the Mac
Plus, so it might have a similarly poor SCSI implementation.  I'm not
aware of any compatibility problem with "SCSI-2" hard drives.  I've used
two of them (Quantum LPS and Quantum Trailblazer) with my Apple
High-Speed SCSI card without problems.

I originally had a Rodime 60 MB hard drive (NZ$3200; ouch!) with an
original Apple SCSI card.  I upgraded to the high-speed card and to a
240 MB hard drive, but don't recall whether I ever used the 240 MB drive
on my original Apple card.  Probably.