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From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.sys.next,comp.periphs.scsi,comp.sys.mac.hardware
Subject: Re: Compatibility of Mac SCSI drives with other systems
Message-ID: <37559@cup.portal.com>
Date: 4 Jan 91 05:01:35 GMT
References: <1991Jan3.225103.14407@wpi.WPI.EDU>
Organization: The Portal System (TM)
Lines: 50

Some observations and questions:

	1. If you buy a drive packaged in an external case, rather
than a raw drive, you may want to check the cabling.  Some vendors
cut the reset line.  This can cause problems with some software.

	2. When in doubt, check to make sure termination power
is being supplied by someone.  This is most likely to be a problem
when going from something else to a Mac plus.

	3. Someone said Next requires drives to support the 10
byte read and write commands.  Why is this?  It takes less than
10 lines of code to make your software work correctly on both drives
that don't support 10 byte commands and those that do, with the
only runtime penalty being an AND and a branch.  Is there any
rational reason not to include this?

	4. The special support some vendors provide for the Mac
is usually an option to not do a UNIT ATTENTION after reset.
This is often selectable via MODE SELECT.  I've seen some
drives (although I don't remember which ones they were) where
this was done, although the MODE SELECT page that controlled
this was not documented by the drive manufacturer.  They sold
two models of drive, one for Mac and one not.  The only difference
between them, I believe, was the setting of this undocumented
option.

	5. The Mac does not put many demands on a SCSI drive
as far as software goes.  If the drive has quirks that could
cause problems on other systems, these quirks might not show
up on the Mac, but would show up on other systems, thus preventing
that drive from working on non-Macs.  For example, the 40 meg
Sony that Apple bundles with many of their machines works fine
on the Mac.  It will fail on some other systems because Sony
screwed up their SCSI implementation.  I don't remember the
exact details, because it's been a few months since I found this,
but what happens is something like this:

	Initiator selects Sony with ATN.
	Sony fetches the IDENTIFY message.
	Initiator still has a message to send (negotiation
	for synchronous data transfer), so Sony asks
	for this message.
	After receiveing the first byte, the Sony changes
	to MSG_IN phase and sends a REJECT message.
	Sony then goes into MSG_OUT phase.
	Initiator sends a NOP message.
	Sony drops BSY!

							Tim Smith