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From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st,comp.sys.atari.st.tech
Subject: Re: Weird TT hard disk thingy
Message-ID: <2917@atari.UUCP>
Date: 29 Apr 91 17:15:23 GMT
References: <1991Apr16.195300.1906@jato.jpl.nasa.gov> <1991Apr17.154845.29875@newcastle.ac.uk> <2910@atari.UUCP> <1991Apr24.153245.19616@unx2.ucc.okstate.edu>
Organization: Atari Corp., Sunnyvale CA
Lines: 36

cummins@unx2.ucc.okstate.edu (John Cummins) writes:
>On my Mega ST... I have no SCSI 0 drive.  I use scsi 4 and 5 so I can
>daisy chain a friends scsi 0 drive and still access my own disks.
>But I can still boot... and use my hard disks w/o any SCSI 0 device
>attached.

>The fellow you've been chatting with says the TT won't recognize SCSI devices
>1-7 on the ASCI bus, at least not without an SCSI 0 in place.  Device 0 MUST
>be used for the first device.  Why????

[Am I "the fellow [he's] been chatting with"?]

I can't speak for what SCSI unit numbers ICD or Supra adaptors will or
won't recognize.  From the data presented here, it sounds like ICD will
handle any SCSI unit number on the drive, and Supra will only talk to SCSI
unit zero. The latter is not unreasonable -- if you've got only one drive
on a SCSI-to-ACSI converter, why include the logic to talk to an arbitrary
SCSI device number?  It would mean logic in the adaptor to see what SCSI
drives are out there.

I have even heard of some adaptors which look at the SCSI unit number of
the drive that's connected, and configure themselves as that ACSI unit
number, further confusing the issue.

As for the ST/TT side of the bus, you have to start with zero on each bus
and increment by one for each drive you connect to that bus.  (Remember,
that's unit numbers as the computer sees them, not as the adaptor sees the
disk it's actually talking to.)  The reasons for this are kind of involved,
and I don't want to go into them; just take my word for it and carry on.

I hope this clears up the questions about busses and unit numbers, and we
can get on with our lives.

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt