Path: utzoo!utgpu!watserv1!watmath!att!dptg!mtunf!rgh
From: rgh@mtunf.ATT.COM (Ronald Hiller)
Newsgroups: comp.sys.mac
Subject: Real SCSI versus Macintosh SCSI
Keywords: SCSI, Macintosh
Message-ID: <1095@mtunf.ATT.COM>
Date: 1 Mar 90 22:52:17 GMT
Organization: AT&T BL Middletown/Lincroft NJ USA
Lines: 23

I have several large (>100 MB) SCSI disks that I would like to attach
to a Macintosh.  However, I have heard that the Macintosh SCSI interface
is not *exactly* the same as is found on a vanilla SCSI drive (eg. 
CDC/Imprimus/Seagate Wren VI).  To try and get to the bottom of this
I opened up an Apple 20 MB external drive box and everything seemed
as expected except that it appeared a diode had been soldered from some
mystery place on the disk drive board (I assume 5 V) to pin 26 on the
50 pin SCSI connector ("terminator power").  What does all of this mean?
Can I simply supply +5V to pin 26 of my Wren VI and have everything 
work?  Also, what is really happening here and why is this necessary.

Seeing the diode, I assumed that if I had my new drive on the SCSI
bus with another official Apple drive that it should supply the
termination power and everything should work.  Well, it didn't.
The Mac did not recognize the drive and the Macintosh HD SC Setup program
couldn't find it.

If you know anything about this please clue me in.

Thanks,
Ron
att!mtunf!rgh
rgh@mtunf.att.com