Xref: utzoo comp.sys.mac.hardware:3817 comp.periphs.scsi:609
Path: utzoo!attcan!uunet!cs.utexas.edu!mailrus!ames!eos!shelby!neon!kaufman
From: kaufman@Neon.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.sys.mac.hardware,comp.periphs.scsi
Subject: Re: Why can't I turn off my tape drive?
Message-ID: <1990Jun8.150226.22946@Neon.Stanford.EDU>
Date: 8 Jun 90 15:02:26 GMT
References: <10567@spool.cs.wisc.edu>
Organization: Computer Science Department, Stanford University
Lines: 22

In article <10567@spool.cs.wisc.edu> tonyrich@titanic.cs.wisc.edu (Anthony Rich) writes:

>1.  Is this normal behavior for a Teac 150MB tape backup unit connected to
>	a Mac II?  Shouldn't I be able to turn the tape drive off while the
>	Mac is on without crashing the Mac?

>I don't think it's a SCSI termination problem, but correct me if I'm wrong.
>It's my understanding that the Teac N50 has its own internal termination,
>and presumably so does the (internal) Wren IV hard drive, so I haven't added
>any SCSI terminators, and the tape drive itself functions perfectly.

Au contraire.  It is definitely a SCSI termination problem.  SCSI terminators
require power.  Generally, with internal termination, that power is provided
by the device (and only a very few devices will also allow the ternimators
to be powered from the bus).  When you turn off the device, you turn off
the terminator power, and effectively short all of your SCSI bus lines to
ground (through 135 ohms).

The answer is to remove the internal termination from the Teac, and add an
external terminator (which will take power from the Mac II).

Marc Kaufman (kaufman@Neon.stanford.edu)