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From: mld@netcom.com (Matthew Deter)
Subject: Re: Switching off SCSI external disk and FreeBSD
Message-ID: <mldCrop16.K1M@netcom.com>
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References: <2tkosa$lpo@news-rocq.inria.fr>
Date: Mon, 20 Jun 1994 07:05:29 GMT
Lines: 39
Herve Soulard (soulard@alix.inria.fr) wrote:
: I would like to know if it is possible to switch on and off the external
: disk while the system is running.
Someone correct me if I am wrong, but in my experience, the SCSI bus
will not tolerate devices being added and removed while the bus is
active. (Our Apollo 425t's @ work will crash *hard* if you
plug/unplug the SCSI tape while the machine is powered.)
Of course, there are are exceptions (RAID disk arrays come to mind)
but in general, it is Bad Juju to muck with the busses when power is
applied. [especially since if you yank the ground line before a
signal I/O line, it is quite possible for that signal line to
become the *power* ground. Signal lines are not designed to sink lots
of current (which is what would happen if the real GND line was
disconnected from the socket and the signal line became the least
resistive (lowest potential) path to ground...). The net result is
that you can cook the chips on the signal lines. This happens on the
425t's mentioned above when mucking with the Keyboard. the little
8051 in the keyboard gets cooked...)
Now, I've known people to yank serial I/O cards in and out of the ISA
bus with their machines fully powered, and then proceed to use them.
But I sure as hell wouldn't do that with *my* machine. :-)
: I ask this question because I would like to add an internal hard disk
: within an extractible thing (I don't know how to call that in english) in
: order to replace the external big box I have to carry with me.
I'm still not clear on what your problem is, but I think that changing
the SCSI bus configuration while the machine is up and running is a
Bad Idea.
--
$$ Matthew Deter -- mld@netcom.com
$$$$
$$ "Whatever road I take, the guiding star is within me; the guiding
$$$$ star and the loadstone which point the way. They point in but
$$ one direction. They point to me."
$$$$
$$ -- from the novel _Anthem_ by Ayn Rand