Subject: Re: scsi tapes
To: Herb Peyerl <>
From: Dave Burgess <>
List: current-users
Date: 02/22/1995 08:55:01
> Chris G Demetriou <Chris_G_Demetriou@LAGAVULIN.PDL.CS.CMU.EDU>  wrote:
>  > > I thought for most devices like this, including scsi, you were supposed to
>  > > power down the whole machine before plugging/unplugging things. Certainly,
>  > > that's the advice we got regarding our suns in case of you blowing the
>  > > fuse on the scsi bus (not that I've ever seen this happen - I think I've
>  > > heard of it, though).  (Hot-swap is obviously built for that.)
>  > 
>  > Just a bit on the odds: i've blown exactly one SCSI fuse in my life
>  > (*smirk* and it was on a sun) and i plug in/unplug SCSI devices from
>  > live hardware not meant for "hot swapping" about 2x a day, on average.
> I think the key to minimizing this sort of thing might be in ensuring 
> that all scsi devices are plugged into the same electrical circuit
> lest you find that different devices have different ground potentials.
> Just something that seems intuitive to me though I can't explain why...

When I was stationed in Germany, it became REAL intuitive to me.  We
were running three phase 440 into the building and then running each of
the circuits in  the building from different phases.  I had a terminal
cable in one hand and reached into a powered down Cromemco to pull a
circuit board and got a 440v kick across the room.  From then on, the
concept of different ground potentials has  seemed very intuitive.

It also explains the brain-damage that would allow someone to
deliberately become a FAQ maintainer.