[ILUG] Re: SCSI questions

[Kenn Humborg]

> > > basically: you really don't want to hot-swap normal scsi
> > connections on a
> > > regular basis. If you do, isolate the device from the bus
> > before removing
> > > the power.
> >
> > Surely, you mean the opposite?  Cut power first, then pull out the
> > connector.
> >
>
> no, cut interface first. Because you need to keep the ground
> lines grounded
> during demating, and because then the only disturbance will be due to to
> interface demate.

Is keeping the grounds mated possible with the Centronics-style
connectors?

In my case, I'm not really worried about disturbances on the bus
causing data errors.  I'll be the only person using this gear, so
I can dismount, sync, etc before playing around.

I am more concerned about frying stuff.  I've just looked briefly
at that DTJ article, and they say it's a wired-OR bus.  That's
good.  It means that the line drivers only pull in one direction,
so it's much more difficult to fry them than push-pull drivers.
So hot swap should be electrically safe, if not data safe.

> if you look at SCA drives, the contacts are of different length. The power
> and electrical ground contacts extend closer to the edge than the SCSI bus
> contacts do, to ensure that the electrical ground contacts mate first and
> demate last, and that the bus contacts mate last, and demate first.

Ahh.  SCA.  How modern :-)

Does anyone know if it is normal for external drives (DEC RZ55's
in this case) to _not_ spin up on power-up?  I haven't got all
my cabling together yet, so I don't know what happens when they
are hooked up to a live SCSI bus.

(They might actually be spinning up very quietly, but that would
be _very_ unlikely.  These are full-height 5.25inch drives.)

Later,
Kenn