Subject: Re: ncr hangs system
To: None <>
From: Wilko Bulte <>
List: port-alpha
Date: 03/18/1999 23:36:21
As Matthew Jacob scribbled...
> > I kind of agree with you here, but my predicate was not only correct, but
> > the actual statement in the spec is even stronger. Since SCSI-2 only had
> > FAST-10, that's what they had to be referring to.
> > 
> > 	From ANSI X3.131-1994 section 5.1 page 8:
> > 
> > 		NOTE 2 Use of single-ended drivers and receivers with the
> > 		fast synchronous data transfer option is not recommended.
> > 
> > 	and in 6.8 (pp 37) `fast ...' is defined as any negotiated data
> > 	transfer period less than 200 nS. (Actually, `ns', SIC.)
> > 
> > That is, they specifically discourage INTERNAL singled-ended SCSI. Talk
> > about an almost universally ignored recommendation! Think of all the trouble
> *SMACK* Am *I* enlightened now. That note was something that escaped my
> notice... But that's also from the same committee that recommended 100ohm
> or greater cables years before cable manufacturers could make them...

Come on, that was 1994 when this was written. I've had lengthy discussions
with one of the fathers of Ultra SCSI and it seems to make a he* of a
difference how well you can design and produce your silicon. And control
your bus impedance, which gets us back to the cable issue.

See for the bus isolator chips. We can now do
20 meters _single ended ultra scsi_ (though point to point only)
using these things. (we == DEC ^H^H^H Compaq storage division).

> > that would have been avoided if everyone had just listened and gone
> > differential.
> I argued for this at Auspex and was told that a >=200$ per drive
> additional cost made this a 'poor idea'.

Take a look at the Storageworks UltraSCSI shelves. The trick used is
to have a isolator chip between the host-shelve cable and the 
actual shelf backplane that houses the disks. The disk / backplane
part is approx 90 cm's long. But the isolator chip ensures you
keep sound signals, even with the electrically undesirable bunch of
stubs (the disk to backplane cabling) at the end of the bus.

Groeten / Cheers,
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