: From: Matthew Jacob <mjacob@feral.com>
: : From: Ross Harvey <ross@ghs.com>
> > 
> > SCSI-2 was never intended to be used in fast synch modes with external
> > devices, and using two external scsi devices is venturing onto somewhat
> > thin ice.  (Sure, people do it all the time, but people walk on thin ice
> > and railroad tracks, too. :-)
>
> Ross- I believe the predicate is wrong, but the conclusion is probably a
> fair statement. Cable lengths and FAST-10 are fine for external devices
> provided really good cabling (high impedance with minimal cable mixing)
> and active termination. It's FAST-20 or higher which is really "no more
> than 4 devices, very short bus length).
>
> That said, the external connector on the Multia's NCR810 is *really* weak,
> but I successfully run several external devices on it. What I have ended
> up doing to minimize signal reflection problems is to remove the (useless
> at 335MB) internal disk from the chain.
>
> -matt

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
that would have been avoided if everyone had just listened and gone
differential.

I also agree: use hi-Z cables of one type, stagger the lengths but keep
them short, and I would add: avoid PVC, use polyolefin or teflon cables.

Of course, if you just go differential then all your SCSI problems will
just vanish, but everyone always wants to use the built-in SCSI and the
cheap drives. This is why I generally buy bare motherboards and add my own
differential controllers and drives. Sometimes you can get great deals on
these things from sellers who don't realize their value.

And I run my Multias diskless...

	Ross.Harvey@Computer.Org