Subject: SCSI info with i386 port

From: Brian Moore <[email-masked]>

I have a question that I was wondering if anybody could give me some info
about.  Does NetBSD-1.0 Beta support Wide SCSI-2, or SCSI-3?  I have seen a few
machines come out recently that say they support Wide SCSI-2, and one that 
claimed support for SCSI-3.  While I doubt the claim of SCSI-3 compliance (
Last I heard, the SCSI-3 committee was still a long way away from agreeing on
the standard, but I could be totally wrong about that ), I would be interested
in getting a Wide SCSI-2 board if they were supported.  Does anybody know of
any SCSI board, of course with the exception of the 'evil, wicked and nasty'
Adaptec boards, that support Wide SCSI-2?

Thanks for any info,
Brian Moore
[email-masked]

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From: "Mark P. Gooderum" <[email-masked]>

Wide SCSI and DIfferential SCSI are idependant options and both are
incompatibile (bus wise) with the absence.

So you have four kinds of SCSI busses,

	"Narrow" Single Ended (Non-differential)
	"Wide" Single Ended 
	"Narrow" Differential
	"Wide" Differential

The single and differential busses use the same connector (for instance
in a 50 pin SCSI connector, all the data, and several other lines each
have a seperate ground, in differential SCSI those lines become +/-).

So a given controller (wide or narrow) may be able to switch between
single and differential SCSI, but a given SCSI bus has to be all one or
the other.  Drives are almost exclusively sold as one or the other
however.  Fror instance, flip through a Seagate drive reference, each
drive has four models, at least the big ones like the Barracudas, with
a model suffix of N, D, WN, or WD (N is single, D is differential, W
means wide).

BTW...many systems don't have a controller <-> host transfer speed to
make full use of the best speed of a controller.

Also, differential doesn't give a huge speed win, although it does 
effect the maximum rate a sync transfer can go, it's big win is giving 
fast and reliable transfers, even with long cables and many drives on
the bus (each drive on the bus add's a bit more load, capacitance, etc).

Wide SCSI gives a much more significant speed win, but is really, really
incompatibile since you're talking more pins.

Finally, beware that not all 50 pin SCSI cables are safe for differential
SCSI.  Many tie the grounds togather, which is a win for normal SCSI but
don't work too well with differential SCSI.
-- 
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---

From: [email-masked] (Donal Tobin)

>the other.
        NCR have a card which converts differential to single, thus using
this you can effectively mix types. They are however still all single on
the controllers cable. Presumeably the NCR card can go the other way aswell
but I am not positive.
        Donal.

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