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.
-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Mark P. Gooderum			   USSnail:  Good Creations
Senior Consultant - Operating Systems Group	     3029 Blackstone Ave. So.
  "Working hard to be hardly working..."	     St. Louis Park, MN 55416
EMail:	     mark@Good.com		   Voice:    (612) 922-3953
Interactive: mark@nirvana.Good.com	   Fax:	     (612) 922-2676
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~