[SI-LIST] : Single-ended SCSI vs. differential SCSI

joel.kolstad@techne-sys.com
Fri, 19 Sep 1997 14:22:31 -0800

I understand the many advantages of differential signaling over twisted =20
pairs -- no need to worry about ground loops or ground bounce (as much), =20
much less crosstalk, far less suseptibility to induced noise and hence =20
better EMI performance. Where I work, we use LVDS (200mV swing) to ship =20
digital camera data about twelve feet at 30MHz. It works quite well (not =20=
=20
that this implies we really know what we're doing or that we employ any =20
SI gurus :-) ).

Anyway, I'm curious as to how well single-ended SCSI works, particularly =20
in comparison to true differential SCSI and also the "fake" 25 wire =20
version of SCSI used by Apple. In the 25 wire version, there's only one =20
ground and no twisted pairs -- clearly making it difficult to have a =20
large amount of signal integrity as your clock rate goes up. But how =20
about the typical single-ended version found in most PCs? Every other =20
wire is a ground wire, and the ribbon cables are generally not twisted =20
pairs. How much of an advantage is this over the single ground wire =20
approach? This is what I figure:

-- The extra ground wire spaces the conductors twice as far apart, giving=20=
=20
you a crosstalk reduction of about four.
-- There seems to be little chance that the current you send down one =20
wire will come back on its associated ground wire (I'm assuming that at =20
the receiver end all the grounds are connected together). This creates =20
varying impedances that probably aren't very friendly to the data edges. =20
Perhaps the premise about "little chance..." is mitigated by the fact =20
that the lowest inductance return paths usually will be the nearest =20
neighbor ground wires and there _will_ be a large amount of roughly =20
equalized ground currents in each return wire?
-- The lack of twists would make the system less immune to induced noise =20=
=20
and produce more EMI.

It seems to me that this isn't really all that much better than, say, one =20=
=20
ground wire with widely spaced connectors. I suspect I'm missing =20
something here and it's actually somewhat better than I'm thinking. =20
Comments?

Interestingly, "wide" SCSI usually uses .025" spaced ribbon cables -- I =20
wonder how its performance over 34 conductor [one ground] .050" spaced =20
ribbon cable would be? Awful given the very large (and uneven) =20
inductance return currents would face?

Finally, let me say that I'm very impressed with the technical knowledge =20
that the people on this list possess. You guys who can send a gigabit =20
per second down a single twisted pair thousands of feet -- that's damned =20
impressive, but the general public is probably never going to recognize =20
that fact!

---Joel Kolstad
Joel.Kolstad@Techne-Sys.Com