SCSI chit-chat

Edward Welbon (
Sun, 9 Mar 1997 05:02:36 -0600 (CST)

On Sat, 8 Mar 1997, Richard B. Johnson wrote:

> Date: Sat, 8 Mar 1997 09:37:53 -0500 (EST)
> From: "Richard B. Johnson" <>
> To: Edward Welbon <>
> Cc: Linux kernel <>
> Subject: Re: your mail
> On Fri, 7 Mar 1997, Edward Welbon wrote:
> > On Mon, 3 Mar 1997, Richard B. Johnson wrote:
> >
> > > This is the SCSI driver. No external drives are attached, and the
> > > internal drives are connected with a SHORT cable properly terminated
> > > on the far end.
> >
> > A too short SCSI cable is a no-no. The problem is that a scsi cable is
> > actually a transmission line and the points where devices connects are
> > essentially points of discontinuity which alter the characteristic
> > impedance of the cable. Depending on the impedance of the the drives and
> > the impedance of the cable, you can have *too* *short* a cable.

> Transmission line mismatches can cause problems when reflections from
> impedance discontinuities interfere with the SCSI bus signals. This
> interference can occur only with LONG cables. These problems can occur
> at odd multiples of 1/4 wavelength at the highest frequency of interest.
> This is the reason for the maximum allowed cable length of 25 meters.

This misses the point entirely. The issue is not whether one can or must
have exceedingly long cables, it is whether cables can be too short. The
truth is that depending on the particular system configuration, scsi
cables can be too short. You have apparently not examined the content of
Annex C from ANSI X3T10 which gives a simple and lucid analysis of the
case against cables that are too short (by way of establishing the minimum
spacing of connectors). In particular, the final paragraph of Annex C

"We can now say that capacitance should not be added at more than twice
the bus distributed capacitance for incident wave switching. For
example, a cabled bus with L=295nH/m (90nH/ft) and C=41pF/M (12,5pF.ft)
and Z=85 ohms, the guideline becomes to add no more than 85pF/m
(26pF/ft) anywhere along the bus. This guideline can be met by 25pF
loads spaced 0,3 m (1 ft) from each other, 50 pF spaced 0,6 m (2 ft)
apart, or 12,5 pF spaced 0,15 m (0,5 ft) apart."

So if you have say 6 drives and these drives present a net capacitance
(including the required connectors) of 12.5 pF, the minimum length of the
cable will be 3.0 ft (with the connectors spaced evenly along the cable).
If you have any less cable length, the net added capacitance per length
will cause the net characteristic impedance of the cable to shift such
that the minimum required assertion level will not be achieved by the
incident wave.

> It is absolutely impossible for interconnections to be too short. In
> Sun Boxes, the SCSI connections are made via very short PWB traces.

This is not a valid counter example. The traces on a PC board can be
easily tuned to have the appropriate characteristic impedance, it is
standard art. And in cases where one wishes to push the limits, it is
prudent to put a scope on at least req, ack and busy to see what they look
like at all of the connectors.

X3T10 Annex C is a guideline that attempts argue reasonably about the
system and expects that no other guidelines are being violated (it
assumes, for example, that the routing of a cable does not result in
substantial contact with a conductive surface (such as the chassis) which
can effect the cable characteristic impedance and thereby signal
propagation). If you follow the Annex C (and the other scsi)
recommendations, you are almost certain to have a functional scsi bus.

> The Adaptec AHA-2940 is a very troublesome card.

> We replaced many of these boards with Buslogic BT-958 boards when we had
> to drive many SCSI devices.

The various members of the 2940 family have all worked fine for me. I have
used at least 5 different instances of these cards and have three
operating flawlessly right now as I type. I don't claim that they are the
fastest but I get very decent performance from my md raid. I don't claim
that they are the most robust but I have operated a 6 drive md raid with a
single 2940 using a 1.5 meter cable without a problem (bandwidth is
obviously better when using multiple scsi cards). I make my own cables
BTW (much cheaper and I can make them exactly as required).

I have no skin in Adaptec, I don't care if Adaptec drops off of the face
of the planet or if the employees are abducted by aliens for disection.
And I also think I would rather have BusLogic or DPT controllers as they
appear to be faster. However, I think that if you follow the rules,
Adaptec scsi stuff works well. I wish everything in the way of computer
hardware worked as well as the 2940.

Ed Welbon;;

Any sufficiently advanced technology is indistinguishable from linux...