[SunRescue] Proper use of VME scsi controllers?

[James Lockwood]

On Wed, 9 Feb 2000, Greg A. Woods wrote:

> For internal cabling, yes, this is probably OK.  I'm not an electrical
> engineer with detailed knowledge of signalling but I do know that even
> one bad connector can fubar even an RS-232 port never mind a 5MHz
> parallel data bus.  Noise on the lines of a single-ended bus is very
> bad.  Lack of good echo cancellation is fatal.

I'd like to add that poor internal SCSI cabling is not a problem unique to
older Suns.  Of the /x30's that I've run (mostly 4/330's) nearly half have
had SCSI problems at some point in their life despite having a nice,
standard esp SCSI controller onboard.

Closed inspection showed that the problem is the internal SCSI cable. 
It's long, has many connectors, and must negotiate many sharp bends and
turns.  There was often no visible difference between ones that worked
well and ones that didn't so we kept a large supply of extra ribbon cable
and ends on hand.  There are also multiple minor impedance mismatches
where the short (and usually tightly turned) HD cable attaches to the
motherboard and into the internal SCSI connector. 

Consider those cables to be like belts in a car.  They may fail randomly
and without warning.  Preventative maintenance is cheap and can save hours
of frustration.

-James