[SunRescue] Hi!

[Greg A. Woods]

[ On Thursday, February 10, 2000 at 10:58:29 (+0100), Peter Koch wrote: ]
> Subject: Re: [SunRescue] Hi!
>
> >I've seen a 3/260 with "si" controller completely fry the TERMPWR line
> >on a ribbon cable when the drive was accidentally rested live on the
> >chassis thus grounding the line.
> 
> So this is maybe a problem of all Sun3, VME or not?!?

You mis-understand.  In the case I refer to the TERMPWR line was
supplying +5vdc as required by the standard and somehow the freefloating
device encountered a ground in some way that caused the big 800W power
supply to push as much current through TERMPWR as was physically
possible thus turning it red hot and eventually opening up somewhere.

The problem on *SOME* Sun systems is that they somehow made the terrible
mistake of connecting TERMPWR to ground instead of +5vdc on the
motherboard (I'd love to have been privy to the discussions of their
engineers before and after this mistake was made!).  In this scenario if
you have your drive jumpered to supply TERMPWR with +5vdc (as you should
if it's near the end of the chain in some cases), then it will be the
source of the current sent back to the motherboard to ground -- i.e. the
direct inverse of the scenario I described.  Along with zapping the
ribbon cable you're likely to let the smoke out of a couple of traces on
the drive's controller card too.

It should be trivial to discover, with an ohm and volt meter, whether or
not your particular machine has the brain-damaged ground connection or
whether it supplies +5vdc as it should, or whether it is not connected
in the motherboard, as it may I think.

> I use flat ribbon cable with either DB-50 or Centronics connectors
> all the time. I cannot say that high quality cables work better.
> The high quality cables are usually 1m or 1.5m long, much longer
> than necessary. I use them where i need the length and otherwise
> i use flat ribbon cables which i can make as short as possible.

Quality in this case also means having proper impedance matching too.

-- 
							Greg A. Woods

+1 416 218-0098      VE3TCP      <gwoods at acm.org>      <robohack!woods>
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