[Beowulf]Infrastruture planning for small HPC 40/100 gigabyet eyhernet or Infiniband?

Jim Lux james.p.lux at jpl.nasa.gov
Mon Jul 28 10:19:10 EDT 2008


Quoting "Robert G. Brown" <rgb at phy.duke.edu>, on Mon 28 Jul 2008  
06:15:44 AM PDT:

> On Mon, 28 Jul 2008, Eugen Leitl wrote:
>
>> On Sun, Jul 27, 2008 at 07:19:56PM -0700, Jim Lux wrote:
>>
>>> bear in mind that ordinary ethernet both coax and twisted pair is
>>> galvanically isolated.
>>
>> This is strange, because I've seen (small) sparks and received (mild)
>> shocks from both, in two different locations.
>
> Ground loop.  Very dangerous.  You go first...;-)
>
>    rgb


Very odd.. I'd be looking for an outright short from the cables to  
something (or, a LOT of capacitive coupling)...

After all, the twisted pairs are isolated at BOTH ends..

Now, there is Power over Ethernet these days.. Basically uses each  
pair of wires as a single conductor (i.e. they feed the juice in at  
the center tap of the isolation transformer)  but, again, that  
shouldn't be sparking/shocking.





>
>>
>> As you say, http://www.apcmedia.com/salestools/FLUU-5T3TLT_R1_EN.pdf
>> claims Ethernet is immune, yet I've read somewhere that Gigabit ethernet
>> is more susceptible than Fast Ethernet. I've got (cheap) UPSen for
>> almost all equipment, maybe they're the problem and not the switching
>> power supplies.
>>
>> In any case I'll have an electrician diagnose the problem. Unfortunately,
>> I anticipate his solution would involve pulling through a new   
>> large-crossection
>> ground wire to several locations. It is at this point that lack of wall
>> conduits will become quite painful.
>

Nope.. shouldn't require a separate grounding conductor, at least not  
along with your cabling.

What you might want to do is see if your electrical safety ground  
(third pin/green wire ground) at the two ends is at a radically  
different voltage.  You might have a miswired receptacle.

You should be able to just drag a single conductor through the house  
and use a multimeter to measure the voltage between the ground pins,  
and it should be zero, or pretty darn close.. use the AC setting, and  
put a small (few K) load resistor across the meter, so you don't get  
fooled by electrostatic/electromagnetic coupling... which will induce  
several volts, at least into an open circuit.



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