[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

Poul-Henning Kamp phk at phk.freebsd.dk
Wed Jan 7 22:56:19 UTC 2009


In message <OF56303512.93B049A7-ON85257537.0079CDE3-85257537.007CC84F at mck.us.ra
y.com>, Joseph M Gwinn writes:

>Could be a differential TX and RX.  I recall that they send a RS422 signal.

Depending on the speed, RS422 works fine with transformers.

>I imagine that the shield is grounded at both ends, if only for
>safety reasons.

That is actually a very unsafe practice, unless there is another
much thicker and reliable ground connection between the two domains.

But you should never let the screen float in the far end, you should
terminate it with a 10M resistor and a sparkgap in parallel to the
local ground.

The resistor takes care of static electricity and the sparkgap will
do lightnings.

>If I had it to do over, I might well use multimode fiber.

Yes, never roll copper more than 100m or between buildings if you
can get away with installing fiber.

>The solution was to use triax.  The 
>outer shield was grounded at both ends.  The inner shield and center 
>conductor together formed the ethernet media.  The inner shield was 
>connected to the outer shield in exactly one place. 

That's technically speaking not triax, that's double shield.  Triax
would have the conductors and one shield.

But yes, double shielding works great, provided you don't have morons
with screwdrivers around.

Poul-Henning

(Who once lost all ethernet interfaces, the access control system
and a few minor computers when a moron first created and then cut
a 600+ A ground loop).


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.



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