[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
magnus at rubidium.dyndns.org
Thu Jan 8 03:35:02 UTC 2009
Poul-Henning Kamp skrev:
> 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.
You want a DC balanced encoding if you send data down the line,
otherwise you will saturate the transformer and saturation works very
well as method of damping. This was actually used for modulation of high
powers and then the transformer was called a transductor, essentially a
transformer with too small core. This is the core at the Grimeton long
wave transmitter (a world heritage site) in south of Sweden. Lovely
thing to visit. The 127 m high and 1,8 km long antenna is not easy to
miss. I beleive the output effect was 200 kW at 17,2 kHz. It has a
definitive steam-engine feel to it. Lovely.
Usually thought the signal is just dampend out. Only transitions
survive. A pure 10 MHz is not a problem at all, but generic RS422 may
>> 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.
Which is what most bonding network standards will describe never the less.
> 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.
You most probably want to use a capacitor from shield to chassi both for
providing a low impedance path for RF and static electricity blasts but
also helps in reducing the RF emission. Clamping an external RF choke on
the cable will be meaningfull when the cap is there as the RF choke is
being properly terminated.
>> 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.
So true. Not that you can't get it to work, but it is tedious to make it
work under all conditions.
>> 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.
The ethernet habit of using vampire clamps provided a great opportunity
for less insightful installation practices.
> (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).
As I said... :)
Remember, we need to support our morons in their daily task. :)
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