NEC question: low-voltage wiring crossing 120v wiring.

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On Sat, 01 Oct 2011 19:26:46 -0500, " snipped-for-privacy@att.bizzzzzzzzzzzz"

So was everyone but then there were a lot of people who said you would never see copper running at a gigabyte. Now it is old technology.

We were doing TDR on a Tektronics 485.
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On Sat, 01 Oct 2011 21:14:12 -0400, snipped-for-privacy@aol.com wrote:

bit?
That's sorta cheating, though. It's no one wire/pair.

What were you using as the pulse generator? I used HP 140s (w/TDR plugins) in college and 7904s (7S10/7S11s) in IBM. There have been many times I wished I had one around.
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On Sat, 01 Oct 2011 20:32:11 -0500, " snipped-for-privacy@att.bizzzzzzzzzzzz"

yes sorry

it is still 500 a pair.

You just use that 18" cable nobody seems to know why the have and couple the trigger out on the side to the" in" with a BNC "T" and bounce that signal down the line. You sync on the fall of the trigger pulse and look for the echo. It really works pretty well but you need a fast scope since you are looking at nanoseconds. You ain't doing it with a 453 unless you have some real long wire. I think a guy in Raleigh figured that out. It was part of the 7800 remote support training. I really only did it once to fix something but I did find the offending drywall screw in a coax on a 100' run in 3 tries. (found the screw heads that were close with a magnet and backed them out with a screwdriver) Third one was the charm. Once I took the screw out it worked so well they left the coax in the wall.
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On Sat, 01 Oct 2011 22:05:39 -0400, snipped-for-privacy@aol.com wrote:

1000BaseT uses four pairs, so it's 250Mb per. Three bits are encoded per pair, per symbol, so the symbol rate is 62.5MHz (MBaud).

Ah, it seems they're using the trigger out as the pulse generator. Neat, but I'm surprised it's fast enough. That rise time is critical. Even the antique HP TDR pulsers were something like 25pS.

Slick. ;-)
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When we moved into this house in 1996, I pulled CAT-3 through it, thinking 10mb is plenty. I am now running gigabit over it just fine. The longest run is maybe 25 feet, so that is a help. But it works. -- Doug
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On Sat, 01 Oct 2011 19:59:33 -0400, snipped-for-privacy@aol.com wrote:

twisted pair is MUCH more forgiving than co-ax. Had a network set up with co-ax and the system was SLOW and unreliable. Found all sorts of 25-50 foot cables coiled up under desks where 5-10 feet would have been more than adequate. When I talked the customer into using "appropriate" lengths, the system came to life.
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On Sat, 01 Oct 2011 20:47:39 -0400, snipped-for-privacy@snyder.on.ca wrote:

I agree, we had a lot of problems with 3270 coax (rg59) but most of it went away when we started using UTP with the appropriate balun.
All that said we still had a lot of coax strung around that worked pretty well. That was another of those things where you want to be sure you only ground the braid on one end. A 3270 terminal was floating on the coax side. Unfortunately that made a great lightning trap. Analog cards used to blow like popcorn in a storm if you tried to span 2 buildings with the coax.
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On Oct 1, 11:56am, snipped-for-privacy@snyder.on.ca wrote:

A metal shield is just that. It shields whatever is inside it from an electric or varying magnetic fields outside it. That is basic physics and requires no ground. So, the shield itself could be an antenna, but the conductor within is shielded.
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On Sun, 2 Oct 2011 05:47:01 -0700 (PDT), " snipped-for-privacy@optonline.net"

Not really true. Whatever (common mode) is inside can couple to the shield, making it the antenna. This stuff is symmetrical, so the same happens outside-in. The key is that the twists make the common mode cancel on the differential signal, within limits. The shield keeps in any common-mode radiation but it must be grounded to do so.
The shield does *nothing* to magnetic fields.
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On Oct 2, 12:14pm, " snipped-for-privacy@att.bizzzzzzzzzzzz"

It most certainly is true. Go Google "Faraday Cage" or look up Maxwell's Equations.

Yes, it's symmetrical, which is why surrounding an antenna with a Faraday cage would render it ineffective.

That's true but unrelated to shielding.

Following that logic, if run a microwave oven on an extension cord without a ground wire, it will spew microwaves all over the kitchen. Of course it doesn't, because it's a Faraday cage, which requires no ground.

That is correct with regard to STATIC magnetic fields. It most certainly does shield varying magnetic fields.
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On Sun, 2 Oct 2011 05:47:01 -0700 (PDT), " snipped-for-privacy@optonline.net"

In the case where the digital data in the cable is interfering with Amature Radio signals, all an ungrounded sheild will do is collect and radiate that signal - in otherwords it will be a capacitively coupled (or possibly even inductively coupled) radiator. With one end grounded, it effectivel traps and stops the signal - keeping it from interfering with the radio. Grounding both ends can cause all kinds of other problems - but apparently there ARE situations where double-end grounding can be called for
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