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Author Topic: 4-Wire Cable Tester  (Read 433 times)

Frank Koenig

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4-Wire Cable Tester
« on: July 07, 2019, 08:11:25 pm »

Everyone should know how to make a 3-wire cable tester with 2 LEDs. (DC is applied to wire 1 at end A. An LED bridges wire 1 to 2 at end B. Another LED bridges wire 2 to 3 at end A. Wire 3 at end B connects to the DC return.) Both LEDs illuminate only if there are no opens, shorts, or any of the 5 possible miswirings (3! - 1) that exclude connecting a wire directly back to the same end of the cable from which it came.

I was trying to extend this idea to 4-wire cables so that I could make a speaker cable tester (all my speaker cables are 4-wire). There are 23 (4! - 1) possible miswirings that fit the criterion above. The obvious thing to try is to use 3 LEDs and bridge pairs of wires at each end so as to connect all wires in series. It turns out that while this approach detects opens and shorts it will not detect all miswirings, usually missing one of the 23 depending on the specific topology. I believe there is one class of topologies, however, that detects all 23 miswirings, an example of which is illustrated below. Please try to poke holes in it. I may be deluding myself.

Anyone actually wanting to build one of these testers should be mindful that a miswired cable applies reverse voltage to the LEDs and their reverse breakdown voltage must be respected by keeping the open-circuit voltage low or using protection diodes. BTW, there are 40,319 ways to screw up an 8-wire cable  :o

And yes, for those of you who don't find this sort of thing amusing (and sort of elegant), buy one of the dozens of commercially available brute-force cable testers.

--Frank


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Andrew Broughton

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Re: 4-Wire Cable Tester
« Reply #1 on: July 07, 2019, 08:39:39 pm »

The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?
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Nathan Riddle

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Re: 4-Wire Cable Tester
« Reply #2 on: July 08, 2019, 09:29:20 am »

The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?

A SR flip-flop would work.
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frank kayser

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Re: 4-Wire Cable Tester
« Reply #3 on: July 08, 2019, 11:37:06 am »

The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?


I'll second the value of intermittency tester function.  Caught more crab cables with that function.
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Steve M Smith

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Re: 4-Wire Cable Tester
« Reply #4 on: July 10, 2019, 03:15:01 am »

Here seems like a good place to post my version of an XLR/Phantom Power tester if anyone is interested...




Steve.
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Mike Caldwell

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Re: 4-Wire Cable Tester
« Reply #5 on: July 10, 2019, 07:58:41 am »

As much as I like a good DIY project and I have built many RAT already makes an NL4 version of their XLR "RAT sniffer" cable tester.

Scott Helmke

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Re: 4-Wire Cable Tester
« Reply #6 on: July 10, 2019, 10:08:05 am »

Here seems like a good place to post my version of an XLR/Phantom Power tester if anyone is interested...




Steve.

Wow, pretty clever design! 

I just carry around a vastly simpler tester, two LEDs on a bare male XLR insert - not as comprehensive, but allows finding many problems.  Plus I can give one away without missing it.
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Steve M Smith

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Re: 4-Wire Cable Tester
« Reply #7 on: July 10, 2019, 10:31:02 am »

Wow, pretty clever design! 

I just carry around a vastly simpler tester, two LEDs on a bare male XLR insert - not as comprehensive, but allows finding many problems.  Plus I can give one away without missing it.

I do have another design which includes an opto transistor and just one LED.  The LED only lights if everything is ok.  All other possible faults cause the LED not to light.  That's all you really need in a live situation.

I designed the three LED version before I was aware of the Dave Rat version. I have had a couple of conversations with Dave via e-mail who was very supportive, rather than most manufacturers who would be protective of their product.  I pointed out that I wasn't competition and I was only making them for friends.


Steve.
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Frank Koenig

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Re: 4-Wire Cable Tester
« Reply #8 on: July 10, 2019, 11:35:27 am »

Here seems like a good place to post my version of an XLR/Phantom Power tester if anyone is interested...

This is very nice as it is diagnostic and not just a go/no-go tester, and I could see it useful in the field. It does not, however, detect perhaps the most insidious cable of all, which is the one with 2 and 3 swapped. Then again, if you start off with correctly wired cables this is unlikely to happen on its own.

I wrote a little R program that can simulate a tester with all permutations of miswiring for any number* of wires and a fixed external arrangement of LEDs. The 4-wire tester above appears to pass.

*Well, not any number, as it's of N! complexity :o

--Frank
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Steve M Smith

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Re: 4-Wire Cable Tester
« Reply #9 on: July 11, 2019, 03:09:05 am »

This is very nice as it is diagnostic and not just a go/no-go tester, and I could see it useful in the field. It does not, however, detect perhaps the most insidious cable of all, which is the one with 2 and 3 swapped. Then again, if you start off with correctly wired cables this is unlikely to happen on its own.


That is absolutely correct.  The Dave Rat/Sound Tools XLR sniffer can't detect that either when used with ordinary phantom power.  However, it can if you use the battery powered sender unit with it as it puts two different voltages on pins 2 and 3.  I could also do that with mine but decided not to as in reality, as you point out, cables don't re-wire themselves, they just short out or go open circuit. The swapped connections it does indicate are just extras which might be useful sometimes.




Steve.
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