Identify unmarked XLR/DMX cables.

[allensmusic]

I have quite a few unmarked 3 wire cables with no identifying marks at all. how can I tell if they are XLR or DMX?
Is there a consistent visual difference in the two types?

[wilson]

I could never find anything. MOST of mine were marked, but had about half a dozen cords that weren't. I ended up checking the ohm resistance on them and marking them myself. Mic = 80. DMX = 120

[allensmusic]

I could never find anything. MOST of mine were marked, but had about half a dozen cords that weren't. I ended up checking the ohm resistance on them and marking them myself. Mic = 80. DMX = 120

how did you check the resistance?

[wilson]

I'd think any multimeter would work. I actually brought over a bunch of my stuff to a friend of mine's to use his really nice label machine. We got to talking about DMX and I mentioned I had some cables I wasn't sure which were which. Looked up online and seen the different resistance and he measured them for me. Just looked like a multimeter to me though. I have one at home and I'll check to see if that works. If not, I'll call him tonight and ask him what it was.

[allensmusic]

the resistance differences should be an inductive resistance and a typical mulimeter will not read it. checking across separate pins would be infinity, and end to end on each wire would be around zero

[wilson]

Mine seems to show .8 for audio and 1.2 for DMX...

[wilson]

PM me your email or number and I'll text or email you images. I can't get to upload. I've checked a bunch now and audio cables come up .8 and .9 and DMX 1.2 and 1.3.

[BillESC]

.8 and 1.2 as it should be.

[ampnation]

And if you don't own a multi-meter, you can get one very cheap at Harbor Freight.
http://www.harborfreight.com/7-function-multimeter-98025.html

[light-o-matic]

Allen is right.. a multimeter is not going to tell you the answer, it only measures DC resistance.

The characteristic impedance of a cable (which is what matters for DMX) is determined by many factors of the cable construction.. eg the type and thickness of insulation, tightness of the twist, and yes the thickness of the wire too. A multimeter doesn't measure these things.

When you are measuring with a multimeter you are measuring DC resistance which is only a measure of the thickness and length of the conductor and it's metalic content. While it is true that DMX cables will tend to be manufactured with thin conductors and thus you might observe slightly higher DC resistance per foot on average than audio cables, this is not something you can count on. I have some DMX cables that are made for professional use, also some that I made myself using quality Alpha and Belden cable and they are relatively low AWG and thus low DC resistance compared to many mic cables, especially lower priced ones.

So yea, in short, a multimeter isn't a reliable method of testing for DMX cables.

[robare99]

My DMX cables just have a different feel to them. I stick to the same brand, and keep them together. I also put a piece of red shrink wrap on the male end, to differentiate them.


Plus, most of them are about 2' long as well.

[allensmusic]

Allen is right.. a multimeter is not going to tell you the answer, it only measures DC resistance.

The characteristic impedance of a cable (which is what matters for DMX) is determined by many factors of the cable construction.. eg the type and thickness of insulation, tightness of the twist, and yes the thickness of the wire too. A multimeter doesn't measure these things.

When you are measuring with a multimeter you are measuring DC resistance which is only a measure of the thickness and length of the conductor and it's metalic content. While it is true that DMX cables will tend to be manufactured with thin conductors and thus you might observe slightly higher DC resistance per foot on average than audio cables, this is not something you can count on. I have some DMX cables that are made for professional use, also some that I made myself using quality Alpha and Belden cable and they are relatively low AWG and thus low DC resistance compared to many mic cables, especially lower priced ones.

So yea, in short, a multimeter isn't a reliable method of testing for DMX cables.

so my safest bet would be to just get rid of the unmarked cables and make sure from now on to get cables that are marked?

[light-o-matic]

They can ALL be used for audio no problem so I wouldn't throw them away. What I would do is try to identify the ones you are pretty confident are DMX.. by nature of their feel or construction eg Teflon and foil shield etc or by markings on the wire itself that you can look up.

If you aren't sure then use them as audio cable.

Here's a really good video that explains how you can measure cable impedance with a signal generator and an oscilloscope. It is worth watching because it visually demonstrates why cable impedance matters.

https://www.youtube.com/watch?v=Il_eju4D_TM&feature=youtube_gdata_player

Sent from my Nexus 5 using Forum Fiend v1.2.2.

[allensmusic]

Thanks Light, I used to work in aviation electronics, and I used signal generators, scopes, TDR's, etc to determine cable properties.
I wasnt going to throw any away, my actual thought was to just list them on ebay as unmarked cablles and start the auction at 99 cents. I am confident they would sell for close enough to new price to make it a moot point.
I allowed my cables to get mixed up, and I started having glitches in my DMX signal because of it.
I didnt know I could use DMX as XLR cables without issues.
Thank you very much for the information.

[light-o-matic]

Oh cool, yea so you know the deal.
But maybe some other people will learn something from that video.

My friend actually just went into the USAF in avionics and she's having a blast learning all this stuff.

I've never done avionics but I have an EE/tech background.. FCC 1st Class Radiotelephone (long lapsed) and have been a ham on and off since I was 20. For a while I was a tech for a paging company, repairing transmitters and all that. And of course the pirate radio station out of my parents house.. I was that kind of kid ;) I still have an HP scope hiding in my house somewhere but haven't used it in forever...

[allensmusic]

I worked with submarine hunting airplanes.
the plane could drop a pattern of buoys. each buoy had a microphone. each microphone could be dropped to a predetermined depth. then each buoy sends a signal back to the airplane. computers on the airplane would filter out natural sounds such as waves and fish fucking.
the remaining sounds were heard through headphones and looked at visually on scopes. if it was thought the be a submarine sound, they would compare the visual signal with other known signals. it was possible to identify specific submarines.

my job was to maintain all the electronic equipment on the plane.

[light-o-matic]

Nice, sounds totally cool :)

Sent from my Nexus 5 using Forum Fiend v1.2.2.

[boxy]

Open the back of the XLR connectors. A mike cable will have the cable screen soldered to the body of the plug as well as pin 1 somewhere. DMX leads never ever ground the cable to the plug and hence fixtures if they're metal as this point is grounded only at the controller end to stop gnd shifts and earth loops.

[light-o-matic]

Open the back of the XLR connectors. A mike cable will have the cable screen soldered to the body of the plug as well as pin 1 somewhere. DMX leads never ever ground the cable to the plug and hence fixtures if they're metal as this point is grounded only at the controller end to stop gnd shifts and earth loops.

That's a good idea.

But it's actually not a good practice to jump the shell to pin 1. It is something that people used to do.. I used to do it when I first started out before I learned better. But thankfully that practice is going away. So when you see a cable where the shell is connected to pin 1, it's definitely an audio cable. But if you don't see that.. then it does not mean it's definitely a DMX cable.