DMX and XLR cables

[SoleJan]

Hello again,
so recently I found out that the existing cables in the school where I work with lighting and sound sometimes, has mixed xlr and dmx cables. The 'build in' lights in the auditorium are definitely dmx' as they've been done by a third party. But all other cables have been used for both sound and lighting. Now I know the difference between the cables is mostly their resistance, 120 ohm at dmx right?
I read online that meters and measuring that level of resistance isn't as easy (and I don't think we got an ohmmeter). 
So is there a way to distinguish these two (is there a reason to?). And yea, all cables are 3pol. 
I kinda want there to be some order.

Thanks
Jan

[Davidmk]

So now you are into something of a grey area - I fully expect this response to attract comment. It especially needs comment from a sound tech as I'm hazy about that side of things.

1. You can often get away with audio cables for DMX but no-one in their right mind would recommend that because you might not get away with it and you can bet the problems would only be apparent mid show and not at fit-up.
2. The easiest way to tell the difference is to see if it is printed on the cable .
3. 150ohms is not the resistance of the cable it is the impedance. Cables do have a resistance but it is dependant on length and is only important if the cable is meant to carry a current (as in mains cables where it is very important and is part of PAT testing).
4. As far as I can tell from researching it, DMX cables should have an impedance of 150ohm as you said). 110ohm is allowed but not recommended. I'm guessing, but I suspect anything below 110ohm is likely to cause problems.
5. The standard for audio is 110ohm but there is a wide range in use - 60ohm to 150ohm is quoted. Apperently, the main thing for audio is that the source and cable should be low impedance and the sink should be high (e.g. 10,000ohms). I Googled "what is the correct impedance for balanced audio cables" and read the AI response to get this information. This would suggest that a "standard" impedance audio cable would be acceptable for DMX but the really low (below 110ohm) cables would not.
6. You can measure the impedance of cables with the right equipment but you can't use the resistance range of a multimeter. Google "how to measure the impedance of a dmx cable" for ways to do it. Your physics lab techs might be of help in this area.

Ultimately it is better to keep your audio and DMX cables apart and mark them with something obvious so you can put them away properly. (That's a big ask in a school I know.)

There is a reason why the standard for DMX is 5pin XLRs even though it only needs 3pins and 3pin XLRs are cheaper.

[kgallen]

An ohmmeter won’t help you. The cables don’t have a different dc resistance they have a different ac impedance. 
 

The most practical way to tell is from markings on the sheath of the cable that will tell you it’s ’DMX’ or ‘digital’ cable maybe. If it says ‘microphone cable’ it’s not really for DMX use.

You can use ‘DMX cable’ for microphones but ideally not ‘microphone cable’ for DMX.

(My response coincided with David’s posting, I’ll read that now!).

[kgallen]

For analogue audio cables, the impedance is not really an issue because the signal frequencies involved are very low. 
 

For digital signals the edge rate is very important and the capacitance of the cable has a big effect on this. Even for DMX at 250kbit/s the required frequency components can be well into the megahertz. Analogue audio cables of any appreciable length as can be found in DMX runs will severely attenuate the high frequencies required to convey a digital signal properly. 
 

For a digital signal like DMX, the cable acts like a ‘transmission line’ (Google that). That’s why we use terminators.  

[Davidmk]

18 minutes ago, kgallen said:

transmission line’ (Google that).

Yeah but Google "what is a transmission line in digital contexts", ignore the AI and go for Wikepedia although you'll need a coffee, some biscuits and a comfy chair.

I think I may be learning more than the original author here!

[Davidmk]

Another point with XLR audio and DMX is the connection of the shield. In DMX  it shouldn't be connected to the XLR shell, only the pin 1. Doesn't help tell them apart but worth checking at the same time if measuring impedance.

[kgallen]

Actually in audio the shell shouldn’t be connected to pin 1 either.  Pin 1 is signal ground, shell is chassis ground and they aren’t the same thing as they deal with different types of interference (electrical, magnetic).

https://www.ranecommercial.com/legacy/note165.html
 

 

[kgallen]

The AI overview on search term ‘what is a transmission line in digital signalling’ seems pretty good. It has characteristic impedance and signal termination sections. 
 

Sorry to the OP, we’ve probably gone a bit over board now! 🤓

[Davidmk]

1 hour ago, Davidmk said:

XLR audio and DMX

😃

Seems like we are in a minority worrying about impedance (although relevant to op).

A pragmatic view is that, if you have a tester that checks framing and has a flicker finder, a DMX source that passes those tests and a normal continuity/wiring tester, then any cable that passes continuity and doesn't introduce flicker or framing errors is a good cable until proved otherwise in practice.