DMX cables

[PantoDame]

A few years ago I installed 6no Par56 LED lights and a small DMX controller in a village hall. I used audio screened cable with 3 pin XLR connectors. I have since read that I should have used lighting type cable for the job. Will it matter?

[Robin D]

If it didn't matter then, it will not now. Might conceivably become an issue if they decide to massively extend the system.

[Don Allen]

You should remove any link between the XLR shell and pin 1 to avoid hum loops when using audio cable on a budget installation. When using audio cables for DMX use a terminator as it shows up any cable problems.

[Jivemaster]

If it works, don't mend it! If you extend the system massively just add in proper DMX cable for the existing lights. Cable for DMX doesn't "go off" with reasonable age, like most electrics it does go off with water!

[david.elsbury]

You should remove any link between the XLR shell and pin 1 to avoid hum loops when using audio cable on a budget installation. When using audio cables for DMX use a terminator as it shows up any cable problems.

could you explains on how hum loops occur when using DMX lights?

[Emmett45]

Every DMX cable I have ever used (in every application) has only ever been bog standard mic cable. The number of issues experienced using this cable? Zero!!

[timsabre]

You should remove any link between the XLR shell and pin 1 to avoid hum loops when using audio cable on a budget installation. When using audio cables for DMX use a terminator as it shows up any cable problems.

could you explains on how hum loops occur when using DMX lights?

 

It is not as silly as it sounds, if everything else is wired right then there will be no problem, but if you get a fault then having DMX ground connected to mains earth can cause some fun and games. I am seeing it a lot at the moment with LED tape drivers, if the LED tape is stuck directly onto a metal surface the +12V can get connected to earth, this can cause smoke if the DMX ground is also earthed.

Though I have to say, I've never seen a pre-made audio XLR with the shell connected to pin 1.

 

[MarkPAman]

Every DMX cable I have ever used (in every application) has only ever been bog standard mic cable.....

 

Actually, when you've used my lighting, all of the cables that are marked as "DMX - 10m" or whatever (which is most of them) are proper!

 

 

 

[timmeh2]

Hi

 

Just to add my 2pence...

 

DMX is far more resilient than most people realise. The standard defines twisted pair conductors with 110ohm impedance but I'm sure we've all used the worst kind of mic cable at some point in our careers, and got away with it. Given that it is a differential signal, providing the conductors are roughly electrically identical you can send it down just about anything. I've fired it down domestic TNE cable with no issues at all.

 

Back in the day, when a fixture's electronics where noisy and only the top-grade stuff had opto-isolation, you could run into trouble. Nowadays even your cheapy LED parcans seem to be mostly immune to interference, unless you do something really naughty. Gone are the days when you had to use terminators and buffer out your LED kit to prevent all sorts of reflection shenanigans, although I still see it from time to time, and it is considered good practise to compartmentalise installations as it makes troubleshooting a lot easier.

 

It also appears that the pin 2-3 reverse issue that often reared its ugly head seems to be a thing of the past.

 

All the best

Timmeh

[timsabre]

Using "proper DMX cable" gives you a nice warm feeling of doing the right thing, but a lot of the cable sold as "DMX cable" by CPC and the like is just audio 2 core screened with "DMX" printed on it. As Timmeh says, DMX will work over just about anything, the problems come when one of the 2 conductors is not connected due to a fault. At this point weird stuff can happen and the type of cable can be blamed, but I have never yet found this to be the case.

[Cheeseweasel]

Every DMX cable I have ever used (in every application) has only ever been bog standard mic cable. The number of issues experienced using this cable? Zero!!

 

It's certainly not standard practice though, and I wouldn't want anyone reading this thread to think it is. Using mic cable for DMX is definitely 'Disco Dave' territory, and I've never come across a serious company that does it. Yes, it works a lot of the time, but it's bad practice and there are solid engineering reasons why it's a bad idea (if anyone wonders what they are, read up on transmission line theory and characteristic impedance).

[timsabre]

Using mic cable for DMX is definitely 'Disco Dave' territory, and I've never come across a serious company that does it.

I am not sure how far I'd agree with this. It is true that all "serious companies" will have separate stocks of audio cabling and DMX cabling, but I have come across plenty of supposed "DMX cable" which has DMX written all over it, but the lay and twist of the cores is pretty random and there is no way it would have a fixed impedance. Equally I have seen some very nice stuff which is braid screened, almost like handling coax.

 

Also, the effects of characteristic impedance and transmission line theory are pretty insignificant at the relatively low frequency DMX runs at.

So I would agree that it is best to install cabling which meets the standard, but it's certainly not worth losing sleep over if you haven't, and beware of more expensive "DMX cable" which isn't really.

 

[Shez]

Though I have to say, I've never seen a pre-made audio XLR with the shell connected to pin 1.

I've seen a few. A colleague bought a batch of XLRs I think from a certain huge German company a few years ago. We saw some odd DMX behaviour and found they all had the shell connected. Ground loops (not hum loops) ahoy. Cutting those links sorted them out.

[adam2]

Use of audio cable for DMX will almost certainly work just fine, and I would not worry about an existing small installation that is working correctly. It would IMHO be preferable to use the correct cable for new or replacement purposes.

 

I have only once experienced a problem with cheap audio cable used for DMX and that was with a most improbable length to some distant LED PARs powered by a car electrical system. It worked fine when tested, but later picked up interference.

 

 

 

[sandall]

I have to say, I've never seen a pre-made audio XLR with the shell connected to pin 1.

Very common with cheap n' Chinese mic cables, which no doubt get used for DMX as well.