DMX Cable vs. XLR Cable What is The Difference?

JimP0771

Active Member
Joined
Jan 13, 2017
Location
Upstate NY
Hi all

First of all I hope everyone is doing ok with the COVID-19 scare going around. I have a quick question. What is the actual difference between DMX and XLR cables? I have a dimmer pack that can be hooked up to a music source something like this https://www.fullcompass.com/prod/54...MIgu_Qos_D6AIVwoFaBR20IAs9EAQYAyABEgLBUvD_BwE It uses a XLR cable. However I have used it with a lighting console that requires and DMX input by using an DMX to XLR 3pin to 5 pin converter. So my question is sense XLR works for this why can't it be used in theatre stage lighting? Or is it? I am not a lighting professional Technician i just do it for a nonprofit theater group and learned everything hands on.

Thanks
 

derekleffew

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DrewE

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Mar 18, 2019
Location
Vermont
While your question is pretty clear, I think it would be helpful to brush up a bit on terminology. XLR refers to a series of connectors, which are use (in different configurations and varieties) for both microphones and other audio applications and for DMX512. More specifically, XLR3 connectors--with three pins--are used for audio, and per the DMX512 specification XLR5 should be used for DMX512 among a couple of other options. The DMX512 specification is quite adamant that XLR3 connectors are not to be used, but that has in no way prevented many low-end makers from using them quite widely....including it seems your dimmer pack. The right solution (perhaps excepting modifying the dimmer to use the correct connector) is to construct some DMX512 cables with XLR3 connectors rather than using microphone cable.

The difference between microphone cables and DMX512 cables mainly has to do with the electrical characteristics of the wire, particularly the capacitance (much greater for microphone cables) and characteristic impedance. These affect signal propagation along the cable. While microphone cable may work in practice for DMX512 for short runs, at some point the greater capacitance and impedance mismatch will degrade the signal enough to cause problems. With DMX512 being a digital protocol, it tends to keep working as the signal degrades right up until a point where it more or less suddenly fails to work, and Murphey says that will happen about five minutes before the big premier of the show. That said, in practice one can indeed often get away with using any old cable for short distances; DMX512 over fence wire or coat hangers would work for at least a few feet.

As I understand it, the primary reason the DMX512 specification is so adamant that XLR3 connectors should not be used is mainly to avoid the temptation or confusion of using microphone cables that are ill-suited to DMX512 data.
 

JimP0771

Active Member
Joined
Jan 13, 2017
Location
Upstate NY
While your question is pretty clear, I think it would be helpful to brush up a bit on terminology. XLR refers to a series of connectors, which are use (in different configurations and varieties) for both microphones and other audio applications and for DMX512. More specifically, XLR3 connectors--with three pins--are used for audio, and per the DMX512 specification XLR5 should be used for DMX512 among a couple of other options. The DMX512 specification is quite adamant that XLR3 connectors are not to be used, but that has in no way prevented many low-end makers from using them quite widely....including it seems your dimmer pack. The right solution (perhaps excepting modifying the dimmer to use the correct connector) is to construct some DMX512 cables with XLR3 connectors rather than using microphone cable.

The difference between microphone cables and DMX512 cables mainly has to do with the electrical characteristics of the wire, particularly the capacitance (much greater for microphone cables) and characteristic impedance. These affect signal propagation along the cable. While microphone cable may work in practice for DMX512 for short runs, at some point the greater capacitance and impedance mismatch will degrade the signal enough to cause problems. With DMX512 being a digital protocol, it tends to keep working as the signal degrades right up until a point where it more or less suddenly fails to work, and Murphey says that will happen about five minutes before the big premier of the show. That said, in practice one can indeed often get away with using any old cable for short distances; DMX512 over fence wire or coat hangers would work for at least a few feet.

As I understand it, the primary reason the DMX512 specification is so adamant that XLR3 connectors should not be used is mainly to avoid the temptation or confusion of using microphone cables that are ill-suited to DMX512 data.

Thanks that makes it totally clear. My guess is the dimmer pack that I have shown in the link in my original post is a low end model that it really does not matter what cable you use and it is really not meant to be used for long runs as it looks like it is used for a DJ gig which is most of the time kept near the table that the DJ has his gear setup at which in turn would be a short run which you would might get away with using a 3 pin DMX/ XLR cable. Am I wrong to think this. I have never used this Dimmer Pack for long runs. As I staten in the original post I have used it with a lighting board the required a 5 Pin Connector by using a 5 Pin to 3 Pin Converter. The board I was using was a Lee Colortran Status 24/48 board. It worked for a short show for 3 lights. I was a 2 night gig and the dimmer pack wis like 3 feet away on the same pole as the lights.
 

RickR

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Joined
Sep 18, 2009
Location
Spokane, WA the great "Inland Northwest"
Hey if it works it will be fine, until is isn't! :pray: Believe it or not, once a system is set up, the electrical environment inside the wires can still change.

The issue is more about reliability that whether any data will pass. Apparently there was an experiment done in the early days of DMX where a barbed wire fence was used. 'Some' data passed and there were many congratulations on how fault tolerant DMX appears. :dance: Does that mean we can abandon our expensive data cables for fencing?

Short runs do help avoid certain issues, but they are not a cure for problems. Electrical things are hard for many people to understand because there is little physical stuff to look at. Seeing less wire is a mental shortcut for a simpler system, but that's an illusion. You'll hear a lot about folks using several splitters to make very short runs because they do other things that cause problems. "Best Practices" are your true path to reliability. 5 pin, good cable, terminators, and equipment that actually follows the standard are top of the list!
 

Chris Pflieger

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Joined
Aug 26, 2016
Location
Indiana
Plus, DMX-512 specs ground on pin 1. Microphone wiring (and the DMX-512 wiring in our church :mad:) use pin 3 for ground.




EDIT: I was wrong, but this post shall remain for all time as a marker of my ignorance.
 
Last edited:

DrewE

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Joined
Mar 18, 2019
Location
Vermont
Plus, DMX-512 specs ground on pin 1. Microphone wiring (and the DMX-512 wiring in our church :mad:) use pin 3 for ground.
Microphones use pin one for the shield, just as DMX512 uses pin one for shield. The polarity of the two other wires for the balanced connection are indeed opposite, but that makes no difference at all for cabling as wires themselves are not inherently polarized or even distinguishable except by the color of the insulation.

If you have microphone cables with the shield connected to other than pin one of the XLR, you're in for some difficulty when using them for audio. They will pick up noise badly. I had some once that were miswired from the factory and had a good bit of head-scratching as to why I could hear an AM radio station through my mixer. Correcting the wiring naturally improved things tremendously.
 

Chris Pflieger

Well-Known Member
Joined
Aug 26, 2016
Location
Indiana
Yeah, I'm an idiot... For some reason I was thinking they were different.


Maybe the installers were just as confused as me, but didn't bother check before firing up their solder irons.
 

Robert F Jarvis

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Joined
Apr 8, 2017
Location
Sebring, FL, USA
XLR or DMX cable - excellent Qs and As here. We try to use the correct cable but due to the number of lamps now sporting only 3 pin sockets do get them mixed up on occasions. That capacitive difference can make a real difference. A set of lights was working just fine for us but to try out a different console I used quite a long coiled up audio XLR cable - and the lights didn't work! Was it the cable? Well, I noticed the last lamp didn't have a DMX 120 Ohm terminator on it so I stuck one in and up came the lights. Removed the temp cable and for a joke the terminator as well (we call them "Arnolds" here) and the lights were OK. Bottom line - take note when folks say to use a terminator on long DMX runs and try your best to use the correct impedance cable for your analog audio and digital DMX signals. Doing the right thing pays in the end.