DMX Fault – Finding and Fixing DMX Problems

Friday, January 18th, 2008 - Featured, Stage Lighting Control - by:

DMX512 is the most common stage lighting control system that can control both intelligent lighting and DMX dimmer systems. A DMX control system is pretty simple and robust and DMX faults are a less common problem in stage lighting, compared to blown lamps etc. If you do have a fault in your DMX chain, it can be tricky to find the source so On Stage Lighting has this guide to troubleshooting DMX, how to spot a DMX fault and how to trace and solve the problems – and without a DMX tester.

Ok, so you’ve got the lighting rig together, dimmers working, moving lights responding and you need to start programming the desk because you have to be ready in a few hours for rehearsal. But, your moving lights start to act strangely. Some of them are fine, some don’t respond to DMX control at all and one just seems to think it is a random search light.
This happened to me on a gig last week, and it is classic DMX fault symptom that was the kick-start of this DMX problem guide. Before reading this your should make sure that you have read DMX Lighting Systems.. and understand how DMX works and how to avoid problems in the first place (using correct cable, termination etc.)

But before we start troubleshooting the DMX…

Non DMX faults

DMX512 is pretty hardy and there are some problems in stage lighting rig that the novice will think is a DMX fault. So, let’s look at their symptoms and eliminate them from our enquiries:

Problem – Nothing Happens / No Control

If you have no DMX control over your lighting rig, then it is likely that:
The Grand Master or DBO button is set to OFF or you need to press clear on the lighting desk.
The lighting desk has not been patched correctly or the DMX output cable has been plugged into the wrong output on the back of the console.
There is an “air gap” (it’s not plugged in) somewhere in your control cable chain. This could be a fault in the DMX cable, but more often than not, turns out to be a error made when connecting up.

Problem – Strange behaviour from your lighting rig.

If your all of your WobblyLiteTM 575s seem to respond to DMX but all do the same weird things, you have patched the console using the wrong fixture personality or have set-up the DMX addresses incorrectly.(See DMX Lighting Systems…)
If only one of your DMX fixtures is behaving oddly, then the problem may well be the address of that fixture. Having checked the DMX addresses, try resetting the fixture.
If one fixture doesn’t do anything, go back and check you didn’t just miss it out when connecting up your DMX cable chain.

Troubleshooting a real DMX problem

Problem : DMX sort of works by fixtures behave erractically

You’ve checked your control cable, patching, fixture personalities, DMX addressing and absolutely every other possible fault (including the most common, human error!) has been eliminated – your DMX problems continue.

It time to get serious but don’t worry, this article is not about DMX Break Rates, Packets, Frames or using an oscilloscope to diagnose DMX problems. If you need to break out that kind of test gear just to get your lighting rig working, it’s time to get some new kit or find a new hire company!!!

There are only really three bits of to a DMX lighting system: The Control, The Cable and The Fixture.

We have mostly discounted The Control as at fault in the previous section, although output circuits on DMX lighting desks have been known to be faulty, they are pretty solid. So, it is the hardware in the rig that is suspect. Let’s send it some DMX signal and get tracing…

Set up a regular effect on the console to all the fixtures in your rig such as open white Tilt Saw or a two-position chase with the shutter open. With all your fixtures doing the same things, you can see your DMX testing have an effect instantly.

The Fixture has an input and output socket that slot them into the DMX control chain. These circuits can fail to pass DMX to the next fixture in the chain, meaning that your rig works fine right up to that fixture and no further. This problem can be verified by disconnecting the DMX in and out cables of the suspect fixture and joining them together, missing out the fixture all together. Simple enough but not as common as our runaway winner…

And the winner is… The Cable!

It’s nearly always a cable. When you get puzzling DMX problems that seem to occur randomly all over the rig a dodgy cable is the cause. Cables get pull about and abused and a dry joint or broken conductor is likely, causing all kinds of strangeness in the DMX signal and confusing your moving lights. DMX dimmers can also suffer from this kind of fault with erratic dimming and flickering but moving lights just go nuts when a faulty cable gets in the DMX chain.

So, where the hell is that cable, then?

Start at the beginning. You’ve got the console sending a regular effect to your moving lights?

Go to the first fixture in your DMX chain and unplug the DMX output cable. This leaves only the first fixture receiving DMX. If it behaves itself, you can assume that the main DMX cable from the console is good – for the moment.

Top DMX Fault Tip:

Using a DMX buffer/splitter to boost or split your DMX signal can mask DMX cable problems. The buffer pushes such strong DMX signals that it can hide the real location of the fault. A cable fault on a buffered DMX chain can be easier to spot if you connect the lighting console directly to the fixtures, so try bypassing the DMX buffer for your tests. Often, an unbuffered signal just seems to stop at the point of the fault, which is much easier to find.

Finding The Cable

To find the fault in the rest of the DMX chain it simply a matter of unplugging output cables from your moving lights, in sequence noting when things start to go wrong, to locate where in the rig the problems lies. Don’t forget that a cable fault further “upstream” can sometimes not show up until you have re-connected more fixtures, which can confuse matters.

When you suspect a cable, just try using a new one for that link in the chain. Don’t bother removing the old cable but do tape over and mark the ends of the dodgy DMX cable in position. You will forget to do it during the get-out and the naughty cable will live to fight another day.

And waste the time of another lampie! It could even be YOU!!!

Good Luck.

P.S Once you have mastered these techniques of tracing a strange DMX fault and are having problems with a large rig, instead of starting from one end of the chain, try splitting the rig in two. Using a long DMX cable, you can “divide and decide” which half of the rig the problem lies and then divide that chain in two and test again. This can seriously cut down the amount of time it takes to check a large lighting rig, but you have to keep your head clear to remember which part of the system you are working on and interpret the results of your elimination.

And, don’t forget to reconnect all the cables when you have finished.

See also:

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Rob is a freelance Lighting Designer and Moving Light Programmer currently lecturing in technical theatre production at Bath Spa University in the UK. He is also the Editor of On Stage Lighting and runs workshops in stage lighting practice.

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21 Comments

  1. Digital Alchemist:

    Great Article !

    Now, to add a little insight on more “modern” problems of DMX-Chains, I’d like to add a

    little anecdote on a problem I recently encountered. My setup was as follows: 2x Eurolite

    PAR56 RGB-LED lamps, and one American DJ DJ Scan 250 EX. Contoller: Velleman DMX VM116. (You

    can tell, I am hobbyist).

    So I sat in front of my PC, a fairly powerful machine usually used for 3D games and such. I

    fired up my Freestyler DMX software and configured all things correctly. THe LED’s worked

    just fine, but when i tried to move the Mirror of my Scanner – nothing worked. The gobo

    wheel was wiggling a bit, but that was about it.

    So, i went looking for cabling errors. Nothing to be found there, all fresh and tidy

    cabling.
    So, i checked the fixtures over and over. Connecting only the Scanner and nothing else to

    the USB DMX controller seemed to work, but as soon as i conneted the LED-PARs, erratic

    behaviour started.
    Note: up to that point, I had only used my Laptop with this USB-DMX-Interface.
    So, I checked the Setup of Freestyler, everything was fine there too. So, last element to

    check – the DMX-controller itself. A Quick inspection did not show any signs of damage or

    such, and connecting it to the laptop, everything went just fine again.

    Finally, while already browsing for a new DMX-USB-Interface, i found the problem: A manual

    of another USB-interface, limits for USB-voltage where described. So i went and measured the

    USB voltage output of my PC compared to my Laptop. Laptop: 5V. Good. PC: 3.9 volts. Aaaah.

    So, looking at all the neat USB devices plugged into my PC, it was more than clear: External

    hard drive, illuminated keyboard, laser gaming mouse… etc. They all want some power and

    whus, even the best motherboard’s USB power will drop by a volt.

    Thus, I’d like to add some more possible failure points for DMX fixture chains that use

    Laptops/PCs and a USB-DMX-Intreface: The USB Connection, the actual USB Interface, and the

    Software.

    (lets hereby just assume that Operating System/Drivers/Laptop Hardware are NOT an issue…

    that would go waaay to far ;) ).

    So, how to check the Software? First of all, make shure that you selected the correct output

    intreface (COM-Port, USB-Device type and so on.) Its different with every Software, but most

    will provide you with good instructions on how to do that. Also, make shure your Software

    actually supports that USB-Intreface you have (or are planning to buy). Most free Lighting

    Software has a large list concerning compatible interfaces.

    And the USB Connection? Just like a DMX connection, a USB connection is composed of 3 parts:

    the “emitter” (PC,Laptop), the cable, and the device. All 3 of them can have problems (the

    PC in my example, providing not enough voltage).

    To circumvent the USB power problem, I recommend using a powered USB Hub (they’re

    cheap) to feed the interface (and ONLY the interface), since most USB-DMX-interfaces seem to

    come without any kind of dedicated power supply.

    Your USB-Cable should be as short as possible, and well shielded. Anything longer than 5 meters is a no-go anyway. Long distance is what DMX and/or Ethernet is for.

    Finally, the USB-Interface. I cannot give too many recommendations on that, having only used a few different ones myself so far. What i can recommend though is to always go for a well-encased device, something “road-proof” (My Velleman VM116 is actually pretty good on that, having a massive black plastic case).
    Also, the USB-Connector on the device should be of either A-Type of B-Type. (http://en.wikipedia.org/wiki/USB#Types_of_USB_connector). All of the “Mini”-plugs are rather fiddly and thus break easily. I cannot wholly recommend the “in-cable” type of USB-DMX-Interfaces, since, when the cable-part of it gets damaged or rips off, its pretty much dead – whereas with a “normal”, block-type of interface, you can screw open the device and try to fix things yourself.

    I hope i was able to add a little further insight into fixing DMX problems.
    Have a good time!

  2. Rob:

    @ Digital Alchemist – The whole USB-DMX side of things is still quite a minefield but it’s great that lighting control is more accessible to even the smallest of budgets. The “hobbyist” (as you put it) market for cheap controllers and LED kit is an area I don’t have as much experience as I would like(?!). Trying to help OSL readers with their latest purchase can be tricky someone who only really gets to play with “pro” kit. Thanks for your valuable insight.

  3. Andy:

    Hi There,

    How’s this for a problem?

    Hi There!

    In the last few years have had purchased 4 new DMX LED lights, 2 Par36 and 2 Panels. I found time to play around with my DMX LED lights and using the 16 Channel Controller.

    While programing I found an anoying glitch. I read the instructional manual but must be missunderstanding something.

    Here’s my lighting setup:
    1st Light: par36 LED, Address 1
    2nd Light: Panel LED, Address 17
    3rd Light: Panel LED, Address 33
    4th Light: par36 LED, Address 49
    All wiring is as instructed and terminator put in place.

    When programing scenes (8 per Bank), and when programing the 4th par36 LED, the 3rd Panel LED seems to want to join in. I deactivated light 3 (depressed FIXTURE 3 button) which should disable the 3rd Panel LED and pressed FIXTURE 4 button to program the 4th par36 LED. It seems the 3rd panel LED maybe using this address as well as the 4th par36 LED. So, I did the next thing, turned off all power and readdressed all lights to 1, 33, 65 & 97 thinking this would do the trick, Unfortunately it didn’t. So I changed them all back again to the original address settings. Then I thought there could be a problem with the panel light so I switched everything off and switched the 2 panel lights over and readdressed them. But nope, the problem was still there. So it’s not the panel.

    Here’s an example of what I wanted:
    Scenes 1 to 8
    A. 1st par36 LED Fixture 1 RED,
    B. Disable Fixture 1 and enable Fixture 2 Panel LED RED,
    C. Disable Fixture 2 and enable Fixture 3 Panel LED RED,
    D. Disable Fixture 3 and enable Fixture 4 par36 LED RED,
    Disable Fixture 4 and repeat from A to D. (BANK 1 Completed).

    “Why is the GREEN coming up on Fixture 3 panel light after it’s been disabled while programming Fixture 4?”.

    The whole exercise for this is to be able to run a scene using the Auto button and using the speed fader to move it fast or slow but that anoying green section in the panel prevents me from having a clear RED Scene running.

    Please, if you can suggest anything that will help, it would be very much appriciated.

    Andy.

  4. Rob Sayer:

    Andy, while a DMX fault can’t be ruled out especially when dealing with certain LED PARS and Panels, you should first check that this ‘fixture overlap’ is happening only when playing back a programmed scene or also during control of the fixtures in general. If the latter, the problem is either addressing at the fixture (which you started to check) or patching/assigning of the DMX channels at the controller.

    If the problem only happens when playing back your sequence, then controller behaviour is more likely, such as what exactly “disable” means in terms of DMX values. If you are in any doubt, I would record scenes like this:

    1st fixture RED @ full
    1st fixture RED @ 0, 2nd Fixture RED @ full

    etc etc.
    Thanks for stopping by.

  5. daniel:

    when i connect my icolors together via dmx on an 8ch controller they freak out , iv tried changing address and leads , but still can’t work it out , they connect one at a time but won’t connect together , please help ?????????????????

  6. Rob Sayer:

    Hi Daniel. Not sure which of the icolor range you are using but the first place you should look is the LEDs mode settings and/or number of dmx channels the fixtures use. If it’s more than 4, then clearly you are going to have an overlap on an 8 way controller. Assume that the desk is nothing more than 8 sliders that control 8 DMX channels (and you can’t change which DMX channels they are, another possible problem).

    I’ll bet it’s a mode or addressing issue, as you say they work fine individually. If you are REALLY sure that all that is good, try DMX cabling them in a different order – a rare problem can exist on the DMX out side of a fixture.

  7. Colin Moore:

    My number one piece of advice is to buy a XLR cable tester and test EVERY cable before it leaves the warehouse and before you use it.

    DMX buffers are vital pieces of kit as well – if something goes badly wrong with a light it protects your desk and means you still have a chance of saving the show.

  8. NeilD:

    I’m confused and could really do with some advice as I’m all out of ideas!!!!
    I have a transcension sdc-6 mk2 DMX controller that I want to use to control a stairville led par 56 I’ve set the par can dipswitches to 1 and connected an xlr cable to the controller but nothing happens.
    Thinking it was a problem with the controller I connected the par can to a usb to dmx cable I have and set the software to start the fixture at dmx channel one by doing this the led par can works fine so the cable ant the can must be good this makes me think the controller is dead so plugged the controller into a scanner I have and as I move the sliders on the controller and the scanner reacts as it should so the controller works ????
    I’ve also tried to add a DMX Terminator to the end of the dmx chain and still no joy
    Any Ideas

    »crosslinked«

  9. Rob Sayer:

    Hi Neil,

    Given your description you seem to have ascertained that each element works but the the controller doesn’t work with the PAR. Does the scanner work with the USB to DMX? Only reason am asking is that if all the elements work but the PAR/controller doesn’t, is there a hot/cold conductor swap issue? Or that the scanner/controller are using a proprietory control signal (ie, not DMX)?

  10. NeilD:

    First of all thanks for the fast reply
    The scanner works with both the dmx to usb and the controller but the can will only work with the usb to dmx the scanner I have is a eurolite ts150 I’ve tried swapping the hot and cold wires on the xlr plug at the controller end but the can just starts going crazy I’ve even linked the controller to the can then onto the scanner with the dip switches set to 1 the scanner reacts as it should but I don’t get anything out of the can.

  11. Paul:

    Hello Rob,
    I’m an audio guy to begin with (4 decades, so old school analogue with an interest indigital). However, a muso friend of mine and I decided to acquire a Eurolight LC2412 the other day to run his led par cans (dmx) and other sundries when needed for his gigs (LED battens etc,and a moving light or two).
    Apart from the company (starts with B) not including a warranty card in the package, I’m having difficulty with addressing what I’ve named number two fixture, both are par 36′s.
    They are both 6 channel fixtures what I’ve seen, with 10 dipwsitches.
    On the desk, Ch 1 brings up all leds, ch 2 gives you red, 3 green, 4 blue, 5 chase, and 6 flash or strobe. That works fine on fixture number 1, but bringing up channels 7 or 8 (for fixture number 2) does nothing. Channel 9 on the desk brings up various permutations on fixture 2, from all colours at start to different combinations when you reach 40% on the fader. Between 40% and 60% there is absolutely nothing, nada, zilch, then at 60% you start to get a chase pattern that gets faster as the fader rises to 100%.
    BIZARRE!
    I (possibly incorrectly) assumed that addressing the second fixture dipswitches to numbers 1 and 3 (incl 10 to allow dmx) would give me 7 as the start address for the second fixture, but these pars have no brand name on them, so can’t ascertain what is what in that regard. He’s dropping two more par 56 LED cans over later today, so hoping they will perform a bit better.
    Is it just the fixture that’s causing the problem in terms of dipswitch setting (or me)? Or is it more likely a problem in the brand new desk?

    If you can shed a light on the problem, much appreciated. Going from audio straight into dmx lighting makes me think I need a brain re-adjustment of some kind :-)

    Cheers,

  12. Rob Sayer:

    Hey Paul

    Have you tried raising faders 1 – 6 with the second unit plugged in? The reason I ask is that, assuming that the DIP switch on the PAR is fairly standard arrangement, switches 1 and 3 ON (plus 10, if you need it to set it to DMX mode) is address 5. Here is more on how to work out DIP switch settings, if you haven’t already read it.

    Don’t forget to check the “good” can using DMX 7 or, as you say you getting more, try them at that address. If they work, that obviously rules out a desk or signal issue. Also, try daisy chaining the cans DMX together in a different order to rule out an problem with any DMX outputs from the cans themselves.

    Hope that’s given you something to think about.

  13. Paul:

    Hello Rob.

    Thanks for the reply, and apologies for my delay in getting back.
    It turned out to be a fault in not only one of the Par 36′s, but also a fault in one of the Par 64′s my friend brought over later. Turned out that 64′s only had 5 channels, while the 36′s had 6 channels of dmx.
    Both lamps were replaced free by the supplier, dropped them over to him, showed him what happened… “No question, here’s some new ones”.

    I’m wondering if there’s any way to trick the LC2412 into running waggly mirror scanners ( I think there some 5 or 6 channel ones around), or 12 channel RGB moving heads (all LED of course).
    I know Behringer (and other people) don’t recommend it, but I’ve always been one for stepping outside the square so to speak.

    Thanks for the help.
    Cheers,
    Paul

  14. Nick thompson:

    Giday Rob

    You do some nice Stuf i like your style of writing.

    God you know you have been doing it too long when go to write “writing” and you end up writing “lighting” LOL :-)

    Anyway
    I have just done a set up for a disco we had a:
    Dimmer
    3 Martin Mx1s
    6 Martin Cx2s
    All on one DMX line termainitated and all

    The rig would work but the the Dmx levles were jumping all over the show :-)

    I would leave the channle fader in one postion the the dimmer levles were jumping upto 30% acording to the display on the dimmer is was the same for the MX1′s and the CX2′s

    I am reading this after i have done the gig but i was wondering what the problem could be in the end we just ran the lights sound to light.

    I think the problem mite of been that havving all the Fixtures/Dimmers, on the one line might of been to much for the desk we were useing With by the way is allmoast as old as me :-) heheh

  15. Larry T:

    Have two Leviton D4-DMX Dimmer Packs, both connected to an Elation Professional DMX Operator – Four Stage Par Can lights are hooked up to the #1 Dimmer Pack and all four operate perfectly. The other dimmer is connected to three Par spot lights. Only one of these operate properly. The other two will not go off even when sliders are turned completely off or Blackout button is activated. Have to turn power off at breaker. They get dim but will not go off. What is my problem. Thanks in advance for any help.
    LT

  16. Rob Sayer:

    Hi Larry,

    Issues with dimmers are commonly down to Triac failure (the bit that does the dimming) but they usually fail either open or closed circuit – they won’t turn the lights on, or stay on whatever you do. If yours are kinda “working” to a certain level, then I’d look at the control circuitry for the individual dimmer channels. That means opening up the dimmer and is best left to a service engineer or competent person. But before you do that, discount your control desk being at fault by setting the broken dimmer to the same DMX address as the working one and checking it still behaving incorrectly. That will eliminate the controller.

  17. Andy:

    Hi there,
    I have a Soundlab, 4 channel dmx scanner. it starts up fine, does what it does then will not respond via sound to light or dmx?? when it has started up nothing else happens, would much appreciate some help or ideas to what it could be. really want this light back up and running. Thanks
    Kind regards

  18. Taylor:

    hi, nice site. i have 4 par 36 that would not respond to preset colors
    foe example if i press “red” it shows blue the only switch that works correctly are the “white” I have six of these can in total and they all are doing the same thing. Any help will be apreciated

  19. allan anaen:

    hi,

    i was assigned to handle the lightning system in our church with little or no knowledge at all..

    we purchased a dmx512 controller w/ 16 channels..my problem is during our congregational singing 3 LED’s randomly flickers while the rest are ok..but during the preaching, all are ok.

    is it the cable that causes the problem?

  20. kev pawson:

    hi i have some moving heads 3x 250′s and 1 x 575
    one of the 250′s and the 575 on start up will go through test run but when they finish the test sequence they both sit down and heat is displayed in the lcd display any ideas how to fix

  21. Richard:

    I just got a Square led rotating light to replace one that went down after 2 years. I connected it to a rotating mini spot unit from the out of the dmx. The square led light control on my control board would not turn the light on but I had control of every other movement. If i connected the Square led straight to the controler it worked fine. I tried other fixtures and had the same outcome. I tried disconnecting one of the dmx wires inside the unit on the dmx out I believe no 1 on the socket and the lights works perfect. I don’t know why. Maybe you might have an idea why. Thanks

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