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DMX issues


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I know this is a bit of a long shot, but...


We are currently in the midst of panto and are having a few issues. I am fairly sure they are to do with DMX, but can't narrow it down.

We don't have a DMX tester so looking at fault finding solutions and ideas.


Desk is an ION but we have previously has some of these issues on our old strand 530.


Rig consists of a lot of generics and then 4x Mac 600's, 4 x Mac 250's and 2x Mac 500's also 6x cheap ish 3 watt LED parcans and a unique 2.1 hazer


It's all on 1 universe running into a 10 channel DMX splitter hub. It's a stage electrics DMX s12. This isn't showing any errors or the error light.


The above fixtures are split over 6 bars and each bar has a different DMX feed from the splitter.


I have firstly been getting random macs lamping off during the show. It's never in the same place, and cannot link it to anything I do on the desk, ie cue or manual sub.


Also they seem to go in fixture types, so I'll get the 2 500's lamping off although they are on a different DMX split and power supply.


Also now the LED's have started to flicker wildly during the show, so much so I have now killed the power to them.


I don't think its the desk as we have had similar problems on the old desk.

Could the splitter be faulty and throwing out odd DMX?

I know we have had problems before with a faulty cable causing issues further up and down the DMX lines, could this be affecting other lines from the split? And I'm surprised a fluctuating DMX signal cold lamp off a Mac as the need such a specific command to lamp them off normally.


I know this is a really difficult thing to answer without a tester, etc, but if anyone has any suggestions it would be greatly appreciated.




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Are any of the lines terminated? Are the MACs displaying any messages when their lamps douse? Do bad things happen from the word go, or is it after a certain amount of time? I've had stuff go nuts before and it was down to a dodgy cable, but having fixtures lamp off is very unlikely for a DMX or cable fault

You could try running a string of fixtures without the splitter in the chain and see if anything improves.

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The first place to start with ANY random issues on a DMX network has to be with the cabling.


A faulty (intermittently so or permanent) DMX cable can cause all sorts of problems on a network of fixtures, and the culprit can be ANYWHERE on a spur of DMX.


In theory, if your splitter is an opto-isolated jobbie, then a fault on one of the outputs shouldn't be able to get back down the line to affect the main feed and/or other isolated feeds. However, a fault on the supply input to that splitter can give all sorts of weird results on any or all of the splitter's output legs.


So - if you're experiencing problems on more than one leg from the split, it's most likely to be a feed cable (or, as you suspect, a fault in the splitter itself, but that's less likely).


If however it's just on ONE leg of the outputs, then start looking between the splitter and the fixtures and checking cables.


There's a tool we used to call the 'Engineer's Tap' back when I used to be a real engineer - consists of a screwdriver held by the blade, and used to tap along circuit boards etc to see if that produces any ill effects. then the 'Engineer's Twist' which is simply wiggling cables and plugs/sockets on (in this case) the DMX lines to again see what happens.


Intermittent faults can be beggars to locate, and require patience and method. Start at one end and work along the route until you find something.






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Sorry but going off on a tangent... but not off topic...


Whilst it doesn't explain the LED issue, 'random' lamping off could easily be a cooling thing. Most moving heads will lamp off if the temperature inside exceeds a set point, and will stay lamped off until that excess heat has gone. Some movers will then re-strike, others will wait for the desk op to do it.


Before blaming DMX, make sure your movers are clean inside. Dust, haze/smoke etc can clog everything up and disrupt the flow of cool air. Additionally the thermistors themselves can develop faults and shut the lamp off below the required temperature.


I'm not saying this IS the problem. But certainly, 'random' lamping off of movers - particularly older units like yours - can simply be down to things over heating.

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Have you tried a reduced DMX refresh rate?


But I think one of the fixtures has a bad control board.


I would bypass every single fixture, by just connecting the DMX in xlr to the DMX out xlr, one by one, and see what happens. I would start with the movers.



BTW: Have you changed some DMX cables lately? I'm asking because some older Martin DMX equipment (from the nineties) has non-standard DMX polarity. The 2 data-lines (pin 2 and pin 3) are reversed.




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Ive had MAC250&550's lamp off in the past due to overheating, I useally turn on the air-con . as for the DMX issues , just do some trouble shooting , Bypass the splitter, different line on the desk or it may be a faulty cable or connection.



Hope this helps

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Any chance it could be an electric supply issue? I'd have thought a large surge or spike could cause lamps to douse (the fixture effectively reseting), although less likely to cause issues with LEDS, it's still possible depending on how tolerent the PSUs are.
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  • 2 weeks later...

I would definitely find one fixture that you are 100% sure works perfectly. Then use this and the desk to test all of the cables seeing you don't have a tester. I would also use this method to test all the branches of the splitter. Then I would set the loop back up and try again, ensuring you remove any of the non-functioning cables. then try isolating fixtures in turn to see if that fixes this problem.

For the LEDs... I've had a problem before when one of them was going into master mode knocking the DMX for anything after that one out, causing them to flash. I would bring them to the deck if possible and look at the displays on each one when they are connected.

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Try and ensure there's no noise on the mains earth.


From experience some of the transceivers in cheaper LED cans are pretty variable in rejecting noise on the 0v rail - in most of these fittings the 0v rail is tied to mains earth. Sometimes a buffer will help bring flickery DMX under control but earth borne noise will always fly straight over the earth connections - this sounds like this might be your problem, somethings being used elsewhere in the venue which is inducing noise on the mains earth.

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This is all a bit long winded,


As mentioned, firstly try swapping out the cable from desk to splitter.


If no luck and its still randomly going doo-lally on various bars then just swap out the splitter.


If then you STILL have wobblies on various bars then its a bit more boring as it could be multiple cable faults (highly unlikely in a pro. facility).


The MACS should be ok, but for certainty, Id make sure that the cheapo DMX LED cans are on a line on their own.


So there you go! Around hopefully 4 or less steps to try. I think some people are overcomplicating it a bit.....

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