DMX Fault – Finding and Fixing DMX Problems
18 January - Featured, Stage Lighting Control - by: Rob Sayer
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:
- DMX Lighting Sytems – Is Anyone Listening?
- DMX PC Lighting Software – Cheap Stage Lighting Controllers
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On Stage Lighting has a ton of stuff like this. You may also like:
- DMX Stage Lighting Systems
- DMX Over Ethernet – The Ethernet To DMX Converter
- Stage Lighting People and Problems – Interactive
- RDM DMX – A Guide to the Basics
- Wireless DMX – WiFi DMX System Review
Rob is a freelance LD and Programmer working the the UK events industry. He is also the Editor of On Stage Lighting
Tags | DMX Dongle, DMX Light Controllers
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December 9th, 2008 at 2:02 pm
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!
December 14th, 2008 at 8:18 pm
@ 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.
March 21st, 2009 at 4:32 pm
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.
March 22nd, 2009 at 2:50 pm
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.
November 24th, 2009 at 8:33 am
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 ?????????????????
November 24th, 2009 at 9:20 am
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.
March 2nd, 2010 at 1:04 pm
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.