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:

Don't Keep It To Yourself

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Rob is a Lighting Designer and Moving Light Programmer and currently Senior Lecturer 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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