Category: General

My DMX lighting fixtures are misbehaving. Help!

If you find that your lighting fixtures are not responding reliably to commands from your DMX controller (be it Show Buddy Active, DMXIS, or any other lighting controller) there are a number of things you can check.

The first thing to realise is this – DMX hardware problems are horribly difficult to diagnose. DMX communication is a very simplistic thing, with absolutely no error correction. When things go bad, they go bad in a confusing manner:

• A poor quality cable connecting two fixtures might cause intermittent faults with a completely different fixture.
• Changing the order of your fixtures could move the problem around randomly. An intermittent connection in a connector might only show up a problem once in a 3 hour show, as the room heats up.
• Cheaply made fixtures might throw out interfere which causes other fixtures to misbehave – yet those fixtures will behave perfectly when connected alone.

Murphy’s Law dictates that these problems will hit you 5 minutes before showtime. So with this in mind, there are a few important guidelines which I recommend you observe.

Don’t use cheap mic cables to connect your lights. Please.

You will read many articles and forum posts from people who swear that they have “never had a problem using mic cables” and that “overpriced DMX cables are just a rip-off”. But the fact is that mic cable and DMX cable is quite different, and the difference becomes more significant as your cable runs increase in length.

DMX is a digital signal, and a long run of cheap mic cable will smudge out the nice sharp digital edges – to the point where  fixtures have difficulty in distinguishing the offs from the ons. DMX cable is designed to carry a digital signal and does not do this (primarily, for the tech geeks out there, due to the lower capacitance of DMX cable).

Also, beware of using really cheap cables sourced from unknown retailers over the Internet. Or if you do, inspect the workmanship INSIDE the connectors yourself before taking them out on a gig. See the picture below, this is the inside of a free DMX cable supplied to me with a brand name fixture from a reputable retailer. Sometimes, cheap cables are cheap for a reason.

My personal opinion is that if you’ve spent hundreds or thousands on your lighting rig, it makes no sense to risk the inevitable problems that cheap cabling could bring you.

Take care of your cables and test them regularly.

Learn how to coil your cables properly (Google for videos on “cable over under”) and don’t tie them in a big knot when you load out at the end of a gig.

Inspect your cables regularly to check for wear & tear, especially at the connectors.

An XLR cable tester box is an invaluable investment for anyone wiring up PAs or lighting rigs on a regular basis. With an XLR tester, you can quickly check a cable for breaks, shorts and – very usefully – intermittent faults.

Have you properly terminated your DMX line?

The final fixture in your DMX chain must be fitted with a DMX terminator. This is simply an XLR connector containing a single resistor, and it prevents reflections of the DMX signal from travelling back up the DMX line, causing errors. It is particularly important to fit a terminator of your DMX cable run is long (over a hundred feet or so).

Again, you can read plenty of people who say that their rig has always worked perfectly without a terminator. But they’re probably just lucky, and one day – eventually – the DMX monster will bite them in the backside.

Clashing fixtures

So, you’ve checked your cabling and terminated properly, but your fixtures are still misbehaving. You might now have a more complex issue with your rig.

Not all fixtures are made equal. Many cheaper fixtures are developed to incredibly tight budgets, and some manufacturers won’t a spend a lot of time or money in testing, making sure that their fixture [a] conforms to the DMX specification and [b] works well in a variety of rigs with other fixtures.

Consequently, it’s quite common to find that when you add a new fixture to your rig, it causes interference and problems with the others. Again, these problems are incredibly hard to diagnose, because often the fixture causing the problem is not the one that exhibits the fault.

In cases like this, a DMX splitter is your friend. A splitter box takes one DMX input (from your controller) and splits it into a number of electrically independent DMX outputs. You then hook up the problem fixtures to one output, and the rest of your rig to another. Problem solved. Splitters also make it easier to route your DMX cabling around the stage. Two things to remember:

• Each chain of fixtures connected to a splitter outout need its own terminator plug (as discussed above)
• Don’t attempt to use a simple audio Y-cable to split a DMX chain into two branches, THIS WILL NOT WORK.

Controller issues

If you’ve ran through all the above checks, and you still have reliability problems with some fixtures, it’s time to investigate if your controller is causing the problem. First, take ONE fixture that is misbehaving, connect it directly to your controller with a short DMX cable and terminate it.

Now, if this single fixture still misbehaves, it’s possible that the controller is transmitting DMX data too quickly. Some fixtures – especially cheaper one – are not engineered to cope with the full 40Hz update rates that the DMX protocol supports. In this case, you can try lowering the transmit rate of your control to say 20-25Hz.

Summary

This is just a list of the most common & easy to resolve causes of fixture problems on a DMX lighting rig. While not an exhaustive list of everything that can go wrong, these are certainly the first things you should check out before deciding whether a fixture is actually defective.

Dave Brown
db audioware