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How Do I Troubleshoot DMX Problems?


DMX Cables

DMX issues can spring up at the worst times! When they do, tracking down the problem can be extraordinarily difficult, especially in larger systems. We know this can be frustrating and we feel your pain, which is why we've compiled a tried and true checklist to get you from "out of whack" to "back on track” in no time.

Starting from a known-good place (i.e, using good cables, adding proper termination, employing DMX splitters where needed, etc.) typically eliminates a fair number of the issues below. But, if you're reading this article you're probably already stuck. Take a look at these common DMX troubleshooting techniques for our best advice!

DMX Troubleshooting Checklist

  • Use the correct wire.
    DMX needs the correct cabling. Its signal won't travel properly if the wrong cable type is used. You should be using "120 Ohm Shielded Twisted Pair" (such as Belden 9829, Belden 9842, or equivalent), or Cat5 or Cat6 UTP cable. See our article on DMX-512 Connector Pinouts for proper wiring information. One of the worst things you can do is to use microphone cable. Microphone cable has the wrong impedance and isn't made from twisted pairs. The DMX signal is a differential signal (meaning it has a Data+ and a Data- component), which must be transmitted over a twisted pair to allow it to properly reject external noise sources.
  • Use the correct topology.
    DMX is designed to be wired in a "daisy-chain" manner only. This means that the signal that comes out of a console or controller is wired to a fixture (meaning a DMX light, dimmer, or device). Each fixture has an input and an output. The output of one fixture goes to the input of the next. You can think of the signal as going in a straight line. It never makes branches and goes in two different directions. Doing this (without the use of a DMX splitter) creates electrical reflections that can easily confuse the receivers.
  • Make sure you're using good splitters.
    If your DMX signal needs to go in more than one direction, you should be using an active splitter (sometimes called an opto-splitter). The DMX signal itself can't be branched (or "Y"ed) without the use of a splitter that actively recreates a new fresh signal for each of your branches. In other words, DMX can't be wired in a "star" topology without the use of a DMX Splitter. And, if you're using a low-quality DMX splitter, this can easily be the source of troubles. Interactive makes a series of small DMX splitters that can help alleviate problems caused by DMX branches.
  • Don't put too many fixtures on a single DMX run.
    The DMX standard is designed for a maximum of 32 fixtures on a single run of DMX. If you have more than this, use one or more DMX splitters to increase your capacity. Some fixtures use low-load DMX receivers in them that can allow you to put more than 32 fixtures on a single DMX run, but it's hard to tell if that's the case with the equipment you're using. It's always much safer to simply use splitters.
  • Don't use wire that's too long.
    The DMX standard specifies a maximum length of 1,800 feet (550 meters) to the last fixture of a run. Going much beyond this will likely cause problems. Consider using a splitter somewhere in the middle of your run. Each time you go through a splitter, the signal is regenerated giving you another 1,800 feet (or 550 meters) to work with. Please Note: If you are using RDM on your DMX system, your maximum cable length drops to 1,000 feet (300 meters).
  • Termination is important.
    Each individual DMX run should be terminated. This means that at or after the last fixture of each run should be terminated by a 120-ohm resistor. Some fixtures have a switch that turns on termination, some do not. If the fixture does not offer built-in termination, you need to add an external DMX Terminator to the DMX output of the fixture. There should only be one terminator per run of DMX. If adding a terminator makes a DMX run stop working, it likely means that a cable or connector problem was present and only barely working before you terminated the run. Don't just take the terminator back off, figure out why a properly terminated run stopped working. Interactive offers both XLR and RJ45 based DMX terminators.
  • Start unplugging things.
    See if any particular run of fixtures causes problems by unplugging one at a time. Once a problem run is identified, remove single fixtures from that run to see if there's a bad ground or ground-loop caused by a single fixture.
  • Change the DMX Output timing.
    A number of DMX consoles and controllers provide a way to change the timing of their DMX output ports, including our CueServer processors. Some fixtures have poorly written firmware in them that requires timing that is a little (or a lot) slower than the maximum allowed by the DMX standard. Try slowing down the DMX output of your controller to see if that helps.
  • Use a DMX tester.
    Use a DMX tester to analyze the signal at the far end of every DMX run. Make sure the update rate is steady and that every packet received is valid and doesn't "flicker".
  • Check the cables and connectors.
    If connections were made in the field, check them out. Wires in terminal blocks can be frayed or not screwed down properly. RJ45 connections made in jacks or punchdown blocks can be improperly connected. Solder joints on XLR jacks can be bad. Also, the wire itself can be broken by being run over by a forklift or severely kinked by being pulled through conduit. Inexpensive Cat5 type cable can be "copper-clad" instead of solid copper, which makes it extremely easy to break internally if bent just a few times.
  • Check the fixture power topology.
    If AC power for the controller and the fixtures is being fed from two or more separate electrical panels, it's possible that that power is coming from two different transformers or transmission lines. This can cause large differences of the relative voltages between the grounds of your controller and/or fixtures. These are commonly called ground loops. A large ground loop in your system can wreak havoc. This is where a DMX opto-isolator can help tremendously. Put an opto-isolator between sections of your installation that are fed from different electrical sources.
  • Check sources of electrical noise.
    Does an unshielded DMX cable pass over any major mechanical equipment? Motors, air-conditioners, pumps, or any highly inductive device can create a lot of electrical noise that can induce currents onto the DMX common. This makes it very hard for DMX receivers to properly decode the incoming signal. Move cables away from these sources, and/or properly shield the cables.
  • Is a fixture bad?
    Check your fixtures. Are they from a reputable source? It is possible for a damaged or poorly engineered fixture to introduce noise or bad signals back onto a DMX run. Try temporarily removing a fixture out of your DMX chain to see if problems disappear.