Written by Terry Doner, August 22 2020. Updated on January 17 2021.
Common questions I see on internet forums are “I am new to lighting and my DMX lights are misbehaving, how do I fix it?”, or “I want to install some theatre lights, what do I do?”. I have written this to try to help people through answering those questions.
There are a few basic things that we can check to minimize DMX problems. Sometimes your system can work while ignoring these points; this is called luck. What it means is that your devices are tolerating noise and poor signal quality. As your equipment ages they may become less tolerant. Or a software update may upset a delicate balance.
But if you are having troubles it is time to start paying attention to the details.
Good design is better than luck.
I have divided this into two sections: System Design and Installation, and Troubleshooting.
The topics in this section are all about building your solution right in the first place. Designing in characteristics that will avoid problems. If you are experiencing problems, take the time to review these and make sure your installation is compliant with these practices.
Your DMX devices are wired in a daisy chain. At the end of the chain you need to have a terminator. You can buy them commercially or make your own - either way as long as you do it. (Some fixtures have a built in terminator, but they are rare. Check your manual to be sure.)
Note:
You will read some dissenting opinions about termination. “I never terminate and never have problems”, or “It was working fine until I added a terminator”.
Good engineering design says terminate. Period.
If you add a terminator and ‘it’ causes a problem, then what is actually happening is you already had a problem and the terminator exposed it.
The DMX signal is a digital signal that needs cabling designed to support its moderate data rates. Standard audio cable is not designed for this job. It is not uncommon for an audio cable to be used for DMX, especially when your DMX equipment has three pin XLR connectors, and it may appear to work. But it degrades the signal and can lead to problems.
The jacket of the cabling should indicate what kind of cable it is. It should say “DMX”, or “Digital Data”, “110 ohm” or “120 ohm”. If you see those words, the cable is of the right type. If not sure, look up the manufacturer's specifications and ensure that it is correct for DMX usage. If the jacket doesn’t say anything then be suspicious; I’ll bet it is an audio cable.
The DMX specification defines the device load that is allowed to be on a DMX network. The limit is 32 devices, of which the transmitter/console is one, leaving room for 31 lights - max.
Note: The above is a simplification. Some fixtures are ‘fractional load’, meaning they only count as half or even quarter. Most lights have a load of 1. Consult your manuals.
Even if your total load is 32 or less, you may want to consider subdividing your network into smaller portions using a DMX splitter. There are several reasons to do this.
Cabling convenience - Running cables to and from every fixture (except last) might not be convenient. You may have a cluster of lights on the ceiling, and another on the stage floor. It might be easier to split the signal and run to different branches (both of which are terminated!)
System Resilience - faults do happen in a system by segmenting your DMX network into sub-networks, if a problem happens in one segment, the other segments should be protected from the fault. Would you rather lose everything during an event or ⅛ of your lights?
Cable length - The insertion of an active splitting device resets your total run length of cable
It bears repeating, if your network load is greater than 32, you need to split it.
See the section below on splitting: 5. Proper Splitting
Assuming you are using standard DMX cable, the total cable length for your DMX (sub) network is 500 metres (or 1600 feet). The closer you get to that maximum length, the more serious you need to take all other design rules.
The cable length is calculated by adding up the length of all cables from the transmitter (console or splitter) through each light or device until you get to the terminator. That length must be less than 500 meters.
You may read in some documentation a length limit of 300 meters. The length of 300 meters or less is a recommended practice. If you need to exceed that, you have to know what you are doing.
If you are using Category cable (cat5 or cat6, etc) then the recommended length distance is 90 meters.
Note: Category cable is often casually referred to as ‘ethernet’ cable. This can lead to confusion in conversations. This is because you can use the same cable in two very different ways:
1) you can use “category cable” directly from fixture to fixture, or
2) you can convert a DMX signal to a network protocol (eg: artnet or sACN) and “category cable” is used to connect the network devices using ethernet.
Describing an installation of the first style, but calling it ethernet cable could lead people to think you are using the second style.
Warning: Do not plug a fixture equipped with a “RJ45-DMX” port into a network switch. There is some lighting gear that may be equipped with a network port that can be plugged into a network switch. Consult your manuals.
As mentioned above, there are good reasons for splitting a DMX line into two or more. But you cannot use a “Y”-cable such a this :
You must use an active splitter/repeater such as this:
There are a variety of device styles to choose from including “opto isolators”, which are typically unidirectional and “RDM compatible” which are bidirectional. There is also a range of prices. Don’t go cheap on your main splitter as would be a critical to system operation.
One advantage of an optical isolator is that they provide some degree of protection from electrical faults, and things like lighting (but not direct strikes).
One easy option that may be available to some, If your console supports multiple universes, you may have an output for each universe. You could divide your fixtures up across those universes. For example, instead of one universe with 30 fixtures, two universes of 15 fixtures each.
“Recommended Practice for DMX512”, By Adam Bennette
Do yourself (and your successor) a favour and document your system. Your documentation should include:
A list of all of the equipment, including make and model numbers. If they have serial numbers, include those as well. You should also include a description of where the equipment is to be found. A splitter hidden in the attic might be a bear for somebody to find if they weren’t told where it was.
A file of all the manuals for your equipment, not as important as it used to be as most manuals can be quickly found online but still a good idea. The manuals could be harder to find when the equipment becomes discontinued. And when in the heat of a problem, having them supper handy is helpful. I have a folder on my computer (actually a dropbox folder) where I keep all of my manuals.
Create a drawing of your installation. How are things connected? Can things on the drawing be located on the the equipment list (#1)? Can you find everything on the drawing in the physical space?
Have somebody else review your documentation, who is not familiar with the details, to ensure it makes sense to them.
Create a list of all userids/passwords needed to manage your system. Of course this needs to be protected and controlled, but it needs to be done. Maybe it is a handwritten list that you keep in the company safe. Keep it up-to-date.
Keep all of your purchase invoices as they might be useful for warranty claims.
Keep a list/file of all purchased software and related serial numbers (and purchase invoices). These might be required for purchasing upgrades, or filling trouble tickets. In particular, keep track of any license codes needed for installation.
Publish your documentation in a place where other people can find it when needed.
Keep it up-to-date.
Many of the topics discussed next are generally applicable to most trouble situations, and some are DMX specific.
There is no required order to follow in these steps, much depends on your situation. Is this a brand new installation? Was it working yesterday? Does nothing work? Are things happening randomly?
As you go through this process, take notes as you do it. Things to include in your notes:
What are your observations?
What did you change and what was the result?
If updating software, What version was installed before and what did you change it to?
As much as is practicable, only change one thing at a time before testing to see if that helped. If you change a whole bunch of things you might not ever know what the problem was.
This is a very good place to start. Make sure your components, all of them have power. And make sure all the cabling is plugged in the correct spot. Simple enough - can you verify it? You can see power lights, you can see a signal coming out?
These concepts can be extended a little further. Are you sending the right signal (ie has it actually been turned on?). If the light fixture is set to channel 1, are you actually sending it a signal on channel 1?
I have used the plug-it-in / turn-it-on rule for decades, and more recently I have added a third … Reboot-it. So much of our systems today are software based that rebooting is often a quickly, useful action. It is not a fix; if it continues to reoccur, then call the vendor and get support.
If your system was working and then stopped working properly, ask yourself what has changed? If you made a change and then things stopped working, undo it. That might be the fastest way to get back into the game.
Is a group of lights flashing on and off? It could be that they are doing exactly what they were told to do. Maybe you have a chase running? This is just one example, so make sure you aren't seeing behaviour that you asked for!
Let me illustrate this with an example. Suppose you have a lighting system that is misbehaving. For example, say you have 16 lights that are all showing the problem.. The divide and conquer approach can be applied this way. Unplug the cable from the console to the light #1 at the light. Move on down the line to light #9 and plug the cable from the console in to light #9. Is the problem visible in that string? (let's say yes) Then repeat the process and move on to light #13. Keep doing this until you are at your last two lights. Hopefully you will see the problem in one and not the other. To confirm this, connect everything back up except the suspect light - everything works properly? Hopefully. Now test the suspect light by itself - hopefully you will see the problem in that light all by itself.
Keep in mind that all of what was described above had at least two components in common for every test: the console and the cable from the console. You may want to explore “Isolate and Test” and “Swap and Test” before you make a conclusion.
The concept here is to substitute out a component and test to see if you still have the problem. For example, replace the cable from the console to the first light with a different good cable. Did the problem stay or go away? If the problem stayed, then the component you swapped out was not likely the problem. Repeat this with every other component until you identify a suspect.
This technique is somewhat similar but opposite. Take one component out of the system and test it in an alternate system (maybe you rent/borrow a console to do this). If it works properly, then repeat for other components.
This is also what you can do if you have access to a DMX tester.
Most consoles have the ability to vary the data rate they use for DMX. It is likely found in the system settings. It might be labelled something like “baud rate”, or “output refresh rate”. It is usually a number in or around 40 Hz. Try a slower speed (a lower number).
If things used to work but now only work at a slower rate, then something in the system may be on the verge of a failure. This might buy you some time, but might not actually fix the problem.
Are you using a computer as your lighting console? Here is a laundry list of things to check:
Reboot it. (just for laughs … https://youtu.be/5UT8RkSmN4k )
Did you apply *ANY* software updates? Undo them and see if that solves the problem.
Note: Do not let your computer automatically update its software. Always do it on our own schedule, and never do it before a big event (unless you are desperate).
Note: Always backup your computer prior to applying an update so you have an easy way to restore to your previous configuration.
Is there an update available for your lighting software? Maybe your problem has been fixed by an update from the vendor? You could read the release notes of the newer releases and see if your problem is likely addressed. Or try it. Always keep a copy of the software versions you install, so if you try a new version and it doesn’t help, you have the option of reinstalling the older version.
Lighting software may be communicating with your lighting network via a USB interface. Check to make sure your your USB drivers are up-to-date, and also check your motherboard driver versions.
Do you use the computer for multiple purposes - that is many programs are running? Try shutting down all other programs and see if the problem goes away. This includes all programs that may autostart on login.
Learn how to use “Task Manager” or “Performance Monitor on Windows, or “Activity Monitor” on MacOS to understand CPU and memory utilisation. Does anything look out of wack? (Disk usage and network usage are not likely to be a concern with lighting software … you should still check it out but it is likely ok)
If you bought your gear from a reputable dealer or manufacturer, they should stand behind their products. Don’t be afraid to call their support line, or send them an email. Many companies are very helpful.
Having your up-to-date system documentation to be able to share with them will usually be helpful. The notes you have taken about what you have changed and your observations will be as well.
If you did something stupid, we all have, don’t hide it. If you spilled a can of coke into your moving light, tell them. Hiding it won’t help.
Recommended Practice for DMX512: A guide for users and installers, 2nd edition
Entertainment Technology -- USITT DMX512-A, Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories
The Worship Ministry Toolkit by Churchfront - Useful is looking for some lighting gear. This has a whole lot more than lighting information, but it is one of the tabs.