Lighting 101: Demystifying DMX

2 04 2013

Next to upgrading a sound system, the most common topic I find myself discussing with churches these days is how to create a more inviting and engaging environment through lighting. We’re going to tackle this topic over the coming months, but before we dive too deep into fixtures and what to do with them, we need to back up a step and discuss one of the more misunderstood topics I come across: DMX. By now, most people involved in church tech have seen these three little letters and know they are the key to controlling your lights, but I find that this is often all that’s known about it. We’re going to change that today.

In the early years of lighting, proprietary communication protocols were pretty much the only way to talk to your dimmer racks from your lighting console. Everyone had their own language, which, as you can imagine, made it really hard to mix and match gear without an “interpreter”. While there were some interpreting pieces of gear out there, it wasn’t until 1986 when the Engineering Commission of United States Institute for Theatre Technology (USITT) created Digital Multiplex, or DMX for short. Driven by the emergence of new “intelligent lights” and the desire to have different brands play nice with each other, we finally had a standard language for all lighting products to speak.

DMX communicates 512 channels of information to everything it is connected to. Every channel is communicating a simple value somewhere between 0 and 255 which we often convert to a percentage of 0-100%. We tell a fixture or dimmer rack what channels we want it to listen to by addressing the fixture/dimmer. For example, if you have a 4 channel dimmer pack addressed to channel 1, it will look for the values on channels 1-4 and provide power to the fixtures plugged in at the value you’ve sent it. An LED fixture in 4 channel mode (Red, Green, Blue, Intensity) addressed to channel 101 will look for data in channels 101-104. Depending on how the manufacturer sets up their firmware, it may see a value for Red in channel 101, Green in 102, Blue in 103 and overall intensity in 104. You tell any given fixture or dimmer which channels it should take its values from and it will, simple as that. DMX travels to fixtures or dimmers via DMX cable which has 120 ohm resistance (as opposed to mic cable which has 75 ohm resistance) and uses either 3 pin or 5 pin XLR connectors. DMX is digital data, so we use cable designed for digital data, not analog mic cables.

So if DMX is just a conduit of information, what generates those values for each of the 512 channels? Your lighting console handles that job, and sort of serves as a highly sophisticated universal remote. You know – those remotes where you select an input and then the remote changes the values for the functions that piece of gear has (like channel, volume, menu settings, etc.). A lighting controller works the same way except it simply says, “OK input 1, set your value to 255. Input 2, to 128. Input 3, to 0.” It does this for all 512 channels constantly, updating the values it sends as you tell it to. And if a fixture has multiple channels of information, it’s just like having more features available on the remote for that source.

One of the beautiful things about DMX is that you can hook up your fixtures in any order and, as long as they are all addressed properly, they’ll all get the values they are looking for. DMX is meant to be daisy-chained, though it’s recommended that you don’t daisy chain more than 30 fixtures together. If you have more than 30 fixtures or your fixtures are spread out so far that daisy-chaining them isn’t practical, you can buy an opti splitter to distribute DMX safely and completely to all of your fixtures. It’s also highly recommended that you terminate your DMX line with a DMX terminator. DMX moves information at a high speed so it’s critical to give it the best signal path possible free from interference and the possibility of getting bogged down. Too many fixtures on one chain, not having your line terminated and using microphone cable instead of DMX are all things that can cause interference in your communication, which can result in lights doing interesting things. Those fixtures then are still receiving communication, it’s just not always the right communication.

Hopefully that clears up any questions or confusion that revolves around DMX for you. It’s really as simple as being a conduit of 512 channels of information, and it’s a critical component to making your lighting systems work. Use it correctly, though, and you will have the power and control to do great things with your lighting.

In future issues of Worship Tools email newsletter, we’ll talk about how to set up a lighting system, types of controllers and talk all about new, low cost, LED lights that we’re using in churches around the country to transform their worship spaces.

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2 responses

5 04 2013
Chad Green

Duke,

So what are we supposed to do with the three pin cable that looks an awful lot like mic cable that may have come with our ADJ fixtures? How do we make/get three pin DMX cable? Thanks.

5 04 2013
dukedejong

Great question! The first thing I’d do is put a colored piece of tape on all of your 3-pin DMX cables to distinguish them from mic cables. As far as how to make cables, it’s pretty simple whether you’re making 5 pin or 3 pin. In fact, if you need to make a 3 to 5 pin cable, this should help too:

XLR-5 pinout
Signal Common
Data 1-
Data 1+
Data 2-
Data 2+

XLR-3 pinout
Ground
Data 1-
Data 1+

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