What You Need to Know When Working with DMX

DMX stands for Digital Multi-PleXing, and it refers to the DMX512 spec, which means 512 channels in a universe. There isn’t a lower number of channels in a universe, as far as I know, so you can use the terms DMX and DMX512 interchangeably. If you are a stickler, there is a more recent spec (circa 1998) known as DMX512A, which is backward compatible with DMX512, but unless you are building the actual DMX circuit boards, it shouldn’t impact you.

The Beginning

It is 1986, and remote lighting control is a clunky mess reserved for live theater installations, large studios, broadcast, and high-budget motion pictures. Each light or fixture is controlled directly from the console, with each function requiring an individual cable run from the board to the fixture. If you want to change the color of a gel, that is a cable run to a motor, and dimming—well, with tungsten bulbs, you’ve got to have a separate room with dimmer packs for each light (LEDs would make this a thing of the past). If you need to trigger a fog machine, then you need a separate control cable. This can easily make for a huge lighting console, with dozens of cables connecting the console to your fixtures, making your setup unwieldly, imposing, and expensive. The more functions your light fixture has, the more control cables you must run.

However, DMX changed that. Instead of running a cable between the control console and the fixture for each function/channel you want to control, DMX allows you to run a single cable to your fixture that allows you to control all the functions of that fixture. Then you can daisy-chain that fixture to another, and another, and another, allowing you to control up to 512 channels.

The Universe

512 channels of control—this means you can control up to 512 different functions spread across whatever number of light fixtures, smoke, or effects fixtures you are running. Because there is only one output cable, this allows for exceedingly small DMX control consoles. Some of these control panels feature a footprint smaller than a 15" laptop, yet can still control up to 512 channels of light and effects. If you need more than 512, then you will need to use a second universe.

How it Works

Each DMX capable fixture gets assigned an ID/address and it uses as many channels to control its functions as it needs. Ideally, each fixture has a unique DMX ID/address, although any fixtures that have the same ID/address will react to the same commands. Every DMX fixture has an input and an output, allowing you to daisy-chain your DMX cable from one light to the next. Just make sure to give each fixture a discrete DMX address for individual control. DMX is robust, and has been around since 1986 for the simple reason that it works.

Wait a Bit; is it 8- or 16-Bit?

DMX sends an 8-bit “word” for each function, this generally provides 256 steps of control per channel. If this is not smooth enough as you dim your fixture, for example, some fixtures support a 16-bit mode that will use two channels; one for coarse adjustment and one for fine adjustment.

Consoles

In the end, a lighting console is what you need to control your fixtures, and the capabilities of your board will define what you can do. While a DMX Universe is up to 512 functions, not all consoles will support that. Most likely, smaller consoles will be limited to between 5 and 12 fixtures, and a limited number of channels per fixture.

Advantages

Obviously, for large installations and lighting setups, DMX is a no-brainer, but even on more intimate shoots, just imagine what a boon it is to be able to adjust the color and brightness of your fixtures without having to leave the camera position.

What Can Go Wrong?

DMX is simple and robust, so very little can go wrong, but that doesn’t mean it can’t fail. Read on for some useful suggestions to avoid catastrophe.

Use a Terminator, or Your Signal Just May Say, “I’ll Be Back”

First, you need a terminator for the DMX signal. Without a signal terminator, your DMX signal will reach the end of the line, and bounce back like a rubber ball off a wall. This is known as a reflection and it will interfere with your DMX signal. Those of you who remember analog video monitors may recall what happened when you forgot to terminate the video signal—you saw a very distorted signal on your monitor. At some point, monitors became auto-terminating and worked very well, and we no longer had to carry terminating BNC connectors to pop onto the monitor’s video out. Auto-termination is still rare on DMX fixtures and can be questionable. A simple terminator on the output of the last fixture in the chain will help prevent the signal from reflecting and traveling back down the chain, causing interference that will affect the smooth operation of your fixtures. Even if the fixture is self-terminating, adding a terminator can’t hurt.

Cables: it’s Digital and it Matters

The DMX protocol calls for 5-pin XLR connectors and low-capacitance cable. You may see fixtures that sport 3-pin connectors or even RJ45 (Ethernet) connectors for DMX. The concern with 3-pin XLR connectors is that they are commonly used for mic cables, which may lead you to think that you can use mic cables. However, most mic cables are high-capacitance. Capacitance may not cause a problem for analog audio signals, but it certainly does affect digital signals. DMX signals are a square wave, and the space between the signals is very important. High-capacitance cables will degrade the signal, because they absorb some of the power within the signal, transforming the nice, clean square wave signal into a saw-tooth signal that may not work properly. So, don’t use mic cables. No matter which connector you are using, you need to use low-capacitance cables.

Distance

So, how long can your cable run be? The DMX specification states the maximum length is 3,281'—but think about it. We are working in the real world; every connection can degrade the signal. A good working practice is to limit your cable run to 1,000'.

Number of Units

32 “loads” is the practical limit. A load is basically a light or effects fixture, and it doesn’t matter how many or how few channels that fixture uses. Could your DMX setup work with 33 or more loads? Probably, but will it be reliable? Why risk it? A simple data splitter comes in handy here. This is not just a simple Y-cable—don’t use a Y-cable, because it introduces noise and splits the signal, weakening it. Always use a data splitter. A data splitter is also useful for extending cable runs longer than 1,000'.

Conclusion

I hope this article has helped make DMX less mysterious, and provided you with some simple guidelines to follow to avoid having problems when using DMX to control your lights/fixtures. Do you use DMX on big lighting setups only, or have you tried using it on small intimate shoots? Please share your experience with DMX below, and thanks for reading.