What is DMX?

DMX is an acronym for Digital Multiplex which is a control language used to control modern lighting and effects equipment.  DMX is a communication protocol designed as a standard between devices regardless of the manufacturer.

A single DMX network, known as a "universe", can identify 512 distinct channels - or addresses.  For larger installations, systems and lightboards can have multiple sets of these universes which can cleanly work together.

The United States Institute of Theatre Technology (USITT) developed the DMX 512 protocol in 1986 as a simple, reliable, and flexible standard for lighting control.
In 1998 USITT transferred preservation of DMX to the Entertainment Services and Technology Association (ESTA) which is a non-profit trade association that represents the entertainment technology industry.
In 2004 the DMX 512 standard was approved by the American National Standards Institute (ANSI).

How does DMX work?

One of the fundamental concepts of DMX is the ability to transmit data on multiple channels over a single cable.  If this was not possible, one would have to have hundreds of cables running from a lighting console throughout the theatre.  In order to accomplish this, a DMX decoder must be built into all of the affected equipment.  The DMX decoder is usually represented by a set of DIP switches or an LED/LCD display.  This control number is commonly referred to as the DMX address.

Traditionally, light boards sent signals to dimmer racks which would then control the power being supplied to light fixtures.  Now, many fixtures (whether they be lights, color scrollers, effects wheels or fog machines) can be controlled by DMX whether a dimmer is involved or not.

Many DMX devices are capable of receiving and utilizing numerous control channels at once.  This capability is accomplished by setting the 'base address' (or the main DMX address) for the piece of equipment, in which case the remaining attributes use the sequential control channels.  Regardless of how many channels a particular device uses; a full set of control information is passed to it, it simply ignores the information not addressed specifically to that device. 

Units are then set up in a daisy-chain fashion where the signal is passed from the controller to each device in a single line.  Up to 32 devices may be connected in a daisy-chain.  You will notice that DMX units have a signal input jack and output jack to aid in this information pass-along. The last device in the chain should make use of a terminator which absorbs leftover signal power so that it's not reflected back into the cable which would degrade the data.

It may be possible to ignore the use of a terminator in short cable runs, but the longer the run the more likely the signal will be reflected back, a probability which can lead to surprising, uncontrolled results, so it's best to use a terminator at the end of any DMX signal chain.

A DMX splitter/repeater (opto-isolator) can overcome a 4,000 cable-run limit by boosting and re-transmitting the signal - and it can increase the number of devices by up to 32 per branch.

Connectors

The DMX standard specifies 5-pin XLR connectors (Cannon X connector with a Latch and Rubber guard).  Only three of these pins have standardized use at this time; the additional two pins were included for future growth.  One pin serves as the ground, and two for data.  Unfortunately, many manufacturers opt to use the less expensive 3-pin XLR connectors which are commonly used for audio cable and microphones.  Some manufacturers also use the currently unused pins to carry voltage, a practice which can be tragic if used with equipment not intended for the extra power.

Cable

DMX 512 requires twisted-pair, shielded, low-capacitance data cable.  The twisted-pair layout helps eliminate interference, and a cable shield also helps protect against interference.  Many people try substituting cheaper balanced audio cable (microphone cable) with inconsistent results.  Audio cable is not designed to support the signal rate of the high speed DMX protocol.  Although it may work over short distances, it is highly vulnerable to interference and degradation leading to unstable results.

Fixture Profiles

Each DMX channel carries a value range of 255.
Dimmers normally use one DMX channel per light - in which case 0 = off and 255 = full on.
Intelligent fixtures use one DMX channel per parameter.  Each fixture has a DMX channel map that matches control channels and parameters.  These maps can vary between manufacturers and even fixtures within manufacturers.  As an example, a relatively simple LED par unit, which doesn't physically move, may only have 3 channels (one for each color).  Channel 1 = red, 2 = blue, 3 = green.
More complex fixtures require more channels because they have more options. As an example: Channel 1 = Pan, Channel 2 = Tilt, Channel 3 = Color, Channel 4 = Gobo, Channel 5 = Dimmer.  Pan, tilt and dimmer functions are linear, but color and gobo functions require more finesse.  In the case of a color wheel with six colors plus open (no color), the fixture might assign a DMX value map to each color:
0-35 Open
36-70 Red
71-105 Cyan
106-140 Green
141-175 Yellow
176-210 Blue
211-255 Magenta
Again, this is just one example which would vary by manufacturer and fixture type.  The DMX Channel and Value Mapping are saved into a computer file for each DMX controller, and this file is called a Fixture Profile.
In order to use a specific fixture with a controller, it's easiest if one can load the profile of your fixtures into the controller (light board).  Many controllers come pre-loaded with hundreds or thousands of fixture profiles from many different manufacturers.

You don't have to have a specific fixture profile; you may just use a generic DMX unit profile, but being able to load the profile into your board can make it easier to keep track of all of the parameters available for you to control.  Profiles, for example, may help you keep track with how many DMX channels each fixture uses. Typically, however, the manual that comes with a fixture when you purchase it offers a guide of which channel controls which parameter.

Master vs Slave

Many fixtures can be set up as a "master" or "slave".  In a slave configuration, the unit(s) function as a copy of the master unit, or as if multiple units have the same DMX address.  As an example, if you had a full set of LED Par cans or color scrollers, you could have them set in a master and slave configuration so that when one unit in that wash is set to red, they all display red.  This is one way to conserve DMX addresses, and can save on programming time.

Some units are also sound-sensitive.  Popular in club or party settings, the units can be placed in this function so they will move or change color and gobo patterns according to the beat of the music.  This is another opportunity where a designer might set one unit to be sound-sensitive and also serve as the master for other slave units, perhaps because one unit will be closer to the source of the music or so that regardless of placement all of the lighting fixtures will at least be in time with each other.

DMX and Power

Normally DMX does not supply power.  The standard is simply a communication protocol.  This means that fixtures need to be plugged into either straight power or a dimmer (depending on the type of fixture, and the amount of control you need over the light) and to receive DMX information.

In the case of color scrollers, projector dousers, and certain other effect fixtures, a 4-pin DMX cable is used to supply both the control information on two of the pins and low voltage on the other two pins.  Be careful to avoid sending voltage through a DMX cable to a fixture that doesn't require it.  Supplying electricity when not requested can damage the fixture and possibly cause a fire or other emergency.
Let’s look at some lights!

For this demonstration I’m going to use a high-end disc jockey (DJ) style DMX control board.  Overall simple in design, it’s highly portable because its simple 2-line LED display is built in, and this board has a joystick which can be fun with moving light fixtures.

This light board has only one DMX 512 output, and uses the less expensive 3-pin XLR connector.  That also means we’ll have to connect all of our lights in one continuous chain.

As for the fixtures, let’s start with some relatively simple Light Emitting Diode (LED) pars.  We need to start by plugging the units in to both power (these units use American standard 120 volt AC with Edison connectors) and to our lightboard via a 3-pin DMX cable.  In order to control both of these units, we’ll need to daisy-chain them by connecting one to our lightboard, then the second unit to the first unit.

To keep our signal clean, we’ll add a terminator to the second fixture.

These par 38s have a few different modes that are available, including sound-activated and slave options; but we’re more concerned with controlling them with DMX, so we need to give them a DMX address. I’m going to set them to address 25.  I’ll explain why I chose that number a little later.  For the moment, so that I can control both of these units at the same time, I’m going to set them both to address 25.

Before we can use these lights, we need to load their fixture profile into our control board.
Many lightboards have different procedures for handling the same information, On this one, we’re going to choose “Fixture” then unit “3”.  This board doesn’t have a profile for these exact Par 38’s, but its generic Par 36 and Par 64 profile will serve us just fine.  We could also use the generic 32 DMX profile, but that would use up 32 channels; when these units only need 3 for our purposes.

Some fixtures use 32 DMX channels; this board breaks those into banks of 8 which can be active at one time.  In this case, we have control over the three primary colors; Red Green and Blue, in that order, from “0” to full intensity.  Remember that DMX has a range of 255; so full in this case is 255, not 100. Programming fixtures via DMX takes a little more planning and effort than standard lighting units and dimmers.  With a standard parcan, we’d only have one color frame in the unit, but we’d only have to set one channel’s worth of information; intensity.  In this case, we have to set 3 channels, but we can choose our colors “on the fly”.

Once we’re happy with our cue, we can record this as a preset.

Perhaps we don’t want both fixtures to be controlled by the same channels; while this control can be useful for a full stage wash, sometimes a designer wants more control than that.  In this case, we can simply change the address of one of the pars.  Let’s make one of them address 26.  Now when we set a level for channel 1, only red from par 1 reacts, as expected.  However, when we bring up channel 2, we get odder results.  And if we jump to one of our presets instead, again we get inconsistent results.

The reason for this is that while par one is set to address 25, it actually makes use of three channels; so DMX address 25, 26 and 27 are controlling par 1.  By setting par 2 to address 26, it also follows information from 26, 27 and 28,  but in the same color order, so we get unintended results.
(play with 4 channels)

To control the units discreetly, we must leave DMX channels 25-27 free.  So, let’s set par 2 to address 28.  Now we have full control over each fixture independently.  Realize that the presets we recorded only affect par 1 because channels 28 through 31 were at zero when we recorded these looks.

Let’s take a look at some even smarter intelligent fixtures.

Moving fixtures fall in to 2 general categories, moving heads and moving mirrors.  There are, of course, advantages and disadvantages to each.  Moving mirrors tend to be less expensive overall, take up less room, and can be quieter.  Their chief disadvantages tend to be that they have less range of motion and often have less features.

This is a LED scanner which uses 8 channels.  Not all scanners are created equally of course.  This one was really designed for use in clubs by DJs, but it does offer us controls for:
Pan
Tilt
Colors
Gobos
Shutter / Strobe
Dimmer / Intensity
12 preset movement washes
And a speed control for those movements.
Some channels (like "3" for color) use different intensities as a way to specify predetermined colors.
This fixture connects the same way, with a  DMX input which we’ve daisy-chained from the pars, and I’ve addressed it to DMX 17.  Because it uses 8 channels, it actually uses channels 17-24, which is why we set our first par to address 25.
Again we have to load a fixture profile into our board.  Into fixture 2 I’m going to set it to the Accuscan 250; which is this particular unit.
This fixture uses a color wheel which has 8 colors plus white.  These colors are created by glass gels which are not changeable, so we can not mix custom colors. 
It does have a rainbow feature where it will continually rotate through the individual colors and then back again, and we can set the speed of that rotation.

This fixture also has 8 gobos and “open” (or no gobo) and offers a “shake” feature which moves the gobo as if in an earthquake.  It can also rotate through the gobos.

It offers strobe shutter effects, but these are not like leko shutters; we can not reshape the light to a square or a doorway.

Its dimmer actually does dim the LED, so it’s quite smooth.

And its movement patterns can be fun for cast parties.

Units with a higher intelligence.

Moving head fixtures are heavier than mirror-based scanners; they also tend to be noisier and more expensive.  On the plus side, however, they usually have a wider range of movement they can cover and typically have more options available to them.  This one’s not an exception.

It uses 16 channels to control
Pan Movement
(fine)
Tilt Movement
(fine)
Color wheel
Rotating gobo wheel
Gobo index
Fixed gobo wheel
3-facet prism control
Focus control
Strobe and shutter functions
Dimmer control
Iris
Frost filter
Speed control
And the obligatory cast-party auto programs

This unit can pan rotate up to 630 degrees and tilt up to 265 degrees
It has a fixed color wheel with 8 colors including a UV filter plus white.  It also offers a rainbow effect which is continual, so it can scroll forever in the same direction.  Our scanner color wheel had to “bounce” back and forth.

It has 7 rotatable gobos and 7 fixed gobos, all of which can be changed, and by combining gobos, we can create a wide array of wonderful effects.

There is a rotating prism,
Focus adjustment

Its shutter, too, is not like a leko; it’s primarily used for a strobe effect.

Its dimmer is mechanical. In this case it’s similar to the closing of venetian blinds, a procedure which is not as smooth or pretty as actually dimming the lamp, but there is no color shift, and the lamp doesn’t need to warm up to full intensity.

It has an iris and frost filter,

And controls for its pre-programmed effects.

I have this unit addressed to channel 1, and it takes 16 to control it, the reason why the other units start at 17.  Because all of our fixtures use less than 512 channels, we can control all of our lights at the same time with this small board.

Lighting fixtures are not the only technology that use DMX.  Many types of effects can as well, such as
Effects wheels which rotate a projection wheel in front of a lighting fixture to create moving effects. 
Color scrollers, which basically sit in the gel frame of a standard lighting unit, then use DMX to tell them which color to scroll to.
There are remote irises and shutters which can remotely reshape the light from a standard or intelligent leko
There are even remote-controlled arms that can make a traditional fixture move like an intelligent fixture.

Another lighting challenge that has crept into the theatre in recent years has been controlling data projectors.  While data projectors as a whole are outside the scope of this CD, lighting designers are often given the task of controlling and supplying power to these fixtures even if the set designer or a specific projection designer is responsible for the images that are projected.
A specific challenge with data projectors is that when they fade to black, they are actually trying to project black, so it’s not a true blackout.  This fade can leave a distracting glow on stage when a blackout is called for and can leave the audience seeing moving silhouettes caused by light spilling out of these darkened units.
City Theatrical, a manufacturer of many DMX controlled lighting effects, has created a projector dowser that rotates a metal flag in front of a projector eliminating the light spillage typically left behind by a projected black.  In this particular unit, a DMX signal runs in to a power supply, which then sends out a 4-pin DMX cable signal:  2 pins for the signal and two pins for powering the unit.
The fixture can then be hung on a side arm in front of the offending data projector or attached beside or on the fixture itself in some cases. 
The metal flag can be attached so that the channel set to zero is either flag open or closed, with the channel at full rotating the flag into its other position.
You can see that DMX can operate a large array of fixtures in the lighting designer’s toolbox.