A Simple Guide to DMX

April 8, 2008 by Mobile Beat Staff Writer

SECTION 1 INTRODUCTION
DMX512 is a communication signal used to control what are commonly referred to as Intelligent Lights by universal or specific DMX controllers. It started out as USITT DMX512, then was abbreviated to DMX512 and is now most often called DMX. The abbreviation DMX512 stands for Digital Multiplex Signal that can control up to 512 channels. This language was created by the United States Institute for Theater Technology (USITT) to simplify and unify theater lighting systems. Before DMX was created, the only lighting control system was analog which required one individual wire to be run from the controller to each light fixture or dimmer pack.

It is important to understand that DMX is a communication signal only and does not supply power to the fixtures. All DMX light fixtures and dimmer packs require their own power supply.

DMX is now recognized world wide as the standard communication language for theater and night club lighting and now even some fog machines.

SECTION 2 FOUR BASIC PARTS
A DMX system has 4 basic parts: A controller that creates and sends the signal, the cable that carries the signal, the light fixtures or dimmer packs that receive the signal and a terminator to stop the signal. These are all connected in a Daisy Chain manner with the controller at the beginning of the line, the light fixtures and dimmer packs in the middle and the terminator at the end of the line.

SECTION 3 CONNECTING THE PARTS
Connecting a DMX system is easy if you follow these three steps.
1. Link the controller to all light fixtures and dimmer packs with high quality cables.
2. Set the DMX address on each fixture or dimmer pack (usually, but not always necessary).
3. Terminate the DMX signal at the end of the line (unless the last fixture has a built in terminator).

These 3 steps do not have to be done in this order, but you must not power up any part of this system until the controller, all light fixtures and dimmer packs and the terminator have been connected and addressed.

SECTION 4 THE CABLES
The most expensive DMX system is no better than the cables you use to connect them. Low grade or damaged cables and their connectors are the main cause of DMX system problems. Bad solder joints and corroded connector plugs can easily interfere with the signal and prevent proper operation.

DMX cables should never be installed in the same conduit or junction boxes as power wires. You wouldn’t even want them running side by side on a temporary basis.

Never attempt to use a Y cable or splitter other than a true DMX splitter.

Not all microphone cables are wired the same as DMX cables!!! We have had many instances where a customer has had a system working fine for months, then adds a new fixture and uses a handy mic cable to connect that new fixture and suddenly his entire light show quit working. Some mic cables have pin 1 tied to the connector body for a ground while others tie pin 3 to the body. You can not use both types of cables for DMX!!

Although a five pin XLR connector started out as the official connector for DMX, the three pin XLR connector has always been the standard in the U.S.A. and is quickly becoming the the most popular everywhere.

Both connectors do the same thing. Pin #1 = ground, pin #2 = negative signal, pin #3 = positive signal. On five pin XLR connectors, pin # 4 and #5 are usually not used.

SECTION 5 SETTING DMX ADDRESSES ON FIXTURES
Each light fixture and dimmer pack must have an address in order to receive data from the controller. Assigning an incorrect address to your light fixture is as fatal as putting the wrong address on your electric bill payment. In both cases the lights won’t come on!! DMX address mistakes are a big cause of problems with new systems and new human operators. Bad addresses on fixtures cause problems such as gobo change instead of mirror movement, color change instead of gobo change, or no response at all.

Setting DMX addresses is sometimes very simple by following manufacturer instructions and sometimes very difficult when no instructions are available.

Most DMX light fixtures have nine DIP switches used to set the address of that fixture.
Address values are easy to remember. Starting on the left and moving right, each dip switch has a value twice as large as the switch at its left.
The address for a DMX light fixture equals the sum of the value for all dip switches in the ON position. In the picture above, the address selected is 1.

Here are a few more examples…
Address is 2
Address is 3
Address is 7
Address is 21

The rule for setting an address with DIP switches is to start with the largest number of a DIP switch and work your way down through the lower numbers. To set an address of 35 you would first flip on DIP switch # 6 for a value of 32, then flip on switch #2 which makes the total value now 34, then flip on switch #1 which makes the total value now 35.

You might find these little switches easier to manipulate with a small screwdriver or a writing pen.

SECTION 6 DETERMINING DMX ADDRESSES ON FIXTURES
This is where the confusion starts. Many intelligent scanning lights use 4 channels; 1 for the gobo wheel, 1 for the color wheel, 1 for left to right mirror movement and 1 for up and down mirror movement. So why does the second fixture need to be addressed 13 instead of 5? I can’t tell you why they do it, but almost all manufacturers make DMX intelligent lights that OCCUPY more channels than they USE. If you don’t have a good owners manual that clearly identifies the addresses of the second, third and fourth units, you can follow the same procedure that we do when we test a new light.

Connect only one fixture to the controller with only one DMX cable. Set the address of this fixture to 1. Turn the power on the controller and fixture and test each slider on the controller to identify which slider activates each item on the light fixture. After you have made a note about each slider and its activity, turn the power off for the fixture and the controller. Run a second DMX cable to one more fixture and set its address to 1. Power up all items and test the sliders again. Both fixtures should do the same thing at the same time. You can have as many fixtures as you like daisy chained together and all set to address #1. This is the simple way to test and run multiple intelligent lights.

If you want to have each light doing something different from the others, you must address them each differently. This is when you need to know how many channels the light OCCUPIES. To do this, turn the power off for the fixtures and the controller. Go to the second fixture and set its address to 2. Power up all items and test the sliders again. If both fixtures don’t work properly and in sync, turn all power off and advance the address of the second unit by 1 and try again. If this still fails to operate properly, power down and advance the address of the second fixture by one again. Keep doing this until you find that the second unit works in sync with the first.

If you know that your light fixture OCCUPIES 6 channels then your first fixture will be address 1, your second fixture will be address 7, the third fixture will be address 13.

SECTION 7 CONTROLLERS
When setting DMX addresses, the design of your DMX controller and each light fixtures channel function need to be observed. For instance our CX-5 has 12 sliders for light operations while our DMX-STEP-ONE has 8 . Each is very convenient for some intelligent lights, while inconvenient for some other lights with fewer channels.

This problem can be overcome by setting DMX addresses imaginatively.
Lets say you have one intelligent scanner that uses channel 4 for left / right mirror movement and channel 5 for up / down mirror movement. You have another light that uses channel 5 for left / right mirror movement and channel 6 for up / down mirror movement. By advancing the address of one fixture, you will have the same slider controlling left / right mirror movement on both lights. You can play with the addresses of every light fixture to make sliders on your controller operate as you would like.

SECTION 8 DMX SIGNAL TERMINATION
In theory all DMX signals must be terminated. I can tell you from personal experience that most of the systems I have operated were never terminated and still worked fine. Termination does reduce signal errors caused by reflection at the end of the DMX daisy chain. The most popular method of termination is to solder a 90 to 120 ohm, 1/4 watt resistor across pin# 2 and pin #3 of a male XLR connector and plug it into the DMX OUT socket of the last light fixture or dimmer pack in your chain. Some newer light fixtures have a built in terminator activated by the flip of a switch.

www.cheaplights.com

Reproduced under permission of cheaplights.com

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Mobile Beat Staff Writer (228 Posts)

This is the general editors account for Mobile Beat Magazine and Website. Who reads Mobile Beat online and in print and attends Mobile Beat events? DJs, VJs and KJs to start with, especially those who own and operate mobile entertainment services. They provide music, video, lighting and a myriad other entertainment choices for corporate events, wedding receptions, dances and innumerable other gatherings.


Filed Under: Lighting