How does DMX work?

Let's say that you have three lighting fixtures. You assign each fixture a unique identification number, 1-3, and then wire them with DMX. Through DMX, you can then say "1:50%", and lighting fixture #1 recognizes his ID number and responds by dimming to 50% brightness. If you say "1:50%, 2:100%, 3:0%", the fixture with ID 1 goes to half-bright, ID 2 turns on full, and ID 3 turns off.

The ID number of a fixture doesn't necessarily have any relationship with its physical position. Fixtures 1 and 4 could be close together, and number two would be way far away.

There isn't necessarily a one-to-one relationship between ID numbers and lighting fixtures. You might have a dozen fixtures that are set to ID 3, all coming on, turning off, and dimming together. And you might have no fixtures at all set to ID 2. The control console doesn't care. It happily calls out "If there is anybody listening to ID 2, go to 75%!"

Now let's go a bit further with that.... Let's say that you have a lighting fixture that is not capable of dimming; it can't even turn on and off. But it can change color. This clever fixture might still be usable via DMX. All it needs to do is use the number that it is given and interpret it, not as a dimmer value, but as the color. Perhaps 0-33% means red, 34-66% means green, and 67-100% means blue.

So, what if you had a fixture capable of both dimming and changing color? Perhaps it listens for two ID numbers. ID 1 might indicate the intensity, and ID 2 tell the color. Thus the command "1:25%, 2:75%" means quarter-bright blue. And from a distance, you can send "1:50%, 2:50%", and the fixture cheerfully goes to half-bright green!

You can even get "intelligent" lighting fixtures that dim, change color, move around, and project gobo shapes - all under DMX control.

That's the power of DMX - since the lighting fixture gets to decide what to do with the number that it is given, you can control almost anything - laser scanners, fog machines, mirror balls - just name it, and somebody probably has it running on DMX.

There are things that you shouldn't use DMX for. DMX has no error detection or correction, so it is possible that a glitch in the transmission might accidentally turn on something when it isn't yet ready. This isn't a big deal if the ID is assigned to a light, which might flicker, and probably won't even be noticed. But if the ID is being used to control pyrotechnic special effects or something else potentially life-threatening, a glitch in the line is potentially disasterous. DMX should never be used to control dangerous things.

More details, please.

• What we have called an ID number is officially known as a "channel".
• The current incarnation of DMX is capable of addressing 512 channels.
• DMX signals are sent over a differential digital line using RS485 serial. This electical interface is not compatible with PC serial ports. If you attempt to hook a DMX cable to your personal computer, you are going to fry something - something expensive.
• The transmission rate is 250k bits-per-second. When using all 512 channels, each channel gets its value refreshed about 40 times per second.
• A numeric value is sent to each channel, but it isn't really a percentage. It is an eight-bit binary number between 0 and 255. The person operating the control console probably sees this as just the position of a sliding control. And the fixture can interpret it any way it wants to.

How do you hook up DMX?

DMX is designed to be easily hooked up as a "daisy chain". Each box that is capable of sending a DMX signal has a female socket. Each box capable of receiving a DMX signal has a male plug. Each cable has a plug on one end and a socket on the other. In order to hook things up, you need merely string cables from each box to the next box.

Here is the output of the Sunn PLC 3200 control console. It sends exactly the same information in three different formats (it also speaks MIDI). We are interested in the DMX output to the right.

This device came with a 5-pin connector for DMX, which has been replaced with the more common 3-pin connector. The DMX output is always female.

Here is the connecting cable. It is male on one end and female on the other.

You will need one for each device you put in the chain.

Here are the connectors on the back of an ADJ X-Cess fixture. Note that it is equipped with both a DMX input and output.

Just take a cable and use it to wire the input of each device to the output of the previous device.

The control console, being only a transmitter of data has a female socket. The terminator at the end, being the ultimate receipient of data, has only a male plug. All of the fixtures, packs, and other gadgets in between are equipped with both female and male connectors, because they take DMX in on one side and pass it out the other side.

DMX signals must travel in a straight line, from the control console on one end, to one unit, then another, to the end. You must not attempt to split the DMX into two directions with a "Y" cable.

Note:

• Every DMX line must be "terminated". This is done by plugging a special terminating plug into the output of the last DMX unit in the chain. You can easily make a terminator by taking a male XLR plug and soldering a 110 ohm 0.5W resistor across pins 2 and 3.
• The official DMX standard calls for a 5-pin XLR connector. Three of the pins have well-defined functions, but the other two are often used for nonstandard, incompatible purposes. As a result, it is common practice in the U.S. to use 3-pin XLR connectors. It's nonstandard, but it is common.
• The electrical standard specifies that not more than 32 devices can be placed on a single DMX chain. It doesn't matter how many channel numbers these devices use, but it matters how many devices you have. In order to work around this limitation, you need a special DMX splitter or signal booster for each additional segment of 32 units.
• For short runs, microphone cable can be used for DMX; you probably even have some sitting around with XLR connectors on it. But DMX is a high-speed digital signal - it works better and more reliably, with proper digital cables.

What is a DMX "pack"?

A "dimmer pack" is a generic DMX target device. It understands DMX, and is capable of dimming. But it isn't a fixture. Instead, you plug the fixture of your choice into it. When you get tired of the controlling the PAR can via DMX, you unplug it and plug a different fixture into the dimmer pack. This adds flexibility and preserves your investment.

Dimmer packs often take advantage of economy of scale by monitoring more than one DMX channel and controlling more than one fixture. Four seems to be a common number - lots of companies make four-channel DMX dimmer packs. Such a unit performs like four generic DMX dimmers, but can be built and sold for a lot less, because all four dimmers share the same case, controls, DMX connectors, and controlling microcomputer.

Dimmer packs are really only suitable for running various kinds of lights. They'll blow out if you attempt to switch heavy loads, motors, or other inductive loads. For control of things other than lamps, you can get a "relay pack", which does not dim, but switches on and off heavy loads.

DMX Rogues Gallery

Do you own stock in ADJ?

What are the haunt applications of DMX?

I should think that there would be plenty of obvious haunt applications for technology that allows you to remotely control lights, lasers, fog machines, and just about anything that runs on electricity. This is especially true if you don't need to justify the purchase of the equipment, because you already have it on-hand for some other use, such as theatrical productions and DJ work.

But if you haven't had your coffee yet this morning, consider what you could do with just a single intelligent lighting fixture: Set the color to green and the aperature to a small spot; use the X/Y controls to move a "Will O'Wisp" around your graveyard. The wandering point of light comes up to something interesting, and grows a bit larger, turning a golden color for a moment. The light wanders towards a tombstone. Suddenly, there is a flash of light, burst of fog, and the tombstone is illuminated with a red light, flickering like flames. By the strange red light, you read "Count Dracula".

Or perhaps you just have some DMX color changers. By using them to illuminate the stones in your graveyard, you can change them from a cold blue to a sickly green, to a blood-red.

Or maybe you want to control the speed of a strobe, without walking up to it and twisting a knob on the back of the unit.

You just have to use your imagination.

For more information on DMX...

NAME	WEB SITE	CONTENT
United States Institute for Theatre Technology	http://www.usitt.org	sells the DMX512 specification
Rosco	http://www.rosco-et.com/dmxtour.htm	Rosco "tour of DMX"
dmx512.com	http://www.dmx512.com	DMX info, products, and projects
Ujjal Kar	http://www.dmx512-online.com	DMX info and projects
DMX-512 mini-FAQ	http://waapa.cowan.edu.au/lx/dmx-faq.htm	DMX-512 mini-FAQ