A common language for lights
DMX512 (or just DMX) is the standard a lighting desk or a computer uses to talk to lighting fixtures: LED PARs, moving heads, strobes, smoke machines, dimmers… Almost every stage fixture has a DMX input, so one controller runs gear from different brands.
The idea is simple: the controller keeps sending a list of numbers, and each fixture listens to the ones meant for it.
Universes and channels
- A universe is one DMX line with 512 channels.
- Each channel carries a value from 0 to 255: 0 is off or the minimum, 255 the maximum.
- The controller sends the whole universe again many times per second (up to about 44), so changes show at once.
If your rig needs more than 512 channels, you use more universes: each one goes out through its own interface or over the network (see Art-Net and sACN).
How many channels each fixture uses
Each fixture takes up several consecutive channels, depending on its mode (or “personality”):
| Fixture | Usual channels | What each one controls |
|---|---|---|
| Simple LED PAR | 3 to 4 | Red, green, blue (and white) |
| Full LED PAR | 6 to 8 | Dimmer, colours, strobe, built-in programs |
| Moving head | 12 to 30 or more | Pan, tilt, colour, gobo, focus, prism, dimmer… |
| Smoke machine | 1 to 2 | Amount of smoke, fan |
You choose the mode on the fixture itself, and the fixture’s manual says what each channel does. That is why control programs use fixture libraries: definitions with the channels of each model, so you don’t have to learn them. DMX Total Control comes with thousands, plus the Open Fixture Library and GDTF import (see Fixtures).
The DMX address
The address is the first channel a fixture listens to. You set it on the fixture itself, on its display or with DIP switches.
For example, with 7-channel PARs:
- The first one, at address 1, uses channels 1 to 7.
- The second one, at address 8, uses channels 8 to 14.
- The third one at address 15, and so on.
Two identical fixtures with the same address do exactly the same thing: sometimes that is what you want (two PARs that always match) and sometimes it is a mistake. When you patch in the program, use the same address and the same mode as on the fixture. The Patch suggests the next free address.
Cables, daisy chain and terminator
- Fixtures are connected in a daisy chain: from the controller to the first fixture, from its output (DMX OUT) to the input of the next one (DMX IN), and so on to the last one.
- Use DMX cable (110 to 120 Ω). Microphone cable has the same XLR connectors, but it isn’t made for data and causes errors on long runs or with many fixtures.
- Connectors can be 3-pin or 5-pin XLR; an adapter takes you from one to the other.
- One line takes about 32 fixtures. For more, or to send the signal in several directions, use a DMX splitter.
- Put a terminator (a 120 Ω resistor in a connector) on the output of the last fixture. Without it the signal bounces back and you get odd flickering.
8 and 16 bits
Some parameters need more precision than 256 steps, above all the pan and tilt of moving heads. For that they use two channels: a coarse one and a fine one (16 bits, 65,536 steps). If the library has the right mode for the fixture, the program takes care of it.
Who wins when two things control the same channel
When two scenes control the same channel, there are two classic rules:
- HTP (highest takes precedence): for intensities. If one scene sets a dimmer at 50% and another at 80%, you get 80%.
- LTP (latest takes precedence): for everything else (colour, position, gobo…). The last scene that touched the channel wins.
DMX Total Control adds priorities from 0 to 10 per scene: the one with the higher priority wins, intensities included. Live explains how.
Next step
With this you are ready to connect your lights to the computer. If you don’t have an interface yet, see how to choose a USB DMX interface.