Basics 1. What DMX is and why you need it

This series is for people who have never worked with stage lighting. You have bought (or are about to buy) a couple of fixtures, you keep hearing the words «DMX», «channel», «address», «universe» — and so far they sound like incantations. Over twelve lessons we will go through all of them: no formulas, no electrical engineering and no assumption that you «already know this».

Let’s start with the main question: what DMX actually is.

Life before DMX

Picture a venue in the early eighties. There are fixtures and there is a person at a control desk. The desk is a box full of knobs, and from every knob a separate wire runs to its own fixture. Want ten fixtures? Run ten wires. Want a fixture that can do more than fade up and down — change colour, say? Run another wire for every trick it knows.

With five fixtures that is bearable. With thirty, each having a dozen parameters, the venue turns into a tangle of cable and the desk into a wall of three hundred knobs. And worst of all: one manufacturer’s desk did not understand another manufacturer’s fixtures. Bought a new fixture — go find the desk that came with it.

So engineers did what computing had already done: they agreed on a common language.

DMX in plain words

DMX (its full name is DMX512) is the language a lighting desk speaks to lighting fixtures. One cable with a stream of numbers running through it, and every fixture picks out «its own» numbers from that stream.

Here is the analogy that works best: imagine a single cable with 512 lanes inside. The lanes are numbered: first, second, third… five hundred and twelfth. One number travels down each lane, and that number is a command: «intensity 200», «red 255», «pan 128». The fixtures all hang on the same cable and listen only to the lanes assigned to them. The first fixture listens to lanes 1 to 6, the second to lanes 7 to 12, and they never get in each other’s way.

In lighting language a lane is called a channel. You will meet this word constantly — the whole of lesson two is devoted to it.

Three facts that everything else follows from

To avoid surprises later, it is worth remembering three characteristics of DMX. Almost every rule and almost every problem you will run into comes from them.

Fact 1. The packet of 512 numbers is sent whole and sent constantly — roughly 40 times per second.

The desk does not send a command saying «fixture number three, turn red». It sends the entire set of 512 numbers, over and over, dozens of times a second. Even when you touch nothing, the stream keeps flowing. That is exactly why light changes smoothly and responds the instant you move a fader: the next packet is already on its way.

Fact 2. The conversation runs one way.

The desk talks — the fixtures listen. A fixture cannot answer «my lamp has blown», «I am overheating» or «I never got that command». Because of this DMX has no delivery check: if the signal is corrupted along the way, nobody finds out — you simply see a fixture that does not work, twitches, or lives a life of its own. Every requirement about cables and connectors comes from this, and lesson eight is about exactly that.

Fact 3. The numbers mean nothing on their own.

What travels down the cable is just «channel 5 = 200». Whether channel 5 is intensity, the colour red or the pan of a moving head is decided by the fixture itself — by the way it is built. The cable, the desk and the software neither know this nor need to. That is why the same command produces different results on different fixtures, and why the two things covered in lessons 3 and 4 matter so much: the address (who the command is for) and the profile (what this fixture takes its channels to mean).

What every DMX system is made of

However much the gear costs, the layout is always the same — four links in a chain.

A computer cannot speak DMX by itself — it has no such socket. That is what the interface is for: it takes the numbers from the software and turns them into an electrical signal in the cable. Interfaces, networking and what to do when 512 channels run out are covered in detail in lesson nine.

What DMX does not do

Three things beginners expect from it — in vain.

• It does not power fixtures. We just said so, but it is mistake number one.
• It does not know what is plugged in. DMX does not interrogate your gear and does not configure it for you. You set the «channel — parameter» mapping in the software by picking the fixture model from a library.
• It does not check that a command arrived. There is no «delivered» receipt. If a connector works loose, the fixture simply freezes in the last state it received (or starts flashing — depends on the model).

What about modern fixtures that run «over the network»?

You have probably seen fixtures and dimmers with an ordinary RJ-45 computer socket. That does not replace DMX, it transports it: the Art-Net and sACN protocols wrap those very same packets of 512 numbers into a normal local network. Channels, addresses and profiles keep exactly the same meaning. The only difference is which wire the numbers travel down, and how many sets of 512 you can run at once.

Key points

• DMX512 is the common language for controlling light, adopted by every manufacturer.
• A DMX line is 512 channels, each carrying its own number-command.
• The packet of 512 numbers goes out whole and constantly, ~40 times per second.
• The conversation is one-way: the fixture never answers, and delivery errors are invisible.
• Numbers only acquire meaning inside the fixture — hence the importance of the address and the profile.
• A DMX cable carries no power; fixtures are plugged into the mains separately.

Coming next: in the following lesson we will take apart the basic unit of this language — the channel and its value from 0 to 255. We will find out why 50 % intensity is 128 rather than 50, and why a single channel can do several different things at once.