DMX, universes, channels and addresses

Every lighting console, including Relackout, talks to fixtures in the same basic language: DMX. Before you patch your first fixture, it helps to know what a universe, a channel and an address actually are — three words you’ll see everywhere once you start. This page is pure background; there’s nothing to click in your own project, just a look at where these ideas show up on screen.

What is DMX?

DMX512 is the standard digital protocol lighting fixtures and effects use to receive their values. A lighting controller sends a continuous stream of channel values, each one a number from 0 to 255, and every fixture on the line listens for the channels it cares about and ignores the rest. DMX itself doesn’t know about colors, positions or gobos — it only knows numbers on channels; a fixture’s own firmware decides what each number means (0 might mean “shutter closed”, 255 “fully open”).

A universe: a line of 512 channels

A single DMX line is called a universe, and a universe always has exactly 512 channels — no more, no less. Whether you’re running one small PAR can or filling a universe with moving heads, the channel count doesn’t change: it’s a fixed-size address space that your fixtures share. Relackout’s address map mirrors this directly: it’s a 512-cell grid, shown 128 channels per page across four pages.

Concept	Value
Channels per universe	512
Value range per channel	0–255
Address map pages in Relackout	4 × 128 channels

A fixture, its start address and its channel block

A fixture’s mode decides how many consecutive channels it needs — a simple RGBW PAR might need 5 (dimmer, red, green, blue, white), a moving head with pan and tilt might need 8 or more. When you patch a fixture, you give it a start address: the number of the first channel it listens to. From there, the fixture claims that many channels in a row, ending at start address + channel count − 1.

Here’s that math with the Demo Venue rig used throughout these guides:

Fixture	Start address	Channels	Channels it occupies
Front 1 (LED PAR RGBW)	1	5	1–5
Front 2 (LED PAR RGBW)	6	5	6–10
Spot Left (Moving Head Spot)	101	8	101–108
Strobe (LED Strobe)	201	2	201–202

Why addresses must not overlap

If two fixtures share even one channel in the same universe, they both react to whatever value is on it — one fixture’s dimmer might end up driving another fixture’s pan, or two PARs fight over the same red channel. That’s why each fixture’s block has to start where the previous one ends, at the earliest: Front 1 occupies 1–5, so Front 2 can start no earlier than channel 6. Relackout checks this for you — patching into an already-used range doesn’t save anything; it shows a conflict warning instead so you can pick a free address.

Beyond the first universe: Art-Net, sACN, USB-DMX

512 channels sounds like a lot until you’ve patched a few moving heads with 20+ channels each — a real rig often needs more than one universe. Art-Net and sACN (also known as E1.31) are two common protocols that carry DMX universes over an ordinary Ethernet network instead of a dedicated DMX cable, so a single network can carry many universes at once; a USB-DMX interface instead turns one universe into a classic 5-pin DMX cable run. Relackout lets you assign universes to any of these three outputs, up to the number your plan allows. The differences between Art-Net and sACN, and how to set up each output, have their own pages.

Where you see it in Relackout

1. 1

   Open Settings from the home screen, then Patch & address map.

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2. 2

   Click a filled cell on the map (11); see the address range and channel count in the Patch details box.

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3. 3

   In the library on the left, select LED Strobe; see the Start address field and the channel count line in the form that opens. The field takes the cell you last clicked on the map (11) as the start address; since 11–15 is already in use, adding the patch from here would give you a conflict warning.

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4. 4

   First close the form with Cancel, then click U2 among the universe buttons at the top right; see that this universe is empty.

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What’s next

Now that you know what a universe, a channel and an address are, Patching and the address map walks through patching fixtures in Relackout step by step, and Fixtures and modes covers how a fixture’s mode decides its channel layout. When you’re ready to send output to real hardware across more than one universe, see Art-Net vs sACN and DMX output: Art-Net, sACN and USB-DMX.