Setting Up The DMX Address And DMX Start Channel

In this tutorial you will learn the meaning of the DMX start channel and the DMX address.

Date: 08/2014

Introduction

The DMX address of a fixture is a combination of the assigned DMX universe and the assigned DMX channel. This DMX channel needs to be set up on the fixture itself (the universe is usually automatically assigned by using the specific DMX line) as well as in MADRIX. Using the right DMX address is very important for DMX fixtures to function correctly.

The DMX start channel also refers to the DMX channel mentioned above. It is the DMX channel where the fixture will start to react. In DMX you can only setup the start channel of a fixture and you will have access to all the other channels of the fixture. To change the assignment of a fixture, it is only necessary to set the first DMX channel anew. All other channels will be changed automatically.

Usually, all fixtures have a unique DMX address so that they all can be controlled individually.

Note: 1 DMX universe includes 512 DMX channels. That means you can control a maximum 170 RGB fixtures (if each fixture needs 3 channels). If you need more channels, it is necessary to use additional universes.

Using Unique DMX Addresses

For example, we want to set up the DMX start channel for a generic RGB fixture to channel 1. Now, we have access to the red color channel with DMX channel 1, to the green color channel with DMX channel 2, to the blue color channel with DMX channel 3.
The next free DMX address is DMX channel 4. When we set up the next fixture to this address, it means we will be able to control this fixture independently from the first fixture.
Let us set up the next generic RGB to start channel 4. That means with DMX channel 4 we are able to control the red color channel, with channel 5 the green color channel, and with channel 6 the blue color channel.
The next free DMX address is 7.

Note: The DMX start channel of a fixture should be an unassigned DMX channel in the specific DMX universe.
Counterexample: Two generic RGB fixtures are configured as follows. Fixture number one is set to DMX start channel 1. Fixture number two is set to DMX start channel 2 in the same DMX universe. In such a configuration the following issue will occur. If you want to control the DMX channel for green of the first fixture, you will inevitably control the DMX channel for red of the second fixture.

Note: It is also possible to set up more than one fixture to the same DMX start address. But in that case, you cannot control every fixture individually.

The Last Usable Start Channel For A Fixture

The last usable DMX start channel for a fixture can be calculated as follows:

(512 available channels) - (DMX channels required by the fixture) + (1).
Is is necessary to add +1 because the start address is also the first channel the fixture occupies.

For instance, if a fixture uses 5 DMX channels:
512 - 5 + 1 = 508
The last useable DMX start channel is 508.

Example 1

Imagine we have an empty DMX universe and we want to patch two fixtures. The first fixture is set up in 4-channel mode (for instance, an RGBW fixture) and the second fixture is a 3-channel RGB light.

The DMX start channel of the first fixture is 1, because the patch is empty and usually the DMX start channel of the fixture is the next, free channel in the universe. Now, the first DMX channel controls the first channel of the fixture, the second DMX channel controls the second fixture channel, the third DMX channel controls the third fixture channel, and the fourth DMX channel controls the fourth fixture channel.
We can see that the next empty DMX channel of this universe is channel 5.
Therefore, we will change the start channel of the second fixture to channel 5. That means that DMX channel 5 controls the first channel of the second fixture, DMX channel 6 controls the second channel of this fixture, and DMX channel 7 controls the third channel of the second fixture.

Example 2
In this example, we want to patch one 3-channel fixture, two 24-channel fixtures, and one 4-channel fixtures. The DMX universe is empty.

The DMX start channel for the first fixture is 1.
Now, we have to calculate the DMX start channel of the second fixture. We know that the first 3 channels will be occupied by the first fixture. That means that the DMX start channel of the second fixture is 4. This fixture occupies 24 DMX channels.
The third fixture should be patched to DMX start channel 28, because the first fixture occupies 3 channels and the second fixture uses 24 DMX channels. Together, the two fixtures occupy 27 DMX channels, starting with channel 1 and therefore the next free channel is 28.
Now, we have only to change the DMX start address of the last fixture. We know that the first three fixtures occupy 51 DMX channels (3 + 24 + 24). The next free DMX channel is 52. That is the DMX start channel of the last fixture in our example.

Example 3
In this example, the DMX universe is occupied until DMX channel 480. We want to add two additional fixtures with 24 channels each.

The next free DMX channel is channel 481. This is the new DMX start channel of the first fixture.
Now, let us calculate the DMX start channel of the second fixture. The occupied DMX channels are 480 + 24 = 504 channels. We know that the next free DMX channel is channel 505 and we can change the DMX start channel of the second fixture to 505.
STOP! This is a mistake because our fixture has 24 channels and 505 + 24 is higher than 512. That means we have to set the DMX start channel to channel 1 and connect this fixture to the next DMX universe.

Congratulations! You have learned to calculate the correct DMX start channels for your fixtures.