DMX

The DMX512 protocol is a standard used by most modern lighting equipment (and other theatre electronics).

Protocol
The DMX protocol is digital, allowing control of up to 512 DMX channels per DMX Universe. Each DMX Channel is assigned a value from 0 to 255 (although a lot of lighting desks scale this from 0% to 100%).

The DMX controller (such as a lighting desk) sends packets at a high frequency to all other devices it is connected to. Each packet has a few bits at the start, to indicate it is a DMX packet, not noise, then it has up to 512 bytes (separated by a few extra bits), representing the value of a channel (an integer between 0 and 255).

Channels
A DMX Channel is a number between 1 and 512 (10 bits) which is sent values between 0 and 255 (8 bits) by a control device.

Multiple devices can read the same DMX Channel, so they will be set to the same level.

Connections
The standard for connecting DMX devices is using either XLR-5 or CAT 5 cables.

XLR-3 cables are sometimes used with cheaper equipment, although this should be avoided, to avoid confusion with audio cables. The use of XLR-3 cables for DMX is prohibited by the DMX512 standard.

This is done by connecting the output of the control device (for example a lighting desk) to the input of the next device (for example, a dimmer rack or intelligent fixture).

Most devices also have a DMX Output, which allows the device to be 'daisy-chained' to the next device. This means you don't need DMX Splitters.

Usually only one DMX Universe is sent over a cable, however it is possible to send a second over the unused pins 4 and 5 in an XLR-5 cable or pins 2 and 6 in a CAT 5 cable. This is required with CAT 5 cables, but optional with XLR-5 cables.

Universes
Most lighting desks support multiple DMX Universes. This allows you to control more than 512 DMX Channels at once. This is useful when using Intelligent Fixtures, as you can connect the first universe to standard dimmers, then daisy chain a second universe between intelligent fixtures.

It is also important to note that some devices may use more than one DMX channel. For example, moving lights often use over 10, for different attibutes. This is overcome by changing the starting address of the device. For example, a dimmer rack with 30 dimmers, with a start address of 31, will look at all dimmers between 31 and 60 (inclusive). You can have multiple devices using the same address though, as the protocol is mainly one way