DMX (Digital Multiplex) is a lighting protocol commonly used in stage lighting, concerts, nightclubs, and other entertainment or professional lighting applications. DMX512 is the most common version of this protocol, first developed in 1986.

Key Features

DMX sends signals from a controller (usually a lighting console or computer) to one or more DMX-compatible lighting fixtures.
512 different "channels" can be controlled over a single DMX line. Each channel can control a different parameter of a fixture (e.g., brightness, color, strobe effect, etc.).

Operating Principle

The DMX system operates between a "master" device and one or more "slave" devices. The master device is typically a DMX console or software that generates all control signals.
The master device sends a value between 0 and 255 for each channel. These values are used by the fixture for various functions (e.g., brightness of an LED or position of a motor).
DMX uses the RS-485 serial connection standard. Cable connections for communication use three-pin XLR-3, XLR-5 sockets, or screw terminal blocks.
 

Applications

Stage and theater lighting
Architectural lighting projects
Concerts and events
Club and bar lighting
TV studios
DMX provides users with full and dynamic control, but the setup and use of the system can be somewhat complex. Especially in large systems with many fixtures and channels, DMX addressing and management require careful attention and expertise.

DMX512 is a standard for digital communication networks that are commonly used to control lighting and effects. Initially designed as a standardized method for controlling stage lighting dimmers, which had previously used various incompatible proprietary protocols, DMX512 quickly became the primary method for connecting control devices (such as a lighting console) to dimmers, smoke machines, and special effects lights.

DMX512 has also begun to be used in non-theatrical indoor and architectural lighting, from decorative lighting for light strings to electronic billboards and stadium or arena concerts. It can now be used to control almost anything, reflecting its popularity in all types of venues.

DMX512 uses unidirectional EIA-485 (RS-485) differential signaling at the physical layer and combines this with a variable-length, packet-based communication protocol. DMX512 does not include automatic error checking and correction, so it is not suitable for control applications where errors could lead to dangerous situations, such as pyrotechnics or the movement of theatrical stage equipment. However, it is still used for such applications. Errors can be created by incorrect triggering, electromagnetic interference, static electricity discharge, improper cable termination, excessively long cables, or low-quality cables.

DMX Network Topology

A DMX512 network typically uses a multi-drop bus topology with multiple nodes, often referred to as a daisy chain. A network consists of a single DMX512 control device – the master controller of the network – and one or more slave devices. For example, a lighting console is often used as the control device for a network of fixtures such as dimmers, smoke machines, and intelligent lights.

In this daisy chain structure, a terminator must be connected to the unused "OUT" connector of the last device in the chain. The terminator is a type of male connector with a 120-ohm resistance. This resistance matches the impedance (resistance) of the cables in the network, making data transmission more stable.

A DMX512 universe, as it is often called, consists of 512 different channels, each with a byte value ranging from 0 to 255. A control device (such as a lighting control console) can manage only one universe with a single OUT connector. However, larger and more complex control devices can control multiple universes simultaneously. Slave devices (such as stage lights) are controlled with different channels from the master control device. Modern control devices can control multiple universes using a single system through the Art-Net protocol and existing Ethernet networks in buildings.

DMX Electrical Connection

DMX512 data is sent over two different cables using a specific voltage level. These electrical characteristics generally comply with the EIA-11-A standard.

A DMX512 network is typically structured to be no longer than 400 meters and can connect up to 32 devices. If more devices need to be connected, the network can be extended using DMX splitters or repeaters. Network cables are terminated with a special termination resistor to prevent signal interference.

DMX512 actually has two sets of cables, but usually only one is used. The second set of cables is currently not defined for any specific function.

Regarding electrical connections, the DMX512 control device's output port should have a low-impedance connection. Receivers should have a high-impedance connection.

Additionally, grounding should be done at only one point to avoid problems in the system. There are strict restrictions on grounding options, and they must be followed strictly.

DMX Cabling

Cables used for DMX512 networks typically use XLR5 type connectors. One end has a male connector, and the other end has a female connector. The male connector is connected to the transmitting device (i.e., the control device), and the female connector is connected to the receiving device (such as a light or smoke machine).

In 2003, a separate project was launched to develop special cabling standards for DMX512. Two different cabling standards were created for portable and permanent installations to meet different requirements for different usage scenarios.

The electrical characteristics of the cables are determined by impedance and capacitance. The impedance of suitable cables for DMX512 is generally 120 ohms. Additionally, Cat5 type cables, commonly used for network and telecommunications, have been tested and found suitable for DMX512. However, microphone and audio cables do not have the necessary electrical characteristics for DMX512 and should not be used. Such incorrect cable selections can lead to irregular operation or errors in the system.

DMX Protocol

The DMX512 control device sends serial data at a speed of 250 kbit/s in the data link layer. The data format is fixed, consisting of a start bit, eight data bits (least significant bit first), two stop bits, and no parity bit.

Each data packet includes the following:

Break state

Mark-After-Break

Channel 0 containing the Start Code

Up to 512 channels, each containing one byte of data

The beginning of a data packet is indicated by a "break" followed by a "mark" (a logical one); this is called "Mark After Break"