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

DMX512 began to be used in non-theatrical indoor and architectural lighting, ranging from strings of lights for decorative lighting to electronic billboards and concerts in stadiums or arenas. It can now be used to control almost anything, reflecting its popularity in venues of all kinds.

DMX512 uses one-way EIA-485 (RS-485) differential signaling at the physical layer and combines it with a variable-sized, packet-based communication protocol. DMX512 does not include automatic error checking and correction and is therefore not a suitable control for hazardous applications like fireworks or the motion of theatrical stage equipment. However, it is still used for such applications. False triggering can be caused by electromagnetic interference, electrostatic discharges, incorrect cable termination, overly 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 main controller of the network – and one or more slave devices. For instance, a lighting console is often used as the control device for a network of slave devices like dimmers, fog machines, and intelligent lights.

This network structure called “Daisy chain” requires a terminator “terminator” to be connected to the “OUT” connector of the last device in this network structure. 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 network is often referred to as a “DMX universe”. A universe consists of 512 different channels, and each channel has a byte value between 0 and 255. A control device (like a lighting control desk) can manage only one universe with a single OUT connector. However, larger and more complex control devices can control multiple universes simultaneously. Slave devices (like stage lights) are managed by different channels coming from the master control device. Modern control devices can control multiple universes with a single system thanks to the Art-Net protocol and the existing Ethernet network in buildings.

DMX Electrical Connection

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

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

DMX512 actually has two sets of cables, but usually, only one is used. The second set of cables is currently undefined for a specific function.

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

Furthermore, grounding should be done at only one point to prevent problems in the system. There are very strict restrictions on grounding options and must be strictly adhered to.

DMX Wiring

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 connects to the transmitting device (i.e., the control device), and the female connector connects to the receiving device (like a light or fog machine).

In 2003, a separate project was launched to develop special wiring standards for DMX512. Accordingly, two different wiring standards have been created for portable and permanent installations. This was done to meet requirements according to different usage scenarios.

The electrical properties of the cables are determined by impedance and capacitance. Cables suitable for DMX512 should typically have an impedance of 120 ohms. Also, Cat5 type cables commonly used for networking and telecommunications have been tested to be suitable for DMX512. However, microphone and audio cables do not have the necessary electrical properties for DMX512 and should not be used. Such wrong cable choices can lead to irregular operation of the system or errors.

DMX Protocol

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

Each data packet contains:

• A Break (Break) status
• Mark after the break (Mark-After-Break)
• Channel 0 containing Start Code (Start Code)
• Up to 512 channels, each containing one byte of data

The start of the data packet is indicated by a “break” and a following “mark”; this is called “Mark After Break” (MAB). This break status marks the end of one packet and the beginning of a new packet. This triggers receivers to start receiving data and provides a frame (position reference) for the data bytes within the packet. The framed data bytes are called “slots”. Following the break status, up to 513 slots can be sent.

The first slot is reserved for a “Start Code” that indicates the type of data in the packet. The 0x00 (hexadecimal zero) start code is the standard value for most lighting hardware and dimmers. Other start codes are used for text packets, system information packets, the RDM extension of DMX, and various proprietary systems. ESTA maintains a database of alternative start codes.

All the slots following the start code are used to control connected devices, such as lighting equipment. The position of a slot within the packet determines which device and function will be controlled, and the data value contained within the slot indicates the type of control to be executed.

DMX Field Application

For the DMX512 system, a 120 Ohm resistor needs to be attached to the very end of the signal line. If this resistor is not correctly installed, issues like flickering lights, uncontrollable movements, or random undesired effects may occur.

Different devices use different methods for termination. Some terminate automatically, while others use a switch. Other devices require the user to manually attach a terminator, such as a resistive XLR-5 male plug.

Therefore, it is crucial for users to check whether their devices have automatic or manual termination features. If they do not check, they might accidentally terminate the line multiple times or not at all, which can lead to problems.

Moreover, if the DMX line is not terminated properly, it often leads to cable issues. For instance, if the ‘Data -‘ cable gets damaged, the terminator might partially function, and the problem becomes evident as soon as the terminator is attached.