The DMX Physical Layer

Physical connection to the DMX bus is specified as a 5-pin XLR connector, which uses pins 1,2 and 3 to carry a EI-485 signal and cable shield. Many designs use a 3-pin XLR connector to carry the same set of signals. Some systems use hard-wiring for permanent installations.

Cable for Use in DMX-512

DMX requires a twisted pair cable (2 conductor) cable with a shield (e.g., foil covering), although only one pair (2 conductor).

The cable is used with balanced transmission using EIA-485 sending a sequence of asynchronously timed 8-bit slots. that form a DMX frame.

The type of cable used in DMX-512 has the following electrical properties:

  1. A pair of data conductors that are tightly twisted together over the cable length. The pair of conductors must have a characteristic impedance in the range 100 to 120 ohms, 120 ohms is preferred.
  2. A ground conductor (sometimes called the "drain") that provides a continuous electrical ground along the length of the cable.
  3. A shield that protects the cable from electromagnetic interference. The shield is normally implemented as a foil wrapped around the conductors and in electrical contact with the ground conductor. The standard allows that the shielding to be either around the individual pairs or more typically an overall shielding covering both pairs, or both of these. A cable screen (earthed at the transmitter) helps eliminate radio frequency interference to the bus signals and emissions from the cable. The screen forms a Faraday cage around the pair of transmission lines.
  4. Capacitance between conductors within a shield must not exceed 65 pF/m.
  5. Capacitance between any conductor and the shield must not exceed 115 pF/m.
  6. A second optional pair of data conductors is supported, but defined by the DMX-512 standard.
  7. An outer protective (PVC) coating to ease handling of the cable.

Shielded balanced cable, as used for DMX-512.

Note: Some application demand specific types of cables, e.g. these can combine DC or AC power condictors with the DMX signal. Specific applications can also require cables with an armoured shield to protect the cable from accidentially being cut (this type of cable is less common).

Note on Grounding: Receivers must not connect the signal shield/earth to their local ground. This could otherwise result in a ground-loop, adding noise to the received signal. It is important to only earth one end of the bus to avoid ground loops. DMX-512 optionally does allow the line conductors at the transmitter to be referenced to the ground level, but then requires that the total resistance to ground is less than 20 Ohms. Professional interfaces isolate the Data conductors using opto-isolators.

Maximum Permitted Cable Segment Length

The capacitance and resistance per metre determine the maximum frequency that can be sent by the cable without severe attenuation (known as the cable bandwidth).

Lengths of DMX cable may be plugged together end-to-end up to the maximum permitted cable segment length.

Since DMX operates at a baud rate of 250 k baud, the EIA-485 standard suggests a maximum total segment length of 300m. A smaller the number of receivers reduces the load on the cable. Low resistance cables can be used to drive much greater distances (up to 1000m with a sender and a single remote receiver). However, this may not be possible in practice, and many practical products are unable to manage to drive a cable of even 300m!

DMX 512 Cable Connection

Receivers to be connected to the DMX cable typically have an input and output XLR connector using male and female sockets. The male and female sockets on each receiver are wired together so that the electrical signal passes straight through the equipment, and the shield is continuous. This ensures that the bus works even if any specific piece of equipment is not powered.

The set of receivers on a bus are connected together using cables with one make and one female XLR plug. These cable are wired so that pin 2 connects to pin 2, pin 3 connects to pin 3. The shield (also known as a shield) is wired to pin 1 at both ends of the cable.

Male Female
Pin 1 (screen)----------------------------- Pin 1 (screen)
Pin 2 (Data -)----------------------------- Pin 2 (Data -)
Pin 3 (Data +)----------------------------- Pin 3 (Data +)

Pinout of 3-pin and 5-pin XLR connectors (pins 4,5 are usually unconnected - but can be connected straight through 4 to 4 and 5 to 5).

Male and Female 3-Pin XLR Connectors

Do Not Use Audio Cables!

The primary reason for the DMX specification using a 5-pin XLR connector for DMX cables was to avoid confusion with audio XLR cables that also use a 3-pin connector. Cables that are intended for use in audio systems (such as microphone cables) often have a 3-pin XLR connector and use balanced cables. However, these cables do not have a specification that matches the requirements of DMX.

Audio signals extend to only a few 10s of kHz, and therefore audio cables often attenuate signals at higher frequencies. This can distort the DMX signal (resulting in attenuation and "ringing"). The result is that these cables provide insufficient signal strength for long cable runs.

Audio cables also do not typically include a shield, since they are not impacted by signals above 100 kHz. This makes them also more vulnerable to radio interference

DMX Termination

Cable terminations are important for consistent operation.

3-pin and 5-pin XLR Terminators for DMX-512

The simplest terminator consists of a resistance that matches the cable impedance. This gives the cable the appearance to a node connected that the cable is of infinite length. When the cable is cut to any length (or lengths of cable added to the cable segment) and terminated, measurements will be identical to values obtained from an infinite length cable.

A DMX terminator therefore has a resistance of 120 ohm +5%/-10% impedance placed between Data+ and Data-. This could simply be a male 5 pin or 3 pin XLR plug with a 120 Ohm resistor soldered across pins 2 and 3. A more sophisticated terminartor incorporates a surge suppressor as well as the resistance.

In the case where the transmitter cannot be connected at one end of the link, then both ends of the link need to be terminated (this is required for RDM).

Since the termination resistance is a nominal 120 Ohms and the line voltage is 5V, this requires a resistor power of v^2/R = 5^2/120 = 25/120 = 0.2W.

Resistor and cable tolerances, among other things, can result in mismatches between the cable and termination impedances. This will result in reflections that increase the noise and could ultimately lead to corruption of data. Similar to radiated emissions, the higher the frequency components and the longer the cable, the more likely it is that reflections will affect the performance.

A Multi-Drop Bus supports multiple receivers

The maximum number of receivers that can be placed across the control bus is determined by the receiver input resistance. In DMX-512, the input impedance of 12 k Ohms restricts this to 32 receivers on a single bus. (32 parallel receivers have an overall impedance of 376 Ohms, safe for a cable of length up to 300m.)

A repeater can be used to divide the bus into multiple cable segments (each terminated at the end). This allows an increase in the total distance that can be reached by the network and to support more than 32 receivers using multiple cable segments. The combination of all cables connected to a bus, including

Since the termination resistance is a nominal of 120 Ohms and the transmitter line voltage is 5V, this requires a resistor power of v^2/R = 5^2/120 = 25/120 = 0.2W.

Other Cable Types suitable for DMX

Other types of balanced cables can also be used to support DMX-512 (e.g., permanent installed wiring): Category 5 UTP, Category 5 Shielded UTP STP/FTP, Category 6 UTP and Category 6 FTP, S/FTP, S/STP and Category 7 S/FTP, S/STP.

The DMX specification can be logically divided into a physical and a link layer:

See also:

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2018)