WHAT IS DMX?
DMX was developed in the United States for stage equipment in 1986. DMX (Digital MultipleX) is a lighting control protocol, origins from the entertainment industry, which is capable of transmitting the levels of up to 512 channels very rapidly, updating many times per second. The DMX control protocol is digital, which means that it sends the level values for each channel as a stream of binary data using a single balanced cable. As such it is ideal for controlling arrays of colour-changing luminaires, where a luminaire may typically require three channels of control at a time: one for the RED level, one for the GREEN level and one for the BLUE level . Therefore to instruct a luminaire to emit purple light, for example, the red and blue channels would be set to a high level whilst the green channel would be instructed to be at zero brightness. The DMX signal is used for controlling a driver, and is not directly connected to the actual LEDs themselves because it is not a power signal. Regulated power for the LEDS (or other lamp types) is provided by the driver or control gear, which may be built into a luminaire or which may be remote.
DMX is wired in a daisy-chain topology, of up to 300m long with the “far end” terminated by connecting a 120R
resistor between the + and – data conductors. There can be upto 32 “unit loads” connected to the DMX bus, where
a typical luminaire has a unit load of “one”. For longer runs, or more luminaires, DMX repeaters/splitters or signal
amplifiers are required. Some products, such as the Floodline RGB from Sylvania have a lower unit load, enabling
longer chains of luminaires.
HOW DOES RGB COLOUR MIXING WORK?
RGB colour mixing is an additive process: optically combining the output of red, green and blue coloured LEDs to create the desired colour. A typical control system can vary the intensity of each of the three channels in steps of 1/255th of full-brightness, leading to 255 red x 255 blue x 255 green combinations, or a spectrum of 16.5 million possible colours.
- Usable data transmission rate: 250 000 bit/s
- Refresh rate: 30 times per second
- Max. 512 channels (addresses) per universe (control circuit)
- Max. of 32 luminaires directly one after the other; for more luminaires, a splitter is required
- Unidirectional: no information is reported
- Two-wire control line (shielded, terminal resistance)
- Application: illumination of façades (large number of lighting points, dynamic light)
FUNDAMENTALS OF DMX512
DMX512A (Digital Multiplex) is a standard for digital communication networks to control lighting and effects such as fog machines and moving lights. DMX512 employs EIA-485 differential signaling at its two-wire physical layer, in conjunction with a variable-size, packet-based communication protocol at 250 kBit/s. It is unidirectional and does
not include automatic error checking and correction. DMX512 is the most used protocol type in lighting control. DMX512 lines can be daisy-chained, the usual connectors are XLR5, XLR3 or RJ45 in professional
environments. The XLR3 plug type is not recommended. XLR5 and RJ45 connections can link two DMX512 lines and so two universes.
A DMX512 data stream in one DMX512 universe can be seen as a sequence of up to 512 data frames (slots) for up to 512 channels. One channel may be a LED, a ballast load or lamp. In one DMX512 message all 512 channels may be sent, or only channels that have changed.
When controlling RGB LEDs every pixel consists of three LEDs and so require three channels from the DMX512 stream to control one triplet.
DMX512 is a channel-based protocol without flow control. One DMX512 channel which means one data frame in the serial stream, controls one recipient and transmits data in eight bits giving 256 steps. Other device types, like effects, beamers or interfaces to other protocols, may use their special channel mapping. Some use three, nine or 15 channels. For three-channel RGB LED fixtures the maximum number of RGB pixels is 170 (512 / 3). Due to the transmission rate of 250 kBit/s the maximum possible refresh rate is ca. 40 Hz. A special feature for DMX512 fixtures is a process called auto addressing. Typically a fixture has a start and end address in the DMX512 data stream, up to 512 addresses in one DMX line, also called a DMX universe, are available. For this reason, the fixture can be adjusted to a fixed start address. Auto addressing assumes that the first channel of the DMX stream is always the start address, the device takes the first addresses for itself and re-addresses the remaining upper channels before they are sent to DMX OUT. Then, the next fixture in the DMX512 line again receives the DMX512 data channels usually beginning with DMX512 address ‘1’.
A DMX512 controller is connected to an EIA-485 rated cable (typically 100-120ohm) in a daisy chain. Category 5e unshielded twisted pair (UTP) cable is also permitted. At the end of the chain of devices the cable is terminated with a resistor that matches the impedance of the cable. In many cases, this termination is built into the receiving device. Plugged connections use 5-Pin XLR connectors. 3-Pin XLR connectors are specifically prohibited in the standard.
The standard excludes other topologies (e.g., stars, or trees). If the physical requirements of a system do not allow for a daisy chain, then DMX512 splitters (also called amplifiers or repeaters) are used.
The use of a high quality continuous EIA-485-A 120 ohm nominal impedance cable of minimal dc resistance theoretically allows for runs as long as 1 kilometer (3281 feet). In practice, runs are typically limited
to ≤500m (1500 ft). If longer runs are required, a signal amplifier (repeater) should be used. ESTA also publishes documents to address cabling; ANSI E1.27-1 – 2006, Entertainment Technology-Standard for Portable Control Cables for Use with USITT DMX512/1990 and E1.11 (DMX512-A) Products and ANSI E1.27-2 – 2009, Entertainment Technology Recommended Practice for Permanently Installed Control Cables for Use with ANSI E1.11 (DMX512-A) and USITT DMX512/1990 Products.
Interoperation with other protocols
Typically interoperation between DMX512-A and other protocols is handled by protocol converters (gateways). Proprietary and open protocols exist which allow DMX512 to be carried over networks, most commonly Ethernet-TCP/IP. ESTA recently published ANSI E1.31, Lightweight Streaming Protocol for Transport of DMX512 using ACN. E1.31 describes a method of transporting DMX512-type data over networks.
Zumtobel’s – The Lighting Handbook | 4th edition, revised and updated: October 2013
Traxon’s – Solution Guide | Traxon Technologies Solution Guide 2014
Sylvania’s – Lighting Controls Catalogue | Lighting Controls Catalogue 2013