DMX512 is a standard for digital communication networks that are commonly used to control lighting and effects. It was originally intended as a standardized method for controlling stage lighting dimmers, which, prior to DMX512, had employed various incompatible proprietary protocols. It quickly became the primary method for linking controllers (such as a lighting console) to dimmers and special effects devices such as fog machines and intelligent lights.
DMX512 has also expanded to use in non-theatrical interior and architectural lighting, at scales ranging from strings of Christmas lights 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 a unidirectional EIA-485 (RS-485) differential signaling at its physical layer, in conjunction with a variable-size, packet-based communication protocol. DMX512 does not include automatic error checking and correction and therefore is not an appropriate control for hazardous applications,United States Institute for Theatre Technology -- Resources > Standards > DMX512 > DMX512 FAQ -- http://www.usitt.org/Resources/Standards2/DMX512/DMX512FAQ#a12 -- ...DMX512 is not an appropriate control protocol for hazardous applications... such as pyrotechnics or movement of theatrical rigging. However, it is still used for such applications. False triggering may be caused by electromagnetic interference, static electricity discharges, improper cable termination, excessively long cables, or poor-quality cables.
The DMX standard is published by the Entertainment Services and Technology Association (ESTA), and can be downloaded from its website.
History Developed by the Engineering Commission of United States Institute for Theatre Technology (USITT), the DMX512 standard (for digital multiplex with 512 pieces of information) was created in 1986, with subsequent revisions in 1990 leading to USITT DMX512/1990.
DMX512-A In 1998, the ESTA began a revision process to develop the standard as an ANSI standard. The resulting revised standard, known officially as "Entertainment Technology—USITT DMX512-A—Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories", was approved by the American National Standards Institute (ANSI) in November 2004. It was revised again in 2008, and is the current standard known as "E1.11 – 2008, USITT DMX512-A", or just "DMX512-A".
Network topology A DMX512 network employs a multi-drop bus topology with nodes strung together in what is commonly called a daisy chain. A network consists of a single DMX512 controller – which is the master of the network — and one or more slave devices. For example, a lighting console is frequently employed as the controller for a network of slave devices such as dimmers, fog machines and intelligent lights.
Each slave device has a DMX512 "IN" connector and usually an "OUT" (or "THRU") connector as well. The controller, which usually has only an OUT connector, is connected via a DMX512 cable to the IN connector of the first slave. A second cable then links the OUT or THRU connector of the first slave to the IN connector of the next slave in the chain, and so on. For example, the block diagram below shows a simple network consisting of a controller and three slaves.
The specification requires a 'terminator' to be connected to the final OUT or THRU connector of the last slave on the daisy chain, which would otherwise be unconnected. A terminator is a stand-alone male connector with an integral resistor connected across the primary data signal pair; this resistor matches the cable's characteristic impedance. If a secondary data pair is used, a termination resistor is connected across it as well. Although simple systems (i.e., systems having few devices and short cables) will sometimes function normally without a terminator, the standard requires its use. Some DMX slave devices have built-in terminators that can be manually activated with a mechanical switch or by software, or by automatically sensing the absence of a connected cable.
A DMX512 network is called a "DMX universe". Each OUT connector on a DMX512 controller can control a single universe. A DMX512 universe is made up of 512 channels, with each channel containing a value between 0 and 255. Each slave device in the chain can "look at" a different set of channels in order to be controlled by the master controller. Smaller controllers may have a single OUT connector, enabling them to control only one universe, whereas large control desks (operator consoles) may have the capacity to control multiple universes, with an OUT connector provided for each universe. Many of the more modern control desks, instead of featuring multiple OUT connectors, have an unshielded twisted pair connector (such as Cat 5, Cat 5e or Cat 6). Such cables and systems can control up to 524,288 universes of DMX512 (32,768 subnets × 16 universes per subnet) using the Art-Net IV protocol, or 65,536 universes using the sACN protocol, and the existing Ethernet in buildings.
Physical layer
Electrical DMX512 data is transmitted over a differential pair using EIA-485 voltage levels. DMX512 electrical specifications are identical to those of the EIA-485-A standard, except where stated otherwise in E1.11.
DMX512 is a bus network no more than long, with not more than 32 unit loads (individual devices connected) on a single bus. If more than 32 unit loads need to communicate, the network can be expanded across parallel buses using DMX splitters. Network wiring consists of a shielded twisted pair, with a characteristic impedance of , with a termination resistor at the end of the cable furthest from the controller to absorb signal reflections. DMX512 has two twisted-pair data paths, although the specification currently only defines the use of one of the twisted pairs. The second pair is undefined but required by the electrical specification.
The E1.11 (DMX512 2004) electrical specification addresses the connection of the DMX512 signal common to earth ground. Specifically, the standard recommends that transmitter ports (DMX512 controller OUT port) have a low-impedance connection between signal common and ground; such ports are referred to as grounded. It is further recommended that receivers have a high-impedance connection between signal common and ground; such ports are referred to as isolated.
The standard also allows for isolated transmitter ports and non-isolated receivers. It also recommends that systems ground the signal common at only one point, in order to avoid the formation of disruptive ground loops.
Grounded receivers that have a hard connection between signal common and ground are permitted, but their use is strongly discouraged. Several possible grounding configurations that are commonly used with EIA-485 are specifically disallowed by E1.11.
Connectors The original DMX512 1990 specified that where connectors are used, the data link shall use five-pin XLR style electrical connectors (XLR-5), with female connectors used on transmitting (OUT) ports and male connectors on receiving ports. The use of any other XLR-style connector is prohibited.
The three-pin XLR connector is commonly used for DMX512 on lighting and related control equipment, particularly at the budget/DJ end of the market. However, using three-pin XLR connectors for DMX512 is specifically prohibited by section 7.1.2 of the DMX512 standard. Use of the three-pin XLR in this context firstly presents a risk of damage to the lighting equipment should an audio cable carrying 48-volt phantom power be accidentally connected. Three-pin XLRs designed for audio applications, rather than DMX512 communications, may also lead to signal degradation and unreliable operation of the DMX network.
DMX512-A (ANSI E1.11-2008) defined the use of eight-pin modular (8P8C, or "RJ-45") connectors for fixed installations where regular plugging and unplugging of equipment is not required. Several manufacturers used other pinouts for RJ-45 connectors prior to this inclusion in the standard.
Other form factors of connectors are permitted on equipment where the XLR and RJ-45 would not fit or are considered inappropriate, for example, on equipment intended for permanent installation.
From ANSI E1.11 - 2008 section 7:
XLR-5 pinout Signal Common Data 1- (Primary Data Link) Data 1+ (Primary Data Link) Data 2- (Optional Secondary Data Link) Data 2+ (Optional Secondary Data Link)
RJ-45 pinout Data 1+ Data 1- Data 2+ Not Assigned Not Assigned Data 2- Signal Common (0 V) for Data 1 Signal Common (0 V) for Data 2
The 8P8C modular connector pinout matches the conductor pairing scheme used by Category 5 (Cat5) twisted pair patch cables. The avoidance of pins 4 and 5 helps to prevent equipment damage if the cabling is accidentally plugged into a single-line public switched telephone network phone jack.
Common non-compliant connectors In the early days of digital lighting control, equipment manufacturers employed various connectors and pinouts for their proprietary digital control signals. The most common of these is the three-pin XLR connector. When DMX512 was ratified, many of these manufacturers then issued firmware updates to enable the use of DMX512 control on their existing equipment by the use of a simple adapter to and from the standard 5-pin XLR-style connector.
As the electrical specification currently only defines a purpose for a single wire pair, some equipment manufacturers continue to use it. Such equipment is not compliant with the DMX standard, but may be sufficiently compatible for operation using simple adapters.
There is a risk of equipment damage if 3-pin XLR audio and DMX signals are plugged into each other.
XLR-3 pinout Section 7 of ANSI E1.11 - 2008 prohibits the use of XLR-3 connectors with DMX512. However, in practice, XLR-3 has been used in the lighting industry with the following pinout commonly seen: Ground Data 1- Data 1+
Since DMX512 over XLR-3 is not officially standardised, there may be devices that use other pinouts.
Other RJ-45 pinouts Color Kinetics has their own version of the RJ-45 connector for DMX, which predates the 2008 official inclusion in the DMX512 standard. The pinout specifically for Color Kinetics LED lighting products is:
Data 1- Data 1+ Shield Optional Optional Optional Optional Optional
Cabling
The standard cables used in DMX512 networks employ XLR5 connectors, with a male connector on one end and a female connector on the other end. The cable's male connector attaches to the transmitting female jack (OUT), and its female connector attaches to the receiving male jack (IN).
Cabling for DMX512 was removed from the ANSI E1.11 standard, and a separate cabling standards project was started in 2003. Two cabling standards have been developed, one for portable DMX512 cables (ANSI E1.27-1 – 2006) and one for permanent installations (draft standard BSR E1.27-2). This resolved issues arising from the differences in requirements for cables used in touring shows versus those used for permanent infrastructure.
The electrical characteristics of DMX512 cable are specified in terms of impedance and capacitance, although there are often mechanical and other considerations that must be considered as well. Cable types that are appropriate for DMX512 usage will have a nominal characteristic impedance of . Also, cables designed for EIA-485 typically meet the DMX512 electrical specifications. Conversely, microphone and line-level audio cables lack the requisite electrical characteristics and thus are not suitable for DMX512 cabling. The significantly lower impedance and higher capacitance of these cables distort the DMX512 digital waveforms, which in turn can cause irregular operation or intermittent errors that are difficult to identify and correct.