DMX512-A

In 1998 the Entertainment Services and Technology Association (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".

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 400 metres (1,300 ft) 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 120 ohms, 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 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 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 which are commonly used with EIA485 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, and secondly encourages the use of cable following analogue audio specifications for DMX, which can 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.
Note: 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:

Cabling

Cable built to the DMX512A specification

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.^[6] 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.^[7]

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 120 ohms. Cat5 cable, commonly used for networking and telecommunications, has been tested by ESTA for use with DMX512A. Also, cables designed for EIA485 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.^[8]

Timing

DMX512 timing parameters may vary over a wide range. The original authors specified the standard this way to provide the greatest design flexibility. Because of this, however, it was difficult to design receivers that operated over the entire timing range. As a result of this difficulty, the timing specification of the original 1986 standard was changed in 1990. Specifically, the MAB, which was originally fixed at 4 μs, was changed to 8 μs, minimum. The E1.11 (2004) standard relaxed the transmitter and receiver timing specifications. This relaxed the timing requirements for systems using controllers built to DMX512-A (E1.11); however, a significant number of legacy devices still employ transmit timing near the minimum end of the range.

Maximum times are not specified because as long as a packet is sent at least once per second, the BREAK, MAB, inter-slot time, and the mark between the last slot of the packet and the break (MBB) can be as long as desired.

A maximum-sized packet, which has 512 channels (slots following the start code), takes approximately 23 ms to send, corresponding to a maximum refresh rate of about 44 Hz. For higher refresh rates, packets having fewer than 512 channels can be sent.

The standard does not specify the minimum number of slots that can be sent in a packet. However, it does require that packets be transmitted so that the leading edges of any two sequential BREAKs must be separated by at least 1204 μs, and receivers must be able to handle packets with break-to-break times as short as 1196 μs.^[11] The minimum break-to-break transmit time can be achieved by sending packets that contain at least 24 slots (by adding extra padding bytes, if necessary) or by stretching parameters such as the BREAK, MAB, Interslot, or Interpacket times.^[12]

Addressing and data encoding

Most data is sent with the default Null Start Code of 00h. Quoting from the standard:

Dimmer packs or racks use a group of slots to determine the levels for their dimmers. Typically a dimmer has a starting address that represents the lowest numbered dimmer in that pack, and the addressing increases from there to the highest numbered dimmer. As an example, for two packs of six dimmers each, the first pack would start at address 1 and the second pack at address 7. Each slot in the DMX512 packet corresponds to one dimmer.

Secondary data link

The Singapore Flyer uses wireless DMX to control the lighting on the pods and rim.^[13]

In the 1986 and 1990 standards the use of the second data pair is not defined other than as an ‘optional second data link’. Both unidirectional and bidirectional use were envisioned. Other proprietary uses have been implemented for these pins. Schemes that use voltage outside of the range allowed by EIA485 are disallowed. Guidance on allowed usage can be found in Annex B of E1.11. Current standard practice is to leave the secondary data link pins unused.

Connectors

DMX512-A specifies that the connector must be a five-pin XLR connector.

DMX512-A uses a single pair of conductors, so can be connected using the cheaper 3-pin XLR connectors. Some manufacturers made units with three-pin XLR connectors, because of their lower cost. However, as 3-pin XLRs are commonly used for connecting microphones and sound mixing consoles, there is a risk of wrongly connecting DMX512 equipment to microphones and other sound equipment. The +48 volt phantom power emitted by mixing consoles could damage DMX512 equipment if connected to it. The DMX512 signals emitted by lighting desks can damage microphones and other sound equipment if connected to it. As a result, the best practice is to use only 5-pin XLRs for DMX512 signals, to avoid risk of confusion with connectors used for sound signals.

Termination

The DMX512 signal lines require a single 120 Ohm termination resistor to be fitted at the extreme end of the signal cable.
Some of the more common symptoms of improper termination are flashing, uncontrollable or incorrect light operation, or other undesired random special effects.

Some equipment has automatic termination, others a physical switch, while the remainder requires a physical terminator (e.g. male XLR-5 plug fitted with a resistor) to be installed by the user.

It is important for users to check whether their devices have automatic or switched termination, as otherwise they may end up with the DMX line being terminated multiple times or not at all when they believed it to be correct.

Additionally, terminating the DMX line often exposes physical cable faults - for example, if the "Data −" wire is broken, an unterminated DMX run may partially work, while fitting the terminator immediately exposes the problem.

Wireless operation

Recently, wireless DMX512 adapters have become popular, especially in architectural lighting installations where cable lengths can be prohibitively long. Such networks typically employ a wireless transmitter at the controller, with strategically placed receivers near the fixtures to convert the wireless signal back to conventional DMX512 wired network signals or wireless receivers built into the individual fixtures.

Although wireless DMX512 networks can function over distances exceeding 3,000 feet (910 m) under ideal conditions, most wireless DMX512 links are limited to a maximum distance of 1,000–1,500 feet (300–460 m) to ensure reliable operation. The first commercially marketed wireless DMX512 system was based on frequency-hopping spread spectrum (FHSS) technology using commercial wireless modems.^[14] Other later-generation systems still used frequency-hopping spread spectrum (FHSS) technology, but at higher bandwidth. FHSS systems tend to disturb other types of wireless communication systems such as WiFi/WLAN. This has been solved in newer wireless DMX systems by using adaptive frequency hopping, a technique to detect and avoid surrounding wireless systems, to avoid transmitting on occupied frequencies.^[15]

Multiple incompatible wireless protocols currently exist. While DMX-over-Ethernet protocols such as E1.31 - Streaming ACN can be used to send DMX data over WiFi, this is not generally recommended due to the highly variable latency of WiFi.