Digital Multiplex - AbsoluteAstronomy.com

DMX512 is a standard for digital communication networks that are commonly used to control stage lighting

Stage lighting

Modern stage lighting is a flexible tool in the production of theatre, dance, opera and other performance arts. Several different types of stage lighting instruments are used in the pursuit of the various principles or goals of lighting. Stage lighting has grown considerably in recent years...

and effects. It was originally intended as a standardized method for controlling light dimmers, which, prior to DMX512, had employed various incompatible proprietary protocol

Proprietary protocol

In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.-Enforcement:Proprietors may enforce restrictions through patents and by keeping the protocol specification a trade secret...

s. However, it soon became the primary method for linking not only controllers and dimmers, but also more advanced fixtures and special effects devices such as fog machine

Fog machine

A fog machine or smoke machine is a device which emits a dense vapour that appears similar to fog or smoke. This artificial fog is most commonly used in professional entertainment applications, but smaller, more affordable fog machines are becoming common for personal use. Fog machines can also be...

s and moving lights, and has also expanded to uses in non-theatrical interior and architectural lighting; DMX512 has been used at scales ranging from strings of Christmas lights to electronic billboards.

DMX512 employs EIA-485 differential signaling at its physical layer, in conjunction with a variable-size, packet based communication protocol. It is unidirectional.

DMX512 does not include automatic error checking and correction, and so is not an appropriate control for hazardous applications, such as pyrotechnics

Pyrotechnics

Pyrotechnics is the science of using materials capable of undergoing self-contained and self-sustained exothermic chemical reactions for the production of heat, light, gas, smoke and/or sound...

or movement of theatrical rigging. False triggering may be caused by electromagnetic interference

Electromagnetic interference

Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...

, static electricity

Static electricity

Static electricity refers to the build-up of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. Static electricity can be contrasted with current electricity, which can be delivered...

discharges, improper cable termination

Electrical termination

Electrical termination of a signal involves providing a terminator at the end of a wire or cable to prevent an RF signal from being reflected back from the end, causing interference...

, excessively long cables, or poor quality cables.

DMX512

Developed by the Engineering Commission of United States Institute for Theatre Technology

United States Institute for Theatre Technology

The United States Institute for Theatre Technology is a membership organization which aims to advance the skills and knowledge of theatre, entertainment and performing arts professionals involved in the areas of design, production and technology, and to generally promote their interests...

(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 Entertainment Services and Technology Association

Entertainment Services and Technology Association

The Entertainment Services and Technology Association is a live entertainment industry organization. From their website:"ESTA is a non-profit trade association representing the entertainment technology industry dedicated to a core mission of Building the Business of Show Business...

(ESTA) began a revision process to develop the standard as an ANSI

Ansi

Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....

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

American National Standards Institute

The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...

(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", and is maintained by ESTA.

Network topology

A DMX512 network employs a multi-drop bus topology with nodes strung together in what is commonly called a daisy chain

Daisy chain (electrical engineering)

In electrical and electronic engineering a daisy chain is a wiring scheme in which multiple devices are wired together in sequence or in a ring...

. A network consists of a single DMX512 controller—which is the sole 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 dimmer

Dimmer

Dimmers are devices used to vary the brightness of a light. By decreasing or increasing the RMS voltage and, hence, the mean power to the lamp, it is possible to vary the intensity of the light output...

s, fog machine

Fog machine

s and intelligent moving lights

Intelligent lighting

Intelligent lighting refers to stage lighting that has automated or mechanical abilities beyond those of traditional, stationary illumination. Although the most advanced intelligent lights can produce extraordinarily complex effects, the intelligence lies with the programmer of the show rather...

.

Each slave device has a DMX512 "IN" connector and, in most cases, a DMX512 "OUT" connector (sometimes marked "THRU") as well. The controller, which 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. The final, empty, OUT or THRU connector of the last slave on the daisy chain should have a terminator plugged into it.

A terminator is a stand-alone male connector with a built-in 120 Ω

Ohm

The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...

resistor — matching the cable characteristic impedance

Characteristic impedance

The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...

— connected across the primary data signal pair. If a secondary data pair is used, then another termination resistor is connected across it as well. Although simple systems, i.e., systems having few devices and short cable runs, may work without a terminator, the standard does require its use.
Some DMX 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.

Each DMX network is called a "DMX universe". Large control desks (operator consoles) may have the capacity to control multiple universes, with an OUT connector provided for each universe.

Electrical

DMX512 data are sent using EIA-485 voltage levels. However, quoting from E1.11, "The electrical specifications of this Standard are those of EIA-485-A, except where specifically stated in this document. Where a conflict between EIA-485-A and this document exists, this document is controlling as far as this Standard is concerned."

DMX512 is a bus network

Bus network

A bus network topology is a network architecture in which a set of clients are connected via a shared communications line, called a bus. There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet...

no more than 1200 metres (3,937 ft) long, with not more than 32 devices on a single bus. If more than 32 devices need to communicate, the network can be expanded across parallel buses using DMX splitters. Network wiring consists of a shielded twisted pair

Twisted pair

Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...

, 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. DMX-512 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.

Some equipment manufacturers such as Chauvet and American DJ have disregarded the formal topology rules, and designed their equipment to use nonstandard 3-pin XLR connectors rather than the proper 5-pin DMX connectors, so as to eliminate the need for the unused second pair and allow for the use of regular microphone audio cables. Similarly, end users can create adapter pigtails to convert from the correct 5-pin DMX to a 3-pin XLR microphone cable. For short cable runs of less than about 45 metres (147.6 ft) with only a few devices, it is sometimes possible to operate without termination. At short distances, cables with higher capacitance and different characteristic impedance such as microphone cable can be used. As the cable length and/or number of devices increases however, following the specification for termination and correct cable impedance becomes of vital importance.

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. Systems with the transmitter port and all receiver ports isolated are fairly common. The standard allows for non-isolated receivers.

Good practice dictates that systems ground the signal common at only one point, in order to avoid the formation of disruptive ground loop

Ground loop (electricity)

In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...

s.

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

DMX512 1990 specifies that where connectors are used, the data link shall use five-pin XLR

XLR connector

The XLR connector is a style of electrical connector, primarily found on professional audio, video, and stage lighting equipment. The connectors are circular in design and have between 3 and 7 pins...

style electrical connector

Electrical connector

An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...

s (XLR-5), with female connectors used on transmitting (OUT) ports and male connectors on receiving ports. The use of a 3-pin XLR connector is specifically prohibited.

DMX512-A (ANSI E1.11-2008) allows the use of eight-pin modular (RJ-45) connectors for fixed installations where regular plugging and unplugging of equipment is not required.

Some DMX512 equipment manufacturers at the dawn of the DMX era employed non-compliant or proprietary connectors and pinouts; eventually, the most common of these became the already common three-pin XLR connector (also called cannon jack in some countries), since the electrical specification currently only defines a purpose for a single wire pair. There may be a risk of equipment damage if XLR 3-pin carrying DMX signal is plugged into an audio signal chain, but nevertheless some lighting fixtures and controllers are fitted with 3-pin XLR connectors exactly like the ones used in audio signal chains. These are primarily cheap or very old designs. Furthermore, some of these non-standard items have an inverse polarity for the data signal, requiring the signal wires to be swapped over.

Also, devices are sometimes fitted with four-pin connectors when both communications and power are sent through a common cable. Note also that non-theatrical uses of DMX512 such as architectural lighting often use non-standard connectors.

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

Category 5 cable

Category 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...

(Cat5) twisted pair

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

Public switched telephone network

The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...

phone jack.

Cabling

The cable has a male connector on one end and a female connector on the other end. The male connector plugs into the transmitting, female jack and the female connector plugs into the receiving, male jack.

Cabling for DMX512 was removed from the 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 120 ohms. Cat5 cable

Category 5 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.

Protocol

At the datalink layer, a DMX512 controller transmits asynchronous serial data at 250 kbaud

Baud

In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...

. The data format is fixed at one start bit, eight data bits, two stop bits and no parity

Parity bit

A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....

.

The start of a packet is signified by a break followed by a "mark" (a logical one), known as the "Mark After Break" (MAB). The break, which signals the end of one packet and the start of another, causes receivers to start reception and also serves as a frame (position reference) for data bytes within the packet. Framed data bytes are known as slots. Following the break, up to 513 slots are sent.

The first slot is reserved for a "Start Code" that specifies the type of data in the packet. A start code of 0x00 (hexadecimal

Hexadecimal

In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

zero) is the standard value used for all DMX512 compatible devices, which includes most lighting fixtures and dimmers. Other start codes are used for Text packets (0x17), System Information Packets (0xCF), for the RDM

RDM (lighting)

Remote Device Management or RDM is a protocol enhancement to USITT DMX512 that allows bi-directional communication between a lighting or system controller and attached RDM compliant devices over a standard DMX line...

extension to DMX (0xCC), and various proprietary systems. ESTA maintains a database of alternate start codes.

All slots following the start code contain control settings for slave devices. A slot's position within the packet determines the device and function to be controlled, while its data value specifies the control setpoint. Multi-byte data values are conveyed in little endian

Endianness

In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...

format in adjacent slots.

Timing

DMX512 timing parameters are allowed to 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.

--	Min Break (μs)	Min MAB (μs)
Transmitted	92	12
Receiver recognize	88	8

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

Refresh rate

The refresh rate is the number of times in a second that a display hardware draws the data...

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 a short as 1196 μs. 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. More information on the DMX Packet is well described here http://www.dmx512-online.com/packt.html.

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.

8-bit versus 16-bit

DMX does not mandate a method of 16-bit encoding for Null Start Code packets, however many moving lights make use of encoding larger than 8 bit numbers. To control position more accurately, some fixtures use two channels each for pan and tilt. This gives a 16-bit value range of 65536, permitting accuracies for each axis down to 0.007° (446°/65536).

DMX in practice

DMX512's popularity is partly due to its robustness. The cable can be abused without any loss of function in ways that would render Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

or other high speed data cables useless, although cable faults can occasionally lead to intermittent problems such as random triggering. Unexpected fixture behavior is caused by addressing errors, cable faults, or incorrect data from the controller.

Secondary data link

Although the two secondary link pins of the five-pin XLR connector were originally intended to support a second DMX512 universe, many other proprietary uses have been implemented for these pins. For example, some DMX512 equipment manufacturers used these pins to carry power (typically 24 volts DC) for their proprietary equipment, even though this violated the DMX512 specification and would potentially damage other, standard DMX512 devices on the network. Consequently the standard practice is now to send additional universes on separate connectors and leave the secondary data link pins unused.

Some manufacturers made units with three-pin connectors because of their lower cost, even though DMX512-A specifies that the connector must be a five-pin XLR connector. There is good reason for this rule: a three-pin XLR can easily be connected to a sound board

Mixing console

In professional audio, a mixing console, or audio mixer, also called a sound board, mixing desk, or mixer is an electronic device for combining , routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer...

. If a DMX512 controller or device were to be accidentally connected to a sound board that is generating 48 volt phantom power

Phantom power

Phantom power, in the context of professional audio equipment, is a method for transmitting DC electric power through microphone cables to operate microphones that contain active electronic circuitry....

, the phantom power sent along the cable could potentially damage the DMX512 equipment.

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.

Although wireless DMX512 networks can function over distances exceeding 3000 feet (914.4 m) under ideal conditions, most wireless DMX512 links are limited to a maximum distance of 1000–1500 ft (304.8–457.2 ) to ensure reliable operation. Early wireless DMX512 systems typically used WLAN

Wireless LAN

A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...

technology. Later generations used frequency-hopping spread spectrum

Frequency-hopping spread spectrum

Frequency-hopping spread spectrum is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver...

(FHSS) technology to attain more reliable data transfers, although 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 and cognitive coexistence, a technique to detect and avoid surrounding wireless systems, to avoid transmitting on occupied frequencies.

Development

Many alternatives to DMX512 have been proposed to address perceived limitations such as the maximum slot count of 512 per universe, the unidirectional signal, and the lack of inherent error detection. The 2004 DMX512-A revision added a System Information Packet (SIP). This packet can be interleaved with Null packets. One feature of SIPs is they allow check sums to be sent for DMX Null data. However, SIPs have rarely been implemented.

The 2004 DMX512, A revision of DMX512, also lays the foundation for the RDM (Remote Device Management) protocol through the definition of Enhanced Functionality. RDM allows for diagnostic feedback from fixtures to the controller by extending the DMX512 standard to encompass bidirectional communication between the lighting controller and lighting fixtures. RDM was approved by ANSI in 2006 and is rapidly gaining popularity.

An Ethernet based protocol can distribute multiple DMX universes through a single cable from a control location to breakout boxes closer to fixtures. These boxes then output the conventional DMX512 signal. ANSI E1.31—2009 Entertainment Technology—Lightweight streaming protocol for transport of DMX512 using ACN, published May 4, 2009, and Art-Net

Art-Net

Art-Net is a proprietary protocol for transmitting the lighting control protocol DMX512-A over the User Datagram Protocol of the TCP/IP suite. The protocol was developed by Wayne Howell and his company, , and is open for implementation with attribution but without charge, and make available a...

are two free-to-use protocols used to achieve this.

External links

Technical Standards Program from ESTA
Entertainment Services and Technology Association
The Entertainment Services and Technology Association is a live entertainment industry organization. From their website:"ESTA is a non-profit trade association representing the entertainment technology industry dedicated to a core mission of Building the Business of Show Business...
USITT
OpenDMX.net

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.