DMX512

A DMX repeater/buffer. It allows many devices that are controlled by DMX to be plugged into one controller, like a lighting console.

DMX512 is a standard for digital communication networks that are commonly used to control stage lighting and effects. It was originally intended as a standardized method for controlling light dimmers which had, prior to DMX512, employed various incompatible, proprietary protocols. 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 machines and moving lights.

DMX512 employs EIA-485 differential signaling at its physical layer, in conjunction with a variable-size, packet based communication protocol. It is unidirectional and does not include automatic error checking and correction. Consequently, it is strongly discouraged for use in safety-critical applications such as controlling pyrotechnics or laser lighting displays, where audience or performer safety is at risk. Instead, MIDI is sometimes used for this task.

History

DMX512

Developed by the Engineering Commission of USITT, the standard was created in 1986, with subsequent revisions in 1990 leading to USITT DMX512/1990.

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. This current standard is also known as "E1.11, USITT DMX512–A", or just "DMX512-A", and is maintained by ESTA.

Physical layer

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 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 dimmers, fog machines and intelligent moving lights.

Each slave device has a DMX512 "IN" connector and, in many case, a DMX512 "OUT" connector (sometimes marked "THRU") as well. The controller, which only has an OUT connector, is connected via a DMX512 cable to the IN connector of the first slave. A second cable then links the OUT connector of the first slave to the IN connector of the next slave in the chain, and so on. The final, empty, OUT connector of the last slave on the daisy chain should have a terminator plugged into it.

A terminator is a stand-alone mating connector with a built-in resistor. The resistor—which matches the impedance of the cabling used (typically 120Ω, ½W)—is 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 reliably without a terminator, it is considered good practice to always use a terminator at the end of the daisy chain. Some DMX devices have built-in terminators that can be manually activated with a mechanical switch or by software, or by automatically sensing the presence of a connected cable.

Each DMX network is called a "DMX universe".^[1] Large control desks (operator consoles) may have the capacity to control multiple universes, with an OUT connector provided for each universe.

Electrical

DMX512 data is sent using EIA-485 voltage levels. This is a bus network not more than 1200 meters long, with not more than 32 devices on a single bus. If more devices need to communicate, the network can be expanded across parallel buses using DMX repeaters.

The bus network ideally consists of two wires in a twisted pair configuration, with 120 ohms of cable impedance, and with termination resistors at both ends of the cable to absorb signal reflections.

However for short cable runs of less than about 200 meters with only a few devices, it is sometimes possible to use non-spec wiring such as XLR microphone cable, and/or without termination resistors, and not experience data errors that can cause devices to function inappropriately. As the cable length and/or number of devices increases however, termination and correct cable impedance becomes more important.

The data is transmitted as a differential signal pair on two separate conductors. Differential-mode noise—which may result in communication errors—can be minimized by keeping the two conductors close to one another and twisted together throughout their cable runs.

The E1.11 (DMX512 2004) electrical specification addresses the connection of DMX512 signal common to Earth ground. Specifically, the standard recommends that transmitter ports (i.e., 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. Conversely, transmitter ports may be isolated and any one receiver port may grounded, with all other receiver ports isolated, but this grounding method is strongly discouraged as it can lead to confusion in all but the simplest DMX512 networks. In all cases, one and only one port should be grounded and all other ports should be isolated so as to avoid the formation of disruptive ground loops.

Connectors

DMX512 1990 specifies that where connectors are used, the data link must use five-pin XLR style electrical connectors (i.e., "XLR-5"), with female connectors used on transmitting (OUT) ports and male connectors on receiving ports. DMX512-A (E1.11) requires the use of an XLR-5 connector unless there is insufficient physical space on the device, in which case an XLR-5 adapter must be supplied. DMX512-A (E1.11-2008) allows the use of eight-pin modular (i.e., RJ-45) connectors for fixed installations where regular plugging and unplugging of equipment is not required (i.e. non-touring systems).

Some DMX512 equipment manufacturers employ non-compliant connectors and pinouts; the most common of these is the three-pin XLR connector, since the electrical specification currently only defines a purpose for a single wire pair. There is risk of equipment damage if a novice unfamiliar with lighting technology accidently plugs XLR 3-pin DMX into an audio device, since the DMX signal voltages are much higher than what audio equipment normally uses.

Also, slave devices are sometimes fitted with four-pin connectors that convey both communications and power through a common cable.

XLR-5 pinout

1. Signal Common

2. Data 1- (Primary Data Link)

3. Data 1+ (Primary Data Link)

4. Data 2- (Optional Secondary Data Link)

5. Data 2+ (Optional Secondary Data Link)

RJ-45 pinout

1. Data 1+

2. Data 1-

3. Data 2+

4. Not Assigned

5. Not Assigned

6. Data 2-

7. Signal Common (0V) for Data 1

8. Signal Common (0V) for Data 2

The RJ-45 connector pinout matches the conductor pairing scheme used by CAT-5 twisted pair patch cables.

Cabling

Cabling for DMX512 was removed from the standard and a separate cabling standards project was started in 2004. 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. CAT-5 cable, which is commonly used for networking and telecommunications, has been tested and approved by ANSI 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. The data format is fixed at one start bit, eight data bits, two stop bits and no parity.

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 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 extension to DMX (0xCC), and various proprietary systems.

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 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 provided some breathing room 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. A packet is required to have at least 24 channels.

Addressing and Data Encoding

Conventional 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. Some dimmers use profiles to interpret the level being received. A linear profile means the output directly corresponds to the received DMX512 level, but other profiles behave differently. A preheat profile might keep the dimmer at a level of 5% until the received DMX512 level exceeds 5%, and respond linearly after that.

Moving lights use adjacent DMX512 channels to control different aspects of their behavior. These attributes may, for example, be laid out as:

1. Intensity
2. Color
3. Gobo
4. Pan
5. Tilt

The gobo channel may allow groups of values to select gobos, i.e. 0-20 No gobo, 21-40 Gobo 1, 41-60 Gobo 2, etc. It may even allow for gobo rotation, i.e. 21-25 Gobo 1 (No rotation), 26-40 Gobo 1 (Slow - Fast rotation). If there are multiple fixtures that require separate control, the starting DMX512 address of each fixture can be set so that there is no overlap. If the DMX512 address of the first fixture is 1 and the DMX512 address of the second fixture is 6, then the situation would be thus:

DMX Address  Fixture  Attribute
 1            1       Intensity
 2            1       Color
 3            1       Gobo
         ...
 6            2       Intensity
 7            2       Color

Modern DMX512 controllers have libraries of data about fixtures telling them how to map attributes to DMX512 channels. The controller could then have separate ways of selecting gobos and gobo rotation, even though on a particular fixture they are controlled by a single DMX512 channel. Although some lights may require different DMX512 values to achieve the same effect, the light operator is presented with a single, consistent control method for all lights. The controller will also work out the correct addresses for the fixtures. If 512 channels will not suffice, then a desk with multiple DMX512 outputs is required. Each output handles a separate 512 channel universe, allowing many more fixtures to be controlled.

The DMX512 output is designed to feed 32 'units' of load. Although a single fixture may represent a fraction of a unit of load, the cabling in between the fixtures can degrade the signal significantly, particularly if it is very long. To deal with this and cable management issues, DMX512 buffers are often used. These have one DMX512 in but many DMX512 outs, all feeding identical data. Each output from the DMX512 buffer can feed 32 units, thereby making it possible to split the signal from a controller to hundreds of fixtures.

It is not recommended to split a DMX512 signal by "Y"ing an output into two inputs. This can cause termination and reflection problems. Any signal arriving at the Y point will be partially reflected and, depending on the final termination resistances, there will be either reflections from the cable ends or an incorrect steady state resistance seen by the controller.

8-bit vs. 16-bit

An 8-bit "instruction" permits only 256 possible values. So, if a single DMX512 channel is used to control pan on a fixture which has 440° of pan, then each pan value increase of 1 would result in a pan movement of 1.7° (446°/256). Over a long throw (the distance between the fixture and the projection surface), this relatively small move can result in significant displacement of the beam.

To control position more accurately, some fixtures use two channels each for pan and tilt. This gives a 16-bit value range of 65,536, permitting accuracies for each axis down to 0.007° (446°/65,536).

Using these types of devices on older lighting controllers would result in two adjacent channel controls being used to adjust a single movement axis. One would be referred to as the coarse and the other as fine, indicating the relative amount of movement control each channel provided. The coarse channel would allow values in multiples of 256, such as 0, 256, 512, 1024, all the way up to 65280. The fine channel allows the addressing of all in-between values, by adding between 0 and 255 to the value obtained by the coarse channel. Thus the fixture's movement can be controlled more accurately.

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 or other high speed data cables useless. Strange behavior of the fixtures is usually due to incorrect addressing, cable faults, or the wrong data from the controller. Cable faults can occasionally lead to intermittent problems such as twitching fixtures.

Although the two secondary data link pins were originally intended for sending a second universe of data, many other uses have been implemented and the general practice is now to send additional universes on separate data links. Some manufacturers made units with three-pin connectors because they are cheaper. DMX512-A specifies that the connector is to be a five-pin XLR connector and cannot be any other kind of XLR connector. There is good reason for this rule: a three-pin XLR can easily be connected to a sound board. If an electronic piece of equipment was accidentally connected to a sound board with phantom power on, the 48 volts of phantom power sent along the cable would probably damage the circuitry inside the light, necessitating the expensive repair or replacement of the light. However, some companies used the extra pins to carry (usually 24 VDC) power anyway, which would again destroy any equipment which used those pins to carry data. For these reasons, DMX512-A forbids using the extra pins to send power or any other use that does not comply with EIA-485 signal levels.

Recently, wireless DMX512 adapters have become popular, especially in architectural lighting installations where cable lengths can be prohibitively long. While wireless EIA-485 signals can be effectively received over distances of 3,000 feet (910 m) or more under ideal conditions, most companies limit their maximum run to 1000 or 1,500 feet (460 m). Wireless DMX512 generally uses WLAN technology to transfer the DMX512 data, with strategically placed converters bridging the signal back to wired links.

Development

Many alternatives to DMX512 have been proposed and implemented to address limitations such as the maximum slot count of 512 per universe, the unidirectional signal, and the lack of inherent error detection. One configuration that has gained popularity is the use of CAT5 to distribute multiple DMX universes through a single cable from a control location to breakout boxes closer to fixtures. These boxes then output the traditional DMX512 signal. Protocols used over the CAT5 are generally proprietary, although ESTA has initiated a project numbered E1.31 to define an interoperable CAT5 transport for DMX512.

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.

References

1. ^ Bennette, Adam (2006). Recommended Practice for DMX512. pg. 89: PLASA. ISBN 978-0-9557035-2-2. 

External links

• ESTA Technical Standards Program
• USITT
• OpenDMX.net
• OLA, a cross platform, multi language, DMX framework
• Rdmx, an open source DMX implementation for the Ruby programming language

See also

• Lighting control systems for a buildings or residences.
• Lighting control consoles for stage lighting and other DMX-512 devices
• Art-Net, a proprietary protocol for transmitting DMX-512 over UDP/IP