DMX512-A
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DMX512-A is an EIA-485 based communications protocol that is most commonly used to control stage lighting and effects.
Developed by the Engineering Commission of USITT, the standard started in 1986, with subsequent revisions in 1990 leading to USITT DMX512/1990. In 1998 ESTA began a revision process to develop the standard as an ANSI standard, including a Public Review process. The 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 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.
DMX512 was originally intended as a 'lowest common denominator' protocol for use between interfaces supporting 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 is unidirectional and does not include automatic error checking and correction, so it is not safe to use for applications involving life safety, such as controlling pyrotechnics or laser lighting display where audience or performer safety is involved. MIDI is sometimes used for this task.
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[edit] Physical layer
DMX512 data is 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." The major electrical requirements of E1.11 (DMX512 2004) are associated with the connection of network common, pin 1 of DMX connector, to ground. A detailed description of the grounding is beyond scope of this article. In summary it is recommended that DMX transmitter ports have a low impedence connection between DMX signal common and electrical ground. Such ports are referred to as ‘grounded' It is recommended that receivers, at low frequencies, have a high impendence connection between signal common and ground. Such ports are referred to as isolated. Transmitter ports may also be of an isolated topology. Grounded receiver ports of two different types are permitted but neither type is recommended and one is strongly discouraged.
[edit] Line Code and Timing
DMX sends asynchronous data at 250 kbaud. The data format is 1 start bit, 8 data bits, 2 stop bits and no parity. Framed data bytes are known as slots. The data is in little endian format. 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 signals end of one packet and the start of the next. It causes the receivers to start reception. After the break up to 513 slots are sent. The first slot is interpreted as a "Start Code" that tells receivers what kind of data will follow. For lighting fixtures and dimmers a start code of zero is normally used. Other start codes are used for Text packets or the System Information Packets (SIP), proprietary systems, or for the RDM extension to DMX.
[edit] Connectors
DMX512 1990 states; " Where connectors are used, the data link shall utilize 5-pin "XLR" style microphone connectors." DMX512-A(E1.11) prohibits use of any connector other than a 5-pin XLR unless there is not physical space on the device for that connector, in which case an adapter must be supplied. Unfortunately non-compliant connectors and pin-outs have been used by several manufacturers. The most common non-compliant connecter is the 3 pin XLR. One of the most inappropriate connectors is the TRS phone plug.
Color scrollers and units that are powered by a data / power cable are an accepted alternate and are often fitted with 4 pin connectors.
The pin out for the 5-pin connectors is:
1. Data Link 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)
[edit] Connector Gender Standard
Female connectors are used on controllers and other transmitting devices and male connectors are on dimmers and other receiving devices.
[edit] Timing:
DMX timings are allowed to vary over a very large range. The original authors thought they were allowing the greatest flexibility of design. However, all receivers must receive any possible timing from the shortest to the longest allowed. This has caused problems. One timing of the original 1986 standard caused enough problems that it was changed in 1990. The MAB was 4us with no variation. That was changed to 8us or longer. The E1.11 (2004) standard now set different requirements for what timings a transmitter must use and what a receiver must receive. This gives some breathing room for systems using controllers built to DMX512a(E1.11); however, a great number of older devices still send timing near the minimum.
| -- | Min Break | Min MAB |
|---|---|---|
| Transmitted | 92us | 12us |
| Receiver recognize | 88us | 8us |
The reason maximum times aren't listed above is that as long as you send a packet once a 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 you want. (This not a detailed description of the timing. Anyone wishing to design a device must consult the standard for the detailed requirements.)
A full packet takes approximately 23 ms to send, corresponding to a refresh rate of about 44 Hz. For higher refresh rates, fewer channels can be sent. This is accomplished by simply starting a new packet before all 512 channels have been sent. The minimum packet length is equivalent to 24 channels.
[edit] Network Layout
A DMX512 controller is connected to fixtures or devices in a multi-drop bus topology commonly called a ‘daisy chain'. Each device has a DMX512 ‘in' and often a DMX512 ‘out' connector - sometimes marked as DMX512 ‘thru'. The DMX512 controller is linked via a DMX512 cable to the ‘in' on the first device. A second cable then links the DMX512 ‘out' on the first device to the next device, and so on. In general, the final, empty, DMX512 out connector should have a terminating connector plugged into it. A terminator is a male connector with a resistor that matches the impedance of the cabling used (usually 120 ohms). The resistor is connected between pins 2 and 3 of the connector. While some simple systems with few devices and short cable runs may work without a terminator, it is good practice to use one. Some DMX devices have built in terminators, activated by a switch or by software.
[edit] Cabling
Most cable types that are appropriate for DMX512 usage will have a characteristic impendence of around 120 Ohms. Cables designed for EIA485 will normally be electrically acceptable. However there are mechanical and other considerations beyond scope of this article. 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 previous issues arising from the differing needs of cables used in touring shows versus cables used for permanent infrastructure. In addition, cable performance is now specified with regard to nominal impedance and capacitance to provide guidance as to what constitutes an acceptable cable. For example, microphone and line level audio cables do not have the correct characteristics and should never be used for DMX512. The significantly lower nominal impedance and significantly higher capacitance of these cables distort the DMX512 data which can cause irregular operation or intermittent errors that are difficult to identify and correct.
[edit] 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 2 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.
[edit] 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 give surreal intermittent problems such as fixtures twitching.
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 3 pin connectors because they are cheaper. DMX512-A specifies that the connector is to be a 5-pin XLR connector and cannot be any other kind of XLR connector. There is good reason for this rule: a 3-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.
[edit] 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 2 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.
[edit] 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.
[edit] External links
[edit] See also
- Lighting control systems for a buildings or residences.
- Lighting control consoles for stage lighting and other DMX-512 devices
