DMX (old versions DMX512, DMX-512/1990 and current version of DMX -512 -A) is a digital control protocol that is used in the stage and event technology for controlling dimmers, "intelligent" lights, moving heads and effects units. The abbreviation DMX is short for Digital Multiplex.
DMX was standardized first by the USITT ( " USITT DMX512 ", " USITT DMX512/1990 " ), followed in 2000, DIN 56930-2 (standard on the basis of DMX512/1990 ) and in November 2004, the ANSI E1.11 ( DMX512 known as -A; with changes from the DMX512/1990 ).
DMX is based on RS- 485, is used a symmetric transmission method, the differential level between ± 1.5 V and ± 5 V and common mode voltage between 7V and 12 V must be tolerated. Due to the symmetrical transmission DMX has a high resistance, since external interference impact equally on both data lines and the receiver not the overall level, but the level difference is evaluated.
To connect are five pin XLR connector required, but frequently the three pole version is used because of the lower price. The free pair of contacts (pins 4 and 5) is provided for other functions as may be specified in more detail in the DMX512 -A. Contrary to the usual practice in audio engineering is located at DMX transmitter on a socket and a plug at the receiver. Thus, the output is also protected against short circuit in contact with metal parts. In addition, the signal lines are reversed (compared to audio signals).
In older three-terminal devices of the manufacturer Martin Professional, that is built in 2000 and before that, the DMX and DMX cables are reversed. With newer Martin device the default assignment is used.
The data transmission takes place by means of an asynchronous serial interface, which is controlled by a Universal Asynchronous Receiver Transmitter (UART). The data frame consists of the DMX at fixed, predetermined parameters of 8 data bits, no parity and 2 stop bits ( 8N2 ) and a symbol rate of 250 kbps. A bit is thus 4 microseconds long, other data rates are not provided.
The transmission is byte-oriented due to the UART: The idle state of the bus ( logic 1) is the start bit (logic 0) interrupted. Then follow the 8 data bits and 2 stop bits (logic 1). After the bus is again at rest and it can be transferred when needed the next byte.
A DMX packet begins with at least 88 microseconds (22 bit length ) low level ( logic 0) - This section is "Break" called. This allows easy identification of the packet start, since virtually every commercial UART signals the break as an invalid data byte with missing stop bits. This is followed by "Mark after Break" with a minimum of 8 microseconds (2 bit length ) high-level / idle state of the bus ( logic 1). In this Mark time slower clocked controller can adjust to a new DMX packet. Then the start byte is transferred with the value 0. Then the channel bytes are sent, starting with the value of channel 1 ( DMX channel count starts at 1, not 0 ). It may, but need not be transmitted every 512 channel bytes. However, addressing of the channel byte is not possible -. The first transmitted Kanalbyte is for the first channel, the second Kanalbyte for the second channel, etc. If the transmission at any one time be interrupted, it can be resumed by sending a new DMX packet be. The break sequence will automatically lead to a reset of all outstanding transfers.
For DMX -A can also be the starting byte values other than 0 (1 to 255) to accept. The receivers (dimmers or other control parameters) are doing all packages that start with a start byte not equal to 0 to ignore. This is the DMX bus, for example, the possibility of requests RDM ( Remote Device Management = feedback ) to start or to load new software into the devices. Since there are DMX devices on the market that bypass the start byte and note the channel bytes even with a start byte is not 0, this functionality is not actually used in all applications. On each channel, serial data streams can be transmitted (eg for running text display). However, this assumes usually that the command controller and the device are directly connected and the signals (eg, HTP- mixing) not interrupted by conversion or settlement.
Sources for detailed specifications can be found in the web links.
The bus structure
The bus topology is constructed in accordance with standard EIA- 485 ( " daisy chain "). To a transmitter to receiver 32 can be connected, then a repeater is required. For branches splitter must be used. Each chain must end by means of a 120 -ohm terminating resistor (also called terminator ) to be completed in order to prevent signal reflections. This task could, for example, the last DMX device in the chain take over ( depending on the manufacturer: 1 switchable terminating resistor, 2 automatic recognition as the last link in the chain, or even 3rd no terminator ).
Since the signal contains high frequencies (125 kHz square wave, cutoff frequency ~ 2.5 MHz), do not use any microphone cable may be used, but only in accordance with shielded cable with 110 Ω impedance according to specification. Preferably, twisted pair cable is concerned, but there are also special microphone cables on the market, their electrical properties are also suitable for DMX transmission. Nevertheless, it is common to use microphone cable, often in conjunction with a stage box.
On each device, the bus address must be set for devices that receive multiple channels, which is the start address ( a 12kanaliger dimmer with the start address 25 thus receives the channels 25 to 36). This is, however, particularly with the use of intelligent lighting, a large source of error. Incorrect addressing of the devices often leads to undesirable behavior, because they respond to control data, which are not intended for them or channels are moved. On the other hand, it is quite possible two devices intentionally to address the same, such as when a total of not enough channels are available, or to simplify the programming of the light show. Identical addressed devices with the same channel assignment, thereby behave completely identical.
The original use was designed for DMX, was the control of lighting circuits on dimmer. This, the number of 512 channels and the resolution of 8 /16 bit ( 255 levels ) appeared to be sufficient. Meanwhile, however, all devices of the stage and effect lighting can be controlled via DMX practical. Examples are dimmers, color changer, strobes, scanners and moving heads. Especially the latter need to control their diverse functions, multiple channels, beyond the resolution of a channel is too low to allow for smooth rides a mirror or headlight. Therefore, usually two channels are used for the two axes of movement Pan and Tilt. The results for many devices in large numbers of channels ( for example, a scanner: 2 channels Pan, Tilt 2 channels, lamp, brightness ( dimmer ), Shutter, 1 gobo wheel, gobo wheel 2, gobo, 1 color wheel, color wheel 2, Effect, focus, zoom, device control = 16 channels for a device ).
The DMX signals are usually generated by a light mixer / controller, but there are also lighting control software that can output DMX via special computer interface ( DMX interface card or USB-DMX devices).
USB-DMX is a control for DMX devices via computer ( USB port )
Through a USB-DMX interface, the commands of control software in the DMX standard be converted. This can be omitted in some applications on a lighting console. In addition DMX data can also be read into the PC for further processing to control about computer programs via DMX with some interfaces.
USB DMX controllers can be divided into two categories:
- Classic lighting consoles that are connected via the USB port to a PC or notebook with a functionally expanding the console software is running. The advantage of these systems: the buttons and faders on the console allow a fast and accurate access to the functions.
- PC software, which relays the commands directly from the USB port to a USB interface and thus does not require classic desk. The advantage of these systems is the almost unlimited range of PC software and the reasonable price because there is no extra panel is needed. However, input devices are often also used in the form of a classic console's because the control of the light software via sliders, and buttons allow a faster and more direct control. In addition, some software manufacturers additional functionalities that go beyond the pure light control. One then speaks of ILPS systems (Integrated Light- Planning and Steering ) system when the lighting design is an integral part of the overall system.
W-DMX ( wireless DMX) enables wireless connection to a DMX device. For this purpose, a transmitting and a receiving station are required. So far, this technique could not prevail due to high latency ( s delay time).
Meanwhile, Ethernet has evolved so widespread even in the industrial sector that the required devices and possibly leads are not only very cheap, but in many cases already exists. In addition, the use of Ethernet technology is almost all users of modern technology familiar, whereas DMX requires a completely different infrastructure. One hand this has led to the Ethernet cables for the direct transmission of DMX signals are used, however, resulting in the risk of damage caused by the confusion of the mechanically identical ( RJ45), but are electrically incompatible connections. On the other hand solutions have been developed to receive the traditional DMX data transmitted over conventional Ethernet. The latter has the advantage that in addition to pure cabling and the other Ethernet devices (mostly switches) can be used, and thus the higher transmission rate of Ethernet can be used, which would not be possible with DMX signals. Solutions such as Art-Net use this to transfer multiple DMX universes through a single Ethernet cable while eg ACN or PSI represent completely new designs with advanced functionality. Since these solutions on the Internet Protocol (IP ) onto (usually using UDP / IP), their use is also not limited to Ethernet, but at least theoretically on all media that support IP, possible.
As another advantage, these alternatives have in common that they require for the direct production on a normal PC only commercially available network card that is standard, whereas special converters are required for DMX. This simplifies the use of PC -based control software and reduces the cost of entry, but the transition to the many existing DMX infrastructure is required an appropriate converter. Another disadvantage arises from higher latency, which then attract attention especially when the lighting system is controlled in synchronization with music.