DMX (lighting)
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DMX512, often shortened to DMX (Digital MultipleX), is a communications protocol used mainly to control stage lighting. It is a form of the RS-485 architecture.
Developed by the Engineering Commission of USITT, the standard started in 1986, with subsequent revisions in 1990 leading to USITT DMX512/1990. ESTA took control of the standard in 1998 and began a revision process. The new 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.
DMX was originally intended for use in linking controllers and dimmers of differing manufacturers, a protocol to be used as a last choice after trying other, more proprietary methods. However, it soon became the primary method for not only linking controllers and dimmers, but also linking more advanced fixtures and special effects devices such as foggers. As DMX512 is an unreliable data transmission system it must not be used for controlling pyrotechnics, MIDI is sometimes used for this task.
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DMX512-A in theory
A DMX512 controller is connected to fixtures or devices in a daisy-chain link. Each device has a DMX in and generally a DMX out connector - sometimes marked as DMX thru. The DMX out on the controller is linked via a DMX512 cable to the DMX in on the first fixture. A second cable then links the DMX out on the first fixture to the next device, and so on. In general, the final, empty, DMX out connector should have a DMX512 terminating plug attached into it, which is simply a 120ohm resistor joining pins 2 and 3 of the connector. Many modern devices negate this requirement as they are capable of auto-terminating the link.
The connectors themselves should be 5 pin XLR, however only 3 pins of the 5 are used. Many manufacturers use 3 pin XLR connectors, in violation of the Standard. Additionally with the acceptance of the DMX512-A standard Cat5 cabling is also an acceptable wireing topology when used in a permanent installation.
Only cable designed for use with DMX512 should be used. Testing by ESTA has shown that CAT5 cable may be used without signal quality compromise. However, due to the common use of 3 pin XLR connectors, microphone cables are often used for DMX512, and this is certainly not a recommended practice.
Data Plus (Pin 3) and Data Minus (Pin 2) are the opposite way round to sound cables, and the signal travels in the opposite direction to the pins (female is out, male is in). The pin layout is:
1. Signal Common 2. Data Minus 3. Data Plus 4. (Not used) Was Data2 Minus 5. (Not used) Was Data2 Plus
Each DMX512 cable transmits up to 512 8-bit values, between 0 and 255, so one cable could control 512 separate dimmers. Because DMX supports only 512 channels, multiple DMX universes can be used in situations where more than 512 control channels are needed. A Universe refers to a dmx512 line from the console, and all of the devices chained to that cable. Many newer lighting consoles support multiple DMX universes, which must be cabled independently.
DMX512 Data are sent using RS-485 voltage levels and cabling practices. The DMX specification refers the reader to RS-485 for information about the electrical signal. Data are transmitted serially at 250 kbit/s and is grouped into packets of up to 513 bytes. Data are sent with 1 start bit and 2 stop bits, LSB first. The start of a packet is signified by a break of at least 88 uS (used to be 44 uS in the 1986 standard). Receivers detect the break and reset their receiving code. Then up to 513 bytes are sent. The first byte is always the "Status" byte. This tells receivers which kinds of data are being sent. For normal dimmer/level data, a status byte of 0x00 is used. Other status bytes are used for proprietary systems or for the RDM extension to DMX.
The remaining bytes make up the actual level data. Up to 512 bytes can be sent, and it is the job of the receiver to count the bytes to keep track of the channels. As there is no error detection or correction in DMX, it is vitally important for receivers not to miss bytes, and to discard packets if framing or buffer overflow errors are detected.
A full packet takes approx. 23 mS to send. This corresponds 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. Most transmitters send all 512 channels though, as many receivers have trouble with shorter packets.
Moving lights use adjacent DMX512 channels to control different aspects of their behaviour. These attributes may, for example, be laid out as:
1. Intensity 2. Colour 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 Colour
3 1 Gobo
...
6 2 Intensity
7 2 Colour
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. The operator is presented with a single consistent control method for controlling lights which require very different DMX512 values to achieve the same effect. 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. A single fixture may represent a fraction of a unit of load, however the cabling in between the fixtures can degrade the signal significantly. 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, so by using DMX512 buffers it is possible to split the signal from a controller to hundreds of fixtures.
It is not recommended to split a DMX512 signal by "wye"ing an output into two inputs. This can cause termination problems. For instance, if both sides of the wye are terminated with 120 ohms each, the combined resistance of both terminators is 60 ohms. This would probably overload the DMX transmitter, causing it to shut down or produce unacceptably low output voltage. In the case where only one, or neither side of the wye is terminated, the unterminated end(s) will create reflected signals, most likely causing corrupted data.
DMX in practice
DMX512's popularity is partly due to its sturdiness. The cable can be abused in ways that would render Ethernet or other high speed data cables useless without any loss in function. Many people do not use the terminating plugs since without them a break in the Data Plus (Pin 3) or Data Minus (Pin2) cable may not affect the operation of the fixtures. Strange behaviour on the parts of the fixtures is usually due to incorrect addressing, cable faults, or the wrong data from the controller. Cable faults can occasionally give very surreal intermittent problems such as fixtures twitching.
The two unused pins on the 5 pin connectors were originally intended for feeding diagnostic data back to the DMX512 controller, however this was never implemented, and some manufacturers made units with 3 pin connectors. Other companies used the extra pins to carry other data or power. However the ESTA forbids using the extra pins to send power, as this could damage devices expecting other types of signals on these pins. Thus many lighting rigs need a number of small adaptors which can be frequent points of failure. Some companies prefer 5 pin connectors as it makes it harder to confuse sound and lighting cables.
If a single DMX512 channel is used to control pan on a Martin Mac 500, which has 440° of pan, then an increase of 1 would result in a movement of 1.7°. Over a long throw (distance between the fixture and the projection surface) this amount of movement can result in a significant movement of the beam. To control position more accurately Macs and other fixtures use 2 channels each for pan and tilt. This gives a 16-bit value between 0 and 65535 for each movement axis.
Using these type 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, reflecting the relative amount of movement control each channel had. 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 1 and 255 to the value obtained by the coarse channel.
Recently, wireless DMX adapters have become popular, especially in architectural lighting installations, where cable lengths would be prohibitively long (Over approximately 1024 feet). Wireless DMX generally uses WLAN technology to transfer the DMX data, with strategically placed converters bridging the signal back to wired links.
Development
Many alternatives to DMX512 have been proposed and implemented by ESTA and others, to address its perceived shortcomings. One configuration which has gained popularity is the use of Ethernet 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 Ethernet link are generally proprietary, specific to individual manufacturers.
A new revision of the DMX512 standard, called DMX512A, was released in 2004 and lays the foundation for the RDM (Remote Device Management) protocol which will add diagnostic feedback from the fixtures to the controller and extend the DMX512A standard to encompass bidirectional communication between the lighting controller and lighting fixtures. In the longer term work continues on ACN, an open, UDP/IP based replacement for DMX512, which runs on standard Ethernet Cable. This standard is also managed by ESTA.