DMX (lighting)

DMX512, often shortened to DMX (Digital MultipleX), is a communications protocol used mainly to control stage lighting.

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.

Contents

DMX512 in theory

DMX512 cable is used to link a DMX512 controller to many DMX512 fixtures. Each fixture has a DMX512 in and usually a DMX512 thru connector. The DMX512 in on the first fixture is connected to the DMX512 out connector on the controller, then the DMX512 thru is connected to the DMX in of the second fixture, and so on. The final, empty, DMX512 thru connector should have a DMX512 terminating plug plugged into it. A DMX terminator is a 120 ohm resistor. Fixtures therefore 'daisy-chain' from one to the other.

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.

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.

Hot and cold 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. Earth
2. Cold
3. Hot
4. (Not used) Was return -
5. (Not used) Was return +

Each DMX512 cable transmits up to 512 8-bit values, between 0 and 255, so one cable could control 512 separate dimmers. Data is sent using RS-485 voltage levels and cabling practices. The DMX spec refers the reader to RS-485 for information about the electrical signal. Data is transmitted serially at 250 kbit/s and is grouped into packets of up to 513 bytes. Data is 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 kind of data is being sent. For normal dimmer/level data, at 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 actually 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, is vitally important for receivers to not 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 this 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 terminations problems. For instance, the termination if both sides of the wye are teminated with 120 ohms each, the combined resistance of both terminators would be 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 wyed cables will cause all kinds of 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 leave off the terminating plugs because without them a break in the hot or cold 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 cable 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°. Position needs to be controlled more accurately than that, and so Macs and other fixtures uses 2 channels each for pan and tilt. This gives a 16-bit value between 0 and 65535 for pan. This means that there is a coarse and fine channel of control. The coarse channel would allow values in increments 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. This fine channel of control is most useful for fine-tuning the static position of light in a scene.

Development

The ESTA and others have developed many alternatives to DMX512 to overcome its faults. One configuration that is gaining popularity is a combination of Ethernet and DMX512. Ethernet is used to communicate between the controller and the stage, then DMX512 breakouts boxes output DMX512 to the rigging. The Ethernet protocols used are often specific to one manufacturer.

A new revision of the DMX512 standard, called DMX512A has been released to address some of the shortcomings, and lay 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, Ethernet based replacement for DMX512. The standard is also managed by ESTA.