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DMX512, often shortened to DMX, is a communications protocol used mainly to control stage lighting. The full official title for the current specification of the protocol is DMX512/1990, as delivered by the USITT Engineering Commission. DMX is assumed to be an abbreviation for Digital MultipleX. The future development of DMX512 is now in the hands of ESTA, the Entertainment Services and Technology Association.
Although originally intended to be used for controlling light intensity (dimmers) it is now used to control moving lights, video projection systems and stage machinery. One vital ingredient in its success has been its lack of association with any one manufacturer. New generations of lighting controllers have become available that increasingly hide the many shortcomings and varied implementations of DMX512 while the entertainment industry has wrangled over a replacement.
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. 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 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 connectos, 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. 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 wiring two outputs to one input.
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. 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 1.2°. 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 65536 for pan.
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.
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