Lighting desk showing a set of control sliders used to control the value sent in DMX slots.
The Digital Multiplex (DMX) standard is a common standard used in the entertainment industry for stage lighting, automation and control. It is widely used and in the course provides an example of a range of simplex unicast transmission systems used for control applications.
Electrical control of light level in theatres and other applications has been used since electrical lighting was first introduced, replacing gas lighting. The first stage lighting dimmers/fixture functions were controlled by levers on the dimmers/fixtures themselves. During a show, it could take several people to move these levers and another person with sight of the stage to coordinate the movements. This kind of control was cumbersome.
As the number of lights increased, it became common to run control wires to each dimmer/fixture from a control console placed in front of the stage or area to be controlled. A small DC voltage (e.g., 0-10V), was used to control the proportion of a the lamp. Adjusting the voltage proportionaly changed the level of dimming. One wire controlling one device. If there were 300 dimmers/fixtures, you needed 300 control wires!! This 'analog' system can still be found all over the world.
This analogue system suffers from two major problems:
The advent of digital microcontrollers started a change in the entertainment industry that have since spread to all parts of our life. The cost of a low-cost microcontrolled is now substantially less than the other parts of the ssytem, making it possible for one to be built into eevry piece of equipment, and opening the possibility of a major simplification of the design of an enetertainment control system (as it also does to the design of a vehicle, airplane, office, or factory).
Manufacturers started introducing digital signals sent down a digital serial control cable. This single cable could replace 100s of cables. This dramatrically reduced the cost of cable, the cost of installation and the ability to reconfigure installed systems. By adding a microcontroller at each device (what we now call a netork node), each device could be customised to respond appropriately to the control signal it was configured to received. We call this a custoim profile for an equipment. It meant that the operator sending the signals did not need to worry what sort of actual equipment was being controlled. The digital control bus was born.
At first, each manufacturer used their own design of signal and specified their own way to interpret the data. This meant that different manufacturer's equipment could not be combined. It also prevented mass-production of even simple components. Although going digital had many advantages, it resulted in high-cost.
Some people realised that more could be done. Especially, they saw the benefit of a standard that could be used by all in the industry.
The original Standard was developed in 1986 by the Engineering Commission of the United States Institute for Theatre Technology, Inc. (USITT). This evolved to become the ANSI/TIA/EIA-485-A-1998 standard. published in 1986.
The Digital Multiplex (DMX) standard specifies a control network that sends upto 512 data bytes using a serial communications bus. Data is sent serially using a twisted pair of wires (a third pin on the connector provided a shiedling ground).DMX is a form of frame-based time division multiplexing. To the designer, DMX results in a cable is cheap and less bulky, and less cumbersome than a multi-conductor cable, and the ICs and methods to use the cable are readily available. The widespread use now means that there are many devices available that use the standard, all of which interwork.
Despite its limitations, DMX quickly flourished around the world as a theatre and stage lighting standard. Equipment could be manufactured to the standard and used in any suitable application. -- Initially manufacturers targeted theatre stages, and rapidly expanded to include film sets. When discotheques emerged, a whole new market could use the same techonlogy, as could places of worshop, the live event industry and finally office/building lighting and a range of other industries. Each application took advantage of the common standard.
DMX 512 has gained international acceptance throughout the entertainment industry. Minor revisions were made in 1990 and 2000. This is now maintained by ANSI-accredited Technical Standards Program, now operating as PLASA's Technical Standards Program (TSP). These standards are now available for free download from PLASA's TSP web site.
The standard was also extensible, and evolved to support new applications..DMX is based on 512 individual slots, each of which can be set to a value between 0 and 255. For exmaple, if a dimmer/function was assigned channel 1, and the level of channel 1 was brought up from 0% to 100%, the control desk would assign this channel to a DMX slot, and DMX bus would carry a value that inirtially started with a slot value of 09, increasing to 255. A receiver attached to the bus, such as a dimmer pack, would receive the DMX frame abnd read the slot value to determine the appropriate power, mapping the values from 0 to 100% light output.
Lighting desk showing a set of control sliders used to control the value sent in DMX slots.
A different channel could be simultaneously be used to control a different function. Such as a colour/gobo or a movement function. Each receiver uses this slot value from from 0 to 255 to control by the corresponding function.
The controllers, sending the DMX frames have also increased in sophistication and diversity. Many advanced lighting controllers include 3D models and integrate video processing and other inputs/outputs. Other designs travel the world with complex programming for major events/bands/shows. While still other controllers intended for building management are fully automated, responding to changes in the environment, time of day, etc.
The DMX specification can be logically divided into a physical and a link layer:
See also:
Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland (2025).