Digital Multiplex 512

This page contains wiring diagrams for various cables used in the labs that have not been standardised.

Before the DMX Standard

The first dimmers/fixture functions were controlled by levers on the dimmers/fixtures. During a show, it could take several people to move these levers and someone else to coordinate them. This kind of control was quite cumbersome. Later, control wires were run from each dimmer/fixture to a control console. If you had 300 dimmers/fixtures, you had to have 300 control wires!!

Lighting desk showing sliders used to control the the setting of DMX channels.

Early electronic control of theatre lighting used a small DC voltage, the proportion of which turned on the lamp to different levels of dimming. This voltage ran along individual wires for individual channels and this 'analog' system is still used all over the world. Different voltages and polarities were used, but the +10 volt system was the most popular.

This system suffers from two major problems:

  1. It is prone to noise and earth loops, if not wired properly over long distances.
  2. It can be very nonlinear with the different kinds of lamps/equipment in use.
  3. It requires one conductor for each parameter being controlled, resulting in large multi-core cables.

To improve on analogue control, manufacturers started using digital signals sent down one control cable. At first, each manufacturer used their own protocols, meaning that different manufacturer's equipment could not be combined.

History of the DMX Standard

DMX-512 became the standard lighting control protocol. DMX-512 is a Digital Multiplexing standard which allows the control of 512 dimmers using a serial communications bus. This is a form of frame-based time division multiplexing. Despite its numerous limitations, it has flourished around the world as a theatre and stage lighting standard.

The A-version of the specification can only be applied to equipment with active electronics conforming to this standard. One key advantage of DMX is that the cable is less bulky, cheaper, and less cumbersome than a multi-conductor cable. It's widespread use now means that there are many device that use the standard, all of which interwork.

DMX is basically very simple. It is based on 512 individual channels, which can be set to a level between 0 and 255. If a dimmer/function was addressed to be on channel 1, and the level of channel 1 was brought up to 255 (100%), the dimmer/function would be sending out at full power. A colour/gobo or other function may also be assigned a number from 0 to 255. DMX-512 requires only a single channel with a 3 or 5 pin connector to connect many devices. Data is sent serially using a pair of wire (the third pin on the connector is the ground). 

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. DMX 512 has gained international acceptance throughout the entertainment industry. The earlier versions of this Standard covered only data used by dimmers. The Standard has since been used by a wide variety of devices. 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 DMX specification may be logically divided into a physical and a link layer:

See also:

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2014)