Archive for May, 2011

Shedding Light On DMX Controllers

Have you ever wondered how the lighting director at an event can control hundreds of light units simultaneously, raising the brightness to a peak, and then dropping them in unison to black out the stage? Or how about changing the colour of light flooding the stage when there is a change in mood or tempo? Then there are the complex sequences of lights achieved from comparatively few LED lighting units, all connected by a single wire. This magic is achieved through the miracle of Digital MultipleX or DMX.

In the days when the brightness of a stage light was controlled by an individual slider or rheostat, the earlier mentioned magic would have required several lighting technicians acting together, and a mass of wiring.  Now a single master slider can control all the lights at an event through the power of DMX.

The original DMX standard was created in 1986 by the Engineering Commission of USITT with subsequent revisions in 1990 leading to USITT DMX512/1990.  This is the standard which governs all DMX signal transmission and cabling, and is usually written as DMX512. A DMX512 network employs a multi-drop bus topology with nodes strung together in what is commonly called a daisy chain. A network consists of a single DMX512 controller, which is the sole master of the network, and one or more slave devices. For example, a lighting console is frequently employed as the controller for a network of slave devices such as dimmers, fog machines and intelligent moving lights.

DMX is a multiplex technique that uses only four conductor shielded cable to control up to 512 dimmers. The DMX controller sends a series of numbers down the wires, starting with a special number called the start code, then up to 512 numbers representing the levels of each dimmer starting with dimmer one. The controller then repeats this series again and again, to maintain the state of all the devices. In practice, only two conductors (one pair) caries the signal, which had led to a non-standard 3-pin XLR system becoming “standard” for low cost DMX controllers and devices, which is the same as used for audio cables such as microphones. As adapters are available to convert from 3-pin XLR to 5-pin, and the other way around, it is possible to use both systems together.

If you are working with DMX controllers and lighting, there are a few simple rules to help you achieve a trouble free show. Follow these, and you reduce the possible causes of problems significantly:

  • Always use dedicated DMX cables
  • Route DMX cables away from high power sources
  • Always terminate each DMX circuits with the correct terminator

Always use dedicated DMX cables
This means use dedicated DMX cables and not microphone (XLR) cables. Microphone cables with XLR connectors look similar to 3-pin DMX cables, and for short runs will often work. However, DMX is a high speed digital data feed that requires a 120 ohm impedance to work properly while microphone cables transfer lower frequency analog signal that is typically in the range of 75 ohms. Always use dedicated DMX cables, and if you work in a mixed environment with both audio and lighting cables in use together, colour code your DMX cables to make them easy to identify in poor light.

Route DMX cables away from high power sources
All high power cables radiate electrical energy, which in the wrong circumstances can introduce a signal in adjacent cables. Because of the risk of interference, leading to data loss and unpredictable behaviors from your DMX equipment, you should route DMX cables away from your high power equipment. The same principle applies to DMX as to computer network cables, so never run data signals next to or in the same conduit as high power sources. If necessary to be near these sources, cross over them at 90 degrees, or keep as much parallel distance between them as possible.

DMX circuits should always be terminated
A DMX signal chain should always be terminated by the use of a DMX Terminator, which consists of a 120 Ohm, 1/2Watt resistor soldered between pins 2+3 of a 3 or 5-pin male XLR connector. This prevents the signal from reflecting back along the cable, and appears to the transmitting device as a cable of infinite length. If the addition of a terminator causes problems in your DMX chain, it is likely down to poor quality DMX Cable, so check that you don’t have a microphone cable in your signal path.

As DMX is a data signal, you can also run DMX512 on CAT5E or CAT6 cable, and if you are planning a static installation, like a church or community hall, this may be the best option. It can handle much higher data rates than standard DMX cable, and you can always use the cable for data later if required. One note of caution: if you are searching for a diagram to wire RJ45 for DMX, ensure you look for RJ45 for a DMX Chart, or DMX512 cabling chart, and not DMX-over-Ethernet cabling. You should also be aware that the some wiring schemes (Color Kinetics) use a different (non-ESTA standard) color code. While these both work, to avoid confusion, they should not be mixed on a single site.

There are a number of design pitfalls to avoid if you want your light show to be spectacular, night after night:

  • Do not use “Y” cables to split DMX signals; use a dedicated DMX splitter box. If you find a Y cable in your DMX kit, throw it away!
  • Do not use more than one controller in a DMX line; if it is necessary use a DMX signal combiner, or get a controller with more channels.
  • Do not run very long DMX lines; use a DMX repeater or buffer box to ensure that there is no loss of signal data.
  • Do not load too many devices on the DMX line; use a repeater or splitter box instead.
  • Do not run DMX lines bundled with power wiring or near transformers or electric motors. Cross such interference sources at 90 degrees

If you want some tips on designing your own DMX lighting setup, or want to buy in some expertise and equipment, follow the link to contact us. Contact Bruce Thompson Events now to hire your DMX Controllers

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