What Is DMX?
DMX-512 is a standard protocol which allows lighting, controllers, and control desks to communicate with other lighting equipment. It was designed to allow equipment from different companies to be used together easily.
The first dimmers/fixture functions were actually controlled by levers on the dimmers/fixtures themselves. 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 function to a control console. If you had 300 dimmers/fixtures, you had to have 300 control wires!! To improve this, 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. Finally, DMX-512 was adopted as the standard lighting control protocol.
Standard DMX 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. Therefore, a color/gobo or other functions are assigned a number from 0 to 255.
DMX-512 - 512 represents the number of channels one DMX cable can carry. It requires only 3 pins. Older systems, known as 5-core DMX, may be seen (two pins will not be used and are reserved for a 2nd data link). The data is sent serially to 2 pins (the third is the ground pin) known as positive phase or hot and negative phase or cold. Pins 4 & 5 are reserved for a 2nd data signal or talkback. At the console end, the channels are fed into a multiplexer (MUX) which takes the individual channels data and encodes it into a format suitable for sending down the DMX line with all the other channels' data. At the dimmer/fixture end, the data is fed into a demultiplexer (DEMUX) and split up again into the individual channels to be fed to their respective circuits. In practice, the MUX & DEMUX will be incorporated into the console and dimmers respectively, though standalone MUX & DEMUX units are available to convert analog systems into DMX.
The DMX Advantage
DMX is advantageous because the DMX cable is less bulky, cheaper, and less cumbersome than a 48 conductor cable. If the cable were to run long distances, any repeaters/amplifiers would only have to amplify 2 signals instead of 48. DMX also allows control of many different pieces of DMX equipment, such as smoke machines, scanners, dimmers, etc. from a central location.
The DMX-512 standard (actually USITT DMX-512 - 1990) is published by the U.S.I.T.T. and is now maintained by ESTA. For more information on standards, go to ESTA.com
DMX-512 is RS485 serial at 250K baud, as such any installation must meet the requirements of EIA-485. In particular, use of suitable good quality cable is important (we would not recommend mic cable for a permanent install, although we have found a good quality mic cable is suitable for mobile use).
At the speed DMX works, correct wiring practice is important. Always arrange wiring such that the data source is at one end of the cable. Never use a Y-connector but rather a DMX splitter.
All DMX lines must
be correctly terminated if reliable operation is to be obtained. If not,
signal loss can occur. Some fixtures have a switchable line-terminating
resistor built in. In these cases, make sure that only the last fixture
in the chain has the terminating switch set to the on position. If the
last item (farthest from the data source) does not have a terminating
switch, then an external line-terminator is needed. To make one you will
need the following parts:
Because the DMX protocol is so basic, each intelligent light's range of controlling DMX channels must be set manually. Each light's DMX address corresponds to the first of the light's channels.
You may want to do a little planning before you start addressing your lights. Put all of your lights in a logical order; it will help you figure out which light is which later. Next, find out how many DMX channels each light uses - the lights manual should tell you this. Now that you know what channel to start addressing your lights at, you can figure each light's DMX address (except when your controller banks channels, see Controllers). Take the first light's DMX address, add the number of channels it uses, and you have the second light's DMX address. Then, take the second light's DMX address, add the number of channels it uses, and you have the third light's DMX address. Do this until you have all the DMX addresses for your lights written down.
give you settings automatically and some controllers use banks of
channels. Whether your fixture uses 4 DMX channels or 10 DMX channels, it
will still require you to set up your fixtures based on it's banks.
Read your fixture's manual to find out how to address your light. Many intelligent lights use binary dipswitches, while other lights may use a small display and buttons to set the DMX address. There is always a minimum of 9 dipswitches if your fixture uses them (there may be 10, but the last one is used for a standalone/self-test/focus/invert/etc.) Each dipswitch has a corresponding value.
If you have a light
that needs a DMX address of 1, then your dipswitches will be set with only
1 in the 'on' position.
The exception to the above is if you have a controller which banks channels, if so, see the controller section above.
We have included a
DMX Chart in pdf format for all 512 channels.
Click here to get this chart.