DMX lights, the basics

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Before DMX:

In the past disco effects were pretty much stand alone, they would either be just switched on or off, or have their own microphone doing their own sound to light.  Of course, different effects in different places would pick up different sound levels, and they would all operate independently.
Stage lighting was even more difficult, with massive "dimmer racks" by the side of the stage, and miles of wires connecting them to a lighting desk further back from the stage.

The DMX solution:

DMX uses a single cable to control 512 channels of light control.  A simple dimmer might use a single channel per light to control the brightness. A scanner might use many channels for a single unit, one channel for each of: brightness, Gobo (pattern) selection, colour, X position, Y position, zoom, rotation speed, etc. etc.  Even with 8 channels per unit you can still control 64 unit from a single cable.

DMX Cabling:

The official DMX standard is for a 5-pin XLR connector, carrying two seperate DMX channels (or two copies of the same signal allowing for a primary and backup).
But most of the lights and controllers you will find on eBay use a 3 pin XLR, which allows the use of microphone cables (not officially, but over reasonable lengths it works fine).  Watch out because there are two incompatible ways to wire the 3-way connector.  For instance to mix Chauvut and Abstact equipment you need a cable with two of the wires swapped over.
Each of the lights has an input and an output. You run a lead from the controller’s output to the first light’s input, then from that lights output to the next input, until all of the lights are daisy chained together.  In theory, you are ment to have a terminator plugged into the last light (but with reasonable cable lengths, it’s usually not a problem).

Simple DMX Controllers: Fine for a disco with a handfull of lights.

The simplest DMX controller is built into some lighting units. The first unit either is a special unit, or goes into a special mode when it realises that there is no DMX signal.  It may use a built in microphone to do a sound activated show.  Some lights allow a mini controller to be plugged into the first light, and allow some extra control over what is done.  These often use a ¼" jack plug, and will only work with the units they are designed for.
The output from the first "master" light, and the output from mini controllers with a DMX output, is usually a manufacturer specific cut-down version of DMX, it uses a special command set which will only work with a limited range of compatible units.

More complex DMX Controllers: For a bigger disco, or stage lighting

These vary from small programmable units with a few sliders to full desks.  The larger controllers have several outputs, allowing control over multiple blocks of 512 channels.
Of course there are PC based solutions, typically using a USB based DMX interface to create the DMX signal for the light units.
These allow complete control over every channel, controlling every aspect of the lighting units connected to them.  They often require substantial setup time.

Switch settings

In more complicated setups the base DMX channel for a unit controls what needs to be set.  With simple controllers, the base might be in steps of 6 or 8. So the first unit would be set as 1, the second unit as 7 (1+6=7), the third unit as 13 (7+6=13), or 1,9,17 for steps of 8.  More complicated contollers, allow the individual units to be fully assigned.
If a lighting unit uses 6 channels to control it (eg brightness, gobo, colour, x position, y position, rotation speed) then we set its starting channel number, and it will a block of 6 channels, starting with the base channel to control it.
Newer units often use a numerical display to display the unit's DMX base address and use button presses to change it.  Older units use a set of "DIP" switches to set the base address.
To work out the channel number for DIP switch units, just add up the values on the switches.  They are in powers of 2, so the various switches are "worth" 1,2,4,8,16,32,64,128,256.  If the 2,4,16,128 switches are on and the rest are off, then the base address is 2+4+16+128=150.  If the unit uses 6 channels to control it, then it will respond to command send to channels 150,151,152,153,154 and 155.
To work out which switches to turn on, start with them all off.  Find the highest switch value which is less than or equal to your required value.  Turn on the switch, and subtract the switch value from the required value.  Repeat until you reach 0.
So for 150.
All off.
150 is smaller than 256 so we skip that switch (leave it off), but greater equal to 128.  
So we turn the 128 switch on, subtract 128 from 150, leaving 22, and repeat.
22 is smaller than 64 and 32 so we skip them but greater equal to 16.
So we turn the 16 switch on, subtract 16 from 22, leaving 6, and repeat.
6 is smaller than 8 so we skip it but greater equal to 4.
So we turn the 4 switch on, subtract 4 from 6 , leaving 2, and repeat.
2 is greater equal to 2.
So we turn the 2 switch on, subtract 2 from 2 , leaving zero so are all done.
To double check, look at what switches we turned on, 128,16,4 and 2. Adding them up we get 128+16+4+2=150 !

Other notes:

You can configure multiple units with the same number if you want them to respond in exactly the same manner.
DMX repeaters are available, to allow splitting the signal maybe to allow one run to each side of the stage without having to run cables from one side to the other. Repeaters also give better protection against broken cables (less of your lights will fail if a cable fails).
Read the manuals!

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