Page 28 - 8. Introduction to DMX
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8.2
The problem with big lighting rigs
Because each dimmer channel needs its own control signal, if you have a
large lighting rig, the control cable can end up being huge, with big
expensive connectors. If you are using intelligent lighting, like scans, this
gets even worse, because some of the more complicated fixtures use 20
channels or more per fixture. You soon need a better way of getting the
control signals from the control console to the equipment.
8.3
The DMX control system
Fortunately, there is an easy to use system for controlling intelligent
lighting and dimmers with large numbers of channels. It is called DMX-
512, which stands for Digital MultipleX, and it sends up to 512 control
signals down a 2 core or 4 core screened cable. The cable can be simply
daisy-chained from each lighting device to the next - all DMX devices have
a loop-through connector which allows you to link to the next device.
Normally a 5-pin XLR connectors is used and in the case of the ART
system, all 5 pins should be connected.
Control consoles using the DMX system send out the level for channel 1,
followed by the level for channel 2, then 3, then 4 and so on, all the way
up to a maximum of 512 (though not all DMX systems send out all the
channels). It then goes back to channel 1 and goes round again.
All the units connected to the DMX receive all the channels, and you tell
them which channels to respond to by setting the “start address” on each
unit. The computer inside the dimmer or fixture waits for the correct
channels to come up, then reads in the levels for those channels and uses
them to set the dimmer levels or the fixture positions.
8.4
A few cautions about using DMX
DMX is a robust system which normally works very well, provided you
keep to a few simple rules. Here are a few tips for making DMX work well.
DMX cabling should be twisted-pair shielded data cable. Microphone
cable is not recommended because it degrades the signal, especially on
long runs.
Always “terminate” the last dimmer or fixture on the DMX line, either
with a special termination plug, or the ART2000 rack has terminator
switches which do this for you. The termination is a 120 ohm resistor
across the DMX data lines which prevents data interference on the line.
It might work without but…
Don’t split the DMX cable. Loop it from one unit to the next. If you
have to split it, use a proper active splitter unit like the Avolites Rack
splitter or Avolites truss splitter, part numbers 33-65-1000 (19”
mounting, 8 outputs) or 33-65-2000 (truss mounting splitter using a
self-fixing strap, 6 outputs).
The DMX specification says you should not have more than 32 unit
loads on one DMX line. Either use several DMX lines (Avolites consoles
usually have more than one) or use a DMX splitter unit. Again, you will
probably get away with using more if you have to, but the DMX signal
will be weaker than it should be.
Don’t run your DMX lines near high voltage cables or neon supplies.
They may pick up interference which will stop the system working.
