What is DMX?
DMX 512 is a standard
protocol by means of which lighting, controllers and control desks
can communicate with lighting equipment. It was designed to allow
equipment from different companies to be used together easily.
Where did DMX come from?
The first dimmers/fixture
functions were actually controlled by levers on the dimmers/fixtures
themselves. During a show, it could take several men 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/functions, you had to have three hundred 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
DMX has 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 (or 100%), the dimmer/function would
be sending out full power. Therefore, a color/gobo or other functions
are assigned a number from 0 to 255.
Channel 1 Gobo
0 = Blackout
10-20 = Gobo 1
20-30 = Gobo 2
30-40 = Gobo 3
This means anywhere between
level 10-20 gobo 1 would be visible. DMX512: 512 represent 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 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 2nd
data signal or talkback. At the console end, the channels are fed
into a multiplexer (mux) which takes the individual channel's 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 individual dimming/function
circuits. In practice, the mux and demux will be incorporated into
the console and dimmers respectively, though standalone mux and demux
units are available to convert analogue systems into DMX required.
The DMX advantage
DMX is advantageous because
the DMX cable is less bulky than a 48-conductor cable and is cheaper
and less cumbersome. If the cable were required to run long
distances, any repeater/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 machine, scanners, dimmers
etc. from a central location.
Who publishes the standard?
The DMX 512 standard
(Actually USITT DMX 512 - 1990) is published by The U.S.I.T.T. it is
now maintained by ESTA for more info on standards go to ESTA.com
Wire and Electrical details
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 (Mic cable is NOT suitable) suitable cables include:
- Belden 9842
- Alpha 5274 This is not a
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 an 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 item in the chain has its terminating switch set to the
on position. If the last item (Farthest from data source) does not
have a terminating switch then an external line-terminating network
is needed. To make one you will need the following parts:
1 Male 3 Pin XLR.
- 1 110 ohm 0.5W resistor.
To make the line terminator
solder the resistor across pins 2 and 3 of the XLR. You may like to
back fill the XLR with epoxy resin to provide protection. Line
termination will not correct a bad cable or interference from running
a DMX cable next to an electrical cable. High power electrical cables
will cause interference. What is addressing?
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.
If you have two intelligent
lights that use five channels each. You set the DMX address of the
first to 1 and the second to 6. Now, the first light is using
channels 1,2,3,4, and 5; and the second is using 6,7,8,9, and
10. You may also have to setup your intelligent light console
for the type of lights you have and setup their DMX addresses on the
console before you can control them.
You may want to do a little
planning before you start addressing your lights. Put all 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 type of light you
have uses. The light's 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 address for your lights written down.
Some controllers (ShowXpress)
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
CX-5 banks all fixtures in 14
channel intervals so your first fixture would always start on channel
1 and fixture 2 will always start on channel 15.
DMX-40 banks all fixtures in
16 channel intervals so your first fixture would always start on
channel 1 and fixture 2 will always start on channel 17. DMX-40 has
no joy stick there for you can use all the channels.
ShowXpress allows you to use
all the channels
Setting dip switches
Read your fixture's manual to
find out how to address your light. Many intelligent lights use
binary dipswitches, while others lights may use a small display and
buttons to set the DMX address. When using dipswitches there is
always a minimum of 9 dipswitches. Each dipswitch has a value as
listed below. If the first light starts at channel 1 (dip 1 on) and
your second light starts at channel 40 (dip 4, 6 are on). Exception:
if your controller works in banks of 14 you first fixture would start
on channel 1 (dip 1 on) and 2nd fixture would start on channel 15
(dip 1,2,4,8 on) etc....
Some fixtures have more then
9 dipswitches. 10 will always be a standalone function/ focus/
invert/self test etc....
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