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|
A Simple Guide To DMX...
|
SECTION 1 INTRODUCTION
DMX512 is a
communication signal used to control what are commonly referred to as
Intelligent Lights by universal or specific DMX controllers. It started
out as USITT DMX512, then was abbreviated to DMX512 and is now most
often called DMX.
The abbreviation DMX512 stands for Digital
Multiplex Signal that can control up to 512 channels. This language was
created by the United States Institute for Theater Technology (USITT)
to simplify and unify theater lighting systems. Before DMX was created,
the only lighting control system was analog which required one
individual wire to be run from the controller to each light fixture or
dimmer pack.
It
is important to understand that DMX is a communication signal only and
does not supply power to the fixtures. All DMX light fixtures and
dimmer packs require their own power supply.
DMX
is now recognized world wide as the standard communication language for
theater and night club lighting and now even some fog machines.
|
SECTION 2 FOUR BASIC PARTS
A
DMX system has 4 basic parts: A controller that creates and sends the
signal, the cable that carries the signal, the light fixtures or dimmer
packs that receive the signal and a terminator to stop the signal.
These are all connected in a Daisy Chain manner with the controller at
the beginning of the line, the light fixtures and dimmer packs in the
middle and the terminator at the end of the line. |
SECTION
3 CONNECTING THE PARTS
Connecting a DMX system is easy if you follow
these three steps.
1. Link the controller to all light fixtures and dimmer packs with high
quality cables.
2. Set the DMX address on each fixture or dimmer pack (usually, but not
always necessary).
3. Terminate the DMX signal at the end of the line (unless the last
fixture has a built in terminator).
These
3 steps do not have to be done in this order, but you must not power up
any part of this system until the controller, all light fixtures and
dimmer packs and the terminator have been connected and addressed.
|
SECTION 4 THE CABLES
The
most expensive DMX system is no better than the cables you use to
connect them. Low grade or damaged cables and their connectors are the
main cause of DMX system problems. Bad solder joints and corroded
connector plugs can easily interfere with the signal and prevent proper
operation.
DMX cables should never be installed in
the same conduit or junction boxes as power wires. You wouldn't even
want them running side by side on a temporary basis.
Never attempt to use a Y cable or splitter other
than a true DMX splitter.
Not
all microphone cables are wired the same as DMX cables!!! We have had
many instances where a customer has had a system working fine for
months, then adds a new fixture and uses a handy mic cable to connect
that new fixture and suddenly his entire light show quit working. Some
mic cables have pin 1 tied to the connector body for a ground while
others tie pin 3 to the body. You can not use both types of cables for
DMX!!
Although a five pin XLR connector started
out as the official connector for DMX, the three pin XLR connector has
always been the standard in the U.S.A. and is quickly becoming the the
most popular everywhere.
Both connectors do the
same thing. Pin #1 = ground, pin #2 = negative signal, pin #3 =
positive signal. On five pin XLR connectors, pin # 4 and #5 are usually
not used.
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SECTION
5 SETTING DMX ADDRESSES ON FIXTURES
Each
light fixture and dimmer pack must have an address in order to receive
data from the controller. Assigning an incorrect address to your light
fixture is as fatal as putting the wrong address on your electric bill
payment. In both cases the lights won't come on!! DMX address mistakes
are a big cause of problems with new systems and new human operators.
Bad addresses on fixtures cause problems such as gobo change instead of
mirror movement, color change instead of gobo change, or no response at
all.
Setting DMX addresses is sometimes very
simple by following manufacturer instructions and sometimes very
difficult when no instructions are available.
Most DMX light fixtures have nine DIP switches
used to set the address of that fixture.
Address
values are easy to remember. Starting on the left and moving right,
each dip switch has a value twice as large as the switch at its left.
The address for a DMX light fixture equals the sum of the value for all
dip switches in the ON position. In the picture above,
the address selected is 1.
Here are a few more examples...
 Address is 2
 Address is 3
 Address is 7
 Address is 21
The
rule for setting an address with DIP switches is to start with the
largest number of a DIP switch and work your way down through the lower
numbers. To set an address of 35 you would first flip on DIP switch # 6
for a value of 32, then flip on switch #2 which makes the total value
now 34, then flip on switch #1 which makes the total value now 35.
You might find these little switches easier to
manipulate with a small screwdriver or a writing pen.
|
SECTION 6 DETERMINING DMX ADDRESSES ON
FIXTURES
This
is where the confusion starts. Many intelligent scanning lights use 4
channels; 1 for the gobo wheel, 1 for the color wheel, 1 for left to
right mirror movement and 1 for up and down mirror movement. So why
does the second fixture need to be addressed 13 instead of 5? I can't
tell you why they do it, but almost all manufacturers make DMX
intelligent lights that OCCUPY more channels than they USE. If you
don't have a good owners manual that clearly identifies the addresses
of the second, third and fourth units, you can follow the same
procedure that we do when we test a new light.
Connect
only one fixture to the controller with only one DMX cable. Set the
address of this fixture to 1. Turn the power on the controller and
fixture and test each slider on the controller to identify which slider
activates each item on the light fixture. After you have made a note
about each slider and its activity, turn the power off for the fixture
and the controller. Run a second DMX cable to one more fixture and set
its address to 1. Power up all items and test the sliders again. Both
fixtures should do the same thing at the same time. You can have as
many fixtures as you like daisy chained together and all set to address
#1. This is the simple way to test and run multiple intelligent lights.
If
you want to have each light doing something different from the others,
you must address them each differently. This is when you need to know
how many channels the light OCCUPIES. To do this, turn the power off
for the fixtures and the controller. Go to the second fixture and set
its address to 2. Power up all items and test the sliders again. If
both fixtures don't work properly and in sync, turn all power off and
advance the address of the second unit by 1 and try again. If this
still fails to operate properly, power down and advance the address of
the second fixture by one again. Keep doing this until you find that
the second unit works in sync with the first.
If
you know that your light fixture OCCUPIES 6 channels then your first
fixture will be address 1, your second fixture will be address 7, the
third fixture will be address 13.
|
SECTION
7 CONTROLLERS
When
setting DMX addresses, the design of your DMX controller and each light
fixtures channel function need to be observed. For instance our CX-5
has 12 sliders for light operations while our DMX-STEP-ONE has 8 . Each
is very convenient for some intelligent lights, while inconvenient for
some other lights with fewer channels.
This problem can be overcome by setting DMX
addresses imaginatively.
Lets say you have one intelligent scanner that uses channel 4 for left
/ right mirror movement and channel 5 for up / down mirror movement.
You have another light that uses channel 5 for left / right mirror
movement and channel 6 for up / down mirror movement. By advancing the
address of one fixture, you will have the same slider controlling left
/ right mirror movement on both lights. You can play with the addresses
of every light fixture to make sliders on your controller operate as
you would like.
|
SECTION 8 DMX SIGNAL TERMINATION
In
theory all DMX signals must be terminated. I can tell you from personal
experience that most of the systems I have operated were never
terminated and still worked fine. Termination does reduce signal errors
caused by reflection at the end of the DMX daisy chain. The most
popular method of termination is to solder a 90 to 120 ohm, 1/4 watt
resistor across pin# 2 and pin #3 of a male XLR connector and plug it
into the DMX OUT socket of the last light fixture or dimmer pack in
your chain. Some newer light fixtures have a built in terminator
activated by the flip of a switch. |
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