DMX Primer - Iluminarc

DMX Primer - Iluminarc
DMX Primer
Introduction
The DMX protocol (USITT DMX512-A) is a networking protocol that enables a
universal DMX controller device to control the features of multiple DMX
compatible fixtures, whether PAR cans, wash lights, moving heads,
followspots, foggers, or proprietary fixture controllers, etc.
As any other networking protocol, the USITT DMX512-A describes the
physical medium, the signals, and the functions they control.
The Physical Medium
DMX compatible fixtures are connected to the DMX controller using a DMX
connection. This connection consists of a series of wired connections between
the DMX controller and the various DMX compatible fixtures, also known as a
daisy chain connection. In this type of connection, the DATA OUT of one
fixture or the DMX controller connects to the DATA IN of the next fixture, and
so on.
Each DMX fixture links to the previous and next DMX fixture or controller using
a DMX cable. This type of cable consists of a section of shielded, twoconductor twisted pair cable with one 3-pin or 5-pin XLR male connector on
one end, and a 3-pin or 5-pin XLR female connector on the other end. The
XLR connectors pin-out is as follows: pin 1 is the Common (shield), pin 2 is
Signal Negative (S-), and pin 3 is Signal Positive (S+).
Note: For DMX, pins 4 and 5 are not used.
The Signals
The DMX signal stream is unidirectional from the DMX controller to the DMX
compatible fixtures. These signals conform to the EIA-485 standard.
The stream of DMX signals consists of 512 individual, sequential channels that
form a frame. The DMX controller constantly sends frames of DMX signals to
the DMX connection, even if not all of the 512 channels are in use.
Because of this constant transmission method, there can be only one DMX
controller in a DMX connection. If not, the DMX signals sent by one controller
would interfere with the signals sent by the other controller(s).
DMX Universes
A DMX universe is the set of DMX compatible fixtures connected to the same
DMX daisy chain using the same set of 512 DMX channels. Each set of 512
channels is referred to as a DMX Universe.
In most cases, an installation will consist of only one DMX universe.
However, you might find it necessary to define two or more universes
because of constrains imposed by distance or the number of features.
Most DMX controllers support only one universe, although some DMX
controllers may support two or more universes. Each universe will have its
own separated DMX daisy chain. A DMX compatible fixture can only be part
of a single DMX universe.
The Functions
Each DMX channel can have any unitary value in the 000~255 range. Each
DMX compatible fixture uses as many consecutive DMX channels as features
the user can control. The sequential numbers assigned to each DMX channel
(1~512) are also known as DMX addresses.
The function each DMX channel has, and the results of assigning a value to
each depend on the personality (or DMX channel layout) of each controlled
fixture. Some fixtures only use a single DMX channel, while others may
require 15 or more DMX channels to control all their functions. Personalities
are discussed in the next section, DMX Configuration.
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DMX Configuration
The DMX fixture configuration consists in determining how many channels
each fixture will need as well as assigning the corresponding DMX channels to
each fixture in order to size correctly the DMX controller.
Personalities
A DMX personality describes what channel or channels control which fixture
parameters. A DMX fixture may have many personalities to choose from. Each
personality requires a different number of channels, based on the number of
features the fixture enables. The number of DMX channels used by a fixture
may vary from only one (usually the general dimmer control) to 15 or more.
When a job does not require using all the fixture’s capabilities, the user can
select a more basic personality (less channels), allowing the DMX controller to
accommodate more DMX fixtures.
Starting Address
For the DMX controller to control each DMX fixture, the user must first
configure each fixture’s personality to determine the number of channels
required to control the fixture. Each channel will have a DMX address
assigned to it.
However, since assigning a particular DMX address to each channel is
impractical, the user will only need to configure the DMX address on each
fixture that corresponds to the fixture’s first channel of control. This is the
fixture’s starting address. The fixture will automatically assign the other
channels to the subsequent DMX addresses.
Once this assignment is complete, and based on the number of channels
used, the fixture will respond to the DMX signals sent to the range of DMX
channels that begins with the starting address.
For example, a fixture that uses six DMX channels with a starting address of
100, will accept DMX data sent by the DMX controller to channels 100, 101,
102, 103, 104, and 105.
Assigning Addresses
The user must carefully assign the starting addresses for each individual
fixture to avoid DMX channel overlapping. If the DMX channels do overlap, the
impacted fixtures could operate erratically.
However, the user may decide to configure two or more similar fixtures with
the same personality and starting address. In this case, all the fixtures with the
same starting address will operate in unison.
DMX Connectivity
Connecting the DMX fixtures to a DMX controller in small to medium
installations is usually a rather simple operation that requires a minimum of
tools and some planning (not including the actual fixture rigging and
configuration).
However, in large installations it may be necessary to plan carefully the
position and cabling of each fixture to avoid unexpected problems.
Fixture Location
The order in which the fixtures connect to the DMX controller is not important
and has no effect on how a controller communicates to each fixture. However,
the user should always define a physical location for the fixtures that provides
for the easiest and most direct cabling to the controller and other fixtures.
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(DMX Connectivity cont.) Number of Fixtures
When using a DMX controller, the combined number of channels required by
all the fixtures on the DMX daisy chain determines the number of fixtures the
DMX controller has to support. Conversely, the number of onboard sliders,
page buttons, and fixture buttons limits the number of discrete DMX channels
a DMX controller can support.
To comply with the EIA-485 standard, which is the base for the USITT
DMX512-A protocol, do not connect more than 32 fixtures without using
an optically-isolated DMX splitter. Doing otherwise may result in
deterioration of the digital DMX signal.
DMX Data Cabling
You must use DMX compliant data cables to link two or more DMX compatible
fixtures. You may purchase DMX cables directly from a dealer/distributor or
construct your own cable.
USITT recommends limiting the total length of the DMX cable (from the
first fixture/controller to the last fixture) to 300~455 m (985~1,500 ft).
Making Your Own DMX
Cable
If you choose to create your own DMX cable, make sure to use data-grade
cables that can carry a high frequency signal and are less prone to
electromagnetic interference. Use a Belden® 9841 or equivalent cable, which
meets the specifications for EIA RS-485 applications. For certain applications,
Cat5, Cat5e, or Cat6 may be appropriate.
Do not use standard microphone cables for DMX applications because
they cannot transmit DMX data reliably over long distances.
DMX Cable
Characteristics
DMX Cable Connectors
The DMX data cable must have the following characteristics:
Type:
shielded, 2-conductor twisted pair
Maximum capacitance between conductors:
30 pF/ft
Maximum capacitance between conductor and shield:
55 pF/ft
Maximum resistance:
20 ohms/1000 ft
Nominal impedance:
100~140 ohms
Each DMX cable must have a male (3-pin or 5-pin XLR connector) on one end
and a female (3-pin or 5-pin XLR connector) on the other end.
DMX Connector Configuration
To DMX Input
(Female)
To DMX Output
(Male)
Common
1
3
2
DMX -
To avoid signal transmission
problems and interference,
connect a DMX signal terminator
to the last fixture in the
DMX daisy chain, as shown.
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1
3
2
DMX +
1
3
2
120 ohm, ¼ W
resistor between
pin 2 (DMX -) and
pin 3 (DMX +) on
the output of the
last fixture.
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(DMX Connectivity cont.) Test all DMX cables with an ohmmeter to verify their correct polarity and
to make sure that there are no short-circuits between any of the pins, or
between any pin and ground.
If the common wire (shield) touched the chassis ground, a ground loop
could form, which may cause the fixture to perform erratically.
3-Pin to 5-Pin
Conversion Chart
If you use a DMX controller or fixture with a 5-pin DMX connector, you will
need to use a 5-pin to 3-pin adapter. The chart below details a proper cable
conversion.
3-Pin to 5-Pin Conversion Chart
Conductor
Ground/Shield
Negative (-) signal
Positive (+) signal
Not Used
Not Used
DMX Connection
5-Pin Male (Input)
Pin 1
Pin 2
Pin 3
Pin 4
Pin 5
Make sure that the fixtures with which you are working can operate in DMX
mode, not in a proprietary connection mode. Refer to the fixtures’ manual to
learn how to enable their respective DMX modes.
The procedure below illustrates a possible DMX connection method.
1.
2.
3.
4.
5.
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3-Pin Female (Output)
Pin 1
Pin 2
Pin 3
Connect the 3-pin, male connector of the first DMX cable to the DMX
Output connector (3-pin, female) of the DMX controller.
Connect the 3-pin, female connector of the first DMX cable coming from
the controller to the DMX Input connector (3-pin, male) of the first DMX
fixture.
Connect the 3-pin, male connector of the second DMX cable to the DMX
Output connector (3-pin, female) of the first DMX fixture.
Connect the 3-pin, female connector of the second DMX cable coming
from the first DMX fixture to the DMX Input connector of the second DMX
compatible fixture.
Continue linking the other DMX fixtures in the same way.
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