DMX 512 is the industry standard protocol used to control lighting equip-ment; it was developed to create a common interface for controlling lighting equipment using a lighting control desk. Until a few years ago, there was an increasingly pressing need to define a standard in this field, as fixtures were either controlled by control voltage sent to each channel, or several different digital or analog protocols; this wide range of signals and the secrecy the monufactures maintained, made connecting different makes of fixtures on insurmountable problem, and in actual fact compelled users to choose one porticular make for all their control equipment. Urged by theatre technicians looking for a standard interface, in 1986 a commission of the LISITT ILinited States Institute for Theotre Technologyl introduced DMX protocol. In 1990, it was updated to the version currently used today DMX 512 is just a communications protocol, so the wiring practice aren’t described in DMX 512 standards, but in the requirements of the EIA48S industrial interface lolso know as RS 4851, which must be followed and specifies the voltages, currents and components to be used. Microphone cables ond other general purpose two-core audio or signal cables are not suitable for use with DMXS12
The importance of cables Cables must be in compliance with E1A485 characteristics and,
as per DMX512 specifications, must be of the 2-pair screened type, even if at present its
sufficient to install lust one screened pair. In fact, the second screened pair is required for future updates,
so cables with one pair don’t comply. The same goes for connectors: those described in the
IJSITT DMX 512 norm are 5-pin XLR; its therefore important to bear in mind, that the
3-pin XLR connectors sometimes used on some units level if operating correctly)
are not in compliance with DMX 512 standards, Another factor, which often isn’t taken
into consideration when preparing a DMX512 signal line, is the “ DMX Terminator”.
This consists in a resistor of suitable value at the end of the signal path.
In order to ensure that once it reaches the end of the line,
the DMX512 signal isn’t reflected bock along the coble, which could cause it to
over-ride and corrupt the original outgoing signal under certain conditions and with
certain lengths of cable run. Another aspect not to be underestimated is,
that it’s not possible to obtain o DMX512 network by splitting the line into various branches.
Should it be necessary to create a node on the line to divide it into two branches,
a DMX splitter must be used, with the task of amplifying and generating numerous signals,
identical to that received on input. The length of the line and the maximum
number of fixtures that can be connected ore two more delicate points to keep in mind.
According to the norms, the maximum theoretical length shouldn’t exceed 500 meters
and if more than 32 units have to be connected, it’s advisable to use 0 DMX distribution
amplifier. Finally, the following is a summary of the fundamental practical procedures to be carried out
preparing a DMX512 line:
Cable: for E1A485 use at 250 baud, 110-Ohm impedance, low capacitance, two screened twisted pairs.
Wiring: S-pin XLR connector wired as follows: - pin 1 : signal common shield - pin 2:
DMX data 1 - - pin 3: DMX data 1 + - pin 4: first conductor of the
second pair - pin 5: second conductor of the second pair.
Termination: l200hm Watt resistor soldered across pins 2 and 3 in the
S-pin mole XLR connector. Must be applied to the lost fixture in the DMX line.
Fixtures connected: max. 32 Maximum cable run: 500 meters Signal cable run
: ovoid running cables alongside power lines
DMX stands for Digital MultipleXed and that means all information, digital, byte after byte, (multiplexed) is send over a balanced cable, connected between a data transmitter (PC or light-console) and a data receiver (dimmers, scanners, color changers, strobes, smoke machines). (The transmission goes so quickly, that it looks like, all happens at the same time).
A balanced cable takes care of the transport of the control-information of 512 channels. The type of cable and connector is also standardized by
the USITT. The current voltage between both inner conductors is approx. +5V or OV. Is the voltage 4 microseconds positive, an “1 ‘ will be sent. Is the
voltage 4 microseconds negative, a ‘0’ will be sent. So all information exists of 0 and 1 (null and one). This smallest information is named a BIT.
A combination of 8 Bits lnulls and ones) is called a BYTE and gives 256 values. For example 00000000 0, 00000001 = 1,
Digifàidirnming of a dimming pack and porcon by a light-controller are 256 steps tva)ues), and the change of the light intensity of one step is for our eyes not perceptible
The data stream is sent as a packet of data, which is repeating continuously. It consists of starting bits of data, which informs the recetvers that the packet is being refreshed and then sends out a stream of serial data corresponding to the magnitude value of each chan starting with Channel (-)
anen ing wit c anne- ora~ lesser channel, depending on the design and size of the light-console. Each channel is separated from th other by specified bits of start and stop data
The whole DMX-512 system works like a city postal system. Each postman has a block of many houses with 512 apartments (channels). Each apartment
(channel) has a unique address. Some houses have only one apartment (1-channel switch-pack) and some houses have many individual apartments
12 / 4-channel dimmer pack or 6-channel scanner). Each postman goes from house to house and delivers the mail laIl 512 letters with data in each box).
But each occupant only opens his mail with his (starting) address. First, each receiving unit is told its address lone of the 512 DMX-ciddressesl. Some units
(scanners and moving heads) have one address as a starting address Ireads the data of this address and all following addresses).
The digital signal consists of many, quick changing different voltages. Some signal distortion can turn up, caused by reflection-waves by cable-or plug-connections and by long leads. The quality of the cable is important. We advise you to use a balanced cable with a nominal impedance of +/-110 Ohm
(DAP DIG- 110
Cables & Connectors
Under optimum circumstances the maximum cable length could be 500
meters, but in practice we advise you to use a splitting-box after 100 meter.
(SHOWTEC DD-2 or SHOWTEC DD-4).
DMX-512 products are always equipped with two 3-pin or 5-pin
Pin 1 is connected with the cable-shield, pin 2 with minus signal
1-), pin 3 with plus signal 1+)
Some DMX products are equipped with 5-pins XLR connectors, pin 4