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Technical Data
- DMX 512
DMX is a lighting industry standard way of controlling lighting equipment.
The whole idea behind DMX as a standard is that it allows equipment from one
manufacturer to talk to that made by another. DMX-512 allows for one controller
(like a lighting desk) to control many lights. DMX is limited to controlling 512
separate parameters, that's where the name DMX-512 comes from. DMX-512 is a
protocol for controlling (at a basic level) the brightness of lights in a
theatre (it is vary capable and can be used for controlling moving lights, smoke
machines, strobe lights etc.) . It basically consists of one 8-bit (on or off)
signal for each light which sets the brightness level (0-100%) of the light to
one of 256 levels.
DMX-512 is the standard interface and protocol used in the theatre and
entertainment industry. DMX-512 allows control systems to communicate with
dimming systems, automated luminaires, colour scrollers and with other equipment.
DMX512 is connected using a daisy-chain methodology where the source connects to
the input of the first device, the output of the first device connects to the
input of the next device, and so on. The standard allows for up to 32 devices on
a single DMX link. The DMX512 communications protocol is very simple and robust.
DMX512 was created in 1986 by the United States Institute for Theatre Technology
(USITT) as a standardized method for
connecting lighting consoles. It was revised in 1990 to allow more flexibility
(this is the version in use today). The Entertainment Services and Technology
Association (ESTA) has assumed control over
the DMX512 standard. ESTA is making revisions to clarify and further extend the
standard. Special care is being taken so that existing DMX512 equipment will
work under any new standard. Although the DMX512 standard is being updated,
existing equipment will still work the same under any new revision.
DMX512 is designed to carry repetitive control data from a single controller
to one or more receivers. This protocol is intended to be used to control
dimmers, other lighting devices and related non-hazardous effects equipment.
Since this Standard does not mandate error checking, DMX512 is not an
appropriate control protocol for hazardous applications. So do NOT use DMX-512
to control devices like moving platforms or pyrotechnics.
Officially DMX is carried on 5 pin connectors, which carry the data (RS485)
on pins 2 & 3, screen (and common mode reference) on 1. There is a second data
link on pins 4 & 5 the format of which has never been well defined and it is
seldom used in practice. This means that at an electronics level, the signal is
sent through 5-core cable (but only 3 are ever used) consisting of 0v, +ve and -ve
signal wires. Cable for DMX-512 installations should be 110 ohm impedance
shielded twisted pair cable. A normal microphone cable (shielded twisted pair)
works OK for short runs, but can cause problems in long runs (so using it is not
recommended).
In DMX-512 world there has been a long debate on 3-pin vs 5-pin connector.
The official standard says 5-pin connector. Unfortunately some manufacturers use
a cheaper 3-pin XLR connector instead of standard 5-pin. Cheaper/budget lighting
kits (like many disco effects) are almost always 3 pin, probably because the
plugs are cheaper! It seems that 3 pin is becoming the modern de-factor norm
(although not conforming to the DMX512 standard) and 5 pin is becoming more
unusual. Current Martin kit uses 'pin 3 +ve' but older Martin stuff is the other
way round. It's therefore worth carrying 'change-over' connectors as well as
3-to 5-pin adaptors if you are working with systems consisting of components
from many different manufacturers.
Standard DMX-512 connector (5-pin XLR) wiring is:
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Pin 1 - Ground (shield)
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Pin 2 - Data complement (Data -)
-
Pin 3 - Data true (Data +)
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Pin 4 - Optional Second Data Link Complement (Data 2 -)
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Pin 5 - Optional Second Data Link True (Data 2 +)
The pins 4 and 5 are reserved for spare data.
There are also some application where 3-pin XLR-connector is used to carry
DMX-512 data. Use of 3-pin connector is not defined in the DMX-512 standard, but
the industry de-facto pinout for 3-pin XLR connector is the following:
NOTE: There are some products using 3-pin XLR connectors
with Data +/-
reversed.
Which one of the connector you see on the equipment depends on the
manufacturer and to what market those equipment are made for. In professional
theatrical lighting applications the users and manufacturers prefer the standard
5-pin XLR connector. Practically all all the theatrical consoles, dimmer packs,
PDU's, etc. use 5-pin connector. The new version of the DMX spec is very clear
on connector issue - 5 pin only, anything else just makes life difficult for the
end user. Go for 5-pin. Anything else will render your product non-compliant
with the USITT DMX512 standard.
However, 3 pin DMX exists and will no doubt continue to be widely used even
in new products. 3-pin XLR connector is typically found on DJ/club type lighting
instruments- One of the big reasons 3-pin XLR has become so popular is because
manufacturers of intelligent lights targeted DJ's, clubs, etc. who already had
3-pin XLR's for their audio gear. 3 Pin XLR do not conform with the USITT
DMX-512(1990) standard. The DMX512 standard has always been clear about it being
a 5-pin XLR. The reason many of the companies were using 3-pin is because they
had their own native protocols that predated DMX and DMX was just an additional
feature. The problem with using 3-pin XLR connectors is that you can easily
confuse the DMX-512 cables with microphone cables that do not meet the needs of
DMX-512 data cable. If you just use "any microphone cable" for DXM-512
connections, you might be able to get it to work with few instruments on short
distances, but when distances get longer and number of instruments increase,
things do not run smoothly anymore. The only rationalization for continuing to
build any new product using 3-pin is to allow the inappropriate use of low-grade
mic cables. Not a good reason in my book. The new version of the DMX512 standard
in the works by ESTA specifically disallows the 3-pin XLR (not that it was ever
allowed to begin with). If you don't comply you can't mark your product as
complying to the new standard.
Because there are this two connector model situation, many people working on
this field need to use 3-to-5 and 5-to-3 pin adapters quite often.
DMX512 uses EIA-485-A (commonly referred to as RS485) which is a balanced
system. Normative references for electrical specifications at ANSI/TIA/EIA-485-A-1998
Electrical Characteristics of Generators & Receivers for Use in Balanced Digital
Multipoint Systems. The electrical specifications of DMX-512-A standard are
those of EIA-485-A with some minor exceptions.
The DMX-512 interface is electrically RS-485 bus where there is one
transmitter all the time transmitting and multiple receives along the bus. There
is always one transmitter (usually lighting desk or signal repeater) and there
can be up to 32 receivers in a single bus. The signal voltage is is between the
2 data lines (pin 2 & 3). The difference between the pin 2 and 3 voltages is
what is important: data high (digital 1)is if pin 3 is at a higher voltage than
pin 2, data low (digital 0) is if pin 2 is at a higher voltage than pin 3.
Typicaly the pin 2 and 3 are at either +5 or -5 Volts, but the RS485 limits are
+12 and -7 Volts. In other words, the data is carried over a twisted pair
(connected to pin 2 & 3). The transmitting device has a RS485 driver
(transmitter) connected to pins 2 & 3, and transmits uses +5 and 0 volt levels
with respects to the transmitting devices ground. In receiver side pins 2 & 3
need to be within a few volts with respect to the receivers 0V reference. (pin
1) There must be a difference of at least 200mV between pins 2 and 3 for the
logic state to be reliably detected.
The main advantage of using a twisted pair is the ability accept a certain
amount of common mode voltage (external noise/interference) and still get the
data through. However there are limits to much common mode voltage it can
accept, and the cable screen is used to help limit how much gets onto the data
lines. The DMX512 standard that is not clear is exactly how pin 1 (shield
ground) should be used. There are guidelines to manufacturers on how to
implement electrical isolation between devices. There is an ideal situation
discussed, and alternatives, but unfortunately not every manufacturer has
implemented the ideal. Ideally, the shell of the connectors should be connected
to the chassis of the device its plugged into. The cable screen should be
connected to pin 1 at both ends, with the transmitting device provide a
connection to ground. The receiving device provides no connection to avoid
problems with devices having different ground references.
The wiring for what DMX-512 is designed to is 120 ohm shielded twisted pair
wiring. DMX-512 is designed to use 5-pin XLR connectors, but some
implementations use 3-pin XLR connectors. A DMX network is restricted to 32
devices in a daisy chain (including source), and the far end of the network
should be terminated with a 120R resistor between pins 2 & 3. If more than 32
devices needs to be connected, suitable active signal repeaters and/or splitters
needs to be used to split the wiring to parts with less than 32 devices in each
part.
For reliable operation of DMX-512 system use right kind of cable. DMX may, or
may not, work with microphone cable. This depends on the cable and on the run
lengths. So microphone cable is not recommended. DMX512/1990 suggests Belden
9841/2 and Alpha 5271/2 cables, depending on if you want one or two pairs. There
are many other cables around which work well with DMX512 system. You need a
shielded cable approved for EIA-485 use. Conductors connected to connector pins
2/3 and 4/5 should be twisted together. At 250K bits per second (DMX-512 data
rate) the max cable length is about 1000 ft for DMX512 in good conditions.
The DMX512 communications protocol is very simple and robust. The protocol
used in DMX-512 bus is similar to normal serial communications (like RS-232 with
8 data bits + 1 stop bit) and operates at 250 kbps speed. Transmitting DMX-512
data involves transmitting a reset condition (indicating the start of a new
"packet"), a start code, and up to 512 bytes of data. Data packets are
transmitted continuously. As soon as one packet is finished, another can begin
with no delay if desired (usually another follows within 1 ms). If nothing is
changing (i.e. no lamp levels change) the same data will be sent out over and
over again. Not all 512 channels need to be output per packet, and in fact, it
is very uncommon to find all 512 used. For example, most simple lighting
consoles only output 16 channels or less. The fewer channels are used, the
higher the "refresh" rate. DMX needs to transmit all Channels up to the highest
Channel-number used. So in theory if you leave gaps in your DMX numbering it
cause any problems but it will lower your Refresh rate (More channels to transmit,
so at fixed speed the transmissions take more time and thus you get less repeats
per time interval.) In practice you don't have this amount of choice because
on nearly all lighting control desks the number of dmx channels is fixed and the
refresh rate is fixed. On expensive control desks you can go
into the setup and alter the maximum DMX channel transmitted and therefore
increase the refresh rate.
DMX is a very accurately timed stream of repeating data that loops
continuously. To create this continual stream of data at 250,000 bits per second
takes a lot of processing power and finely tuned software, and as such most
commercial PC to DMX modules use on board memory and a processor to churn the
data out continuously leaving the PC free to work on levels and update the
module as required. The receivers for DMX-512 needs to be carefully designed,
because a DMX512 receiver MUST properly decode ANY possible DMX512 transmitter.
If the standard does not say that a transmitter can't do something someone has
designed one that does!
The SIGNALLING in DMX is a "real" standard - virtually anything can coexist
with virtually anything else and DMX will control it all and things stay out of
each other's way nicely. On the other hand, the cabling isn't since two devices
that use the spare pair differently might object to being on the same
daisy-chain.
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