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DMX512 standard LED lighting
DMX512 standard LED lighting, DMX512 LED lights,
DMX 512 LED
The communications standard "DMX512" covers
digital multiplexed signals. It is the most common communications standard used
by lighting and related stage equipment.
DMX512 provides up to 512 control "channels" per data link. Each of these
channels was originally intended to control lamp dimmer levels. You can think of
it as 512 sliders on a lighting console, connected to 512 light bulbs. Each
slider's position is sent over the data link as an 8-bit number having a value
between 0 and 255. The value 0 corresponds to the light bulb being completely
off while 255 corresponds to the light bulb being fully on.
DMX512 specifics
DMX512 data is transmitted at 250,000 bits per second using the RS-485
transmission standard over two wires. As with microphone cables, a grounded
cable shield is used to prevent interference with other signals.
There are five pins on a DMX connector: a wire for ground (cable shield), two
wires for "Primary" communication which goes from a DMX source to a DMX
receiver, and two wires for a "Secondary" communication which goes from a DMX
receiver back to a DMX source. Generally, the "Secondary" channel is not used so
data flows only from sources to receivers.
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. Although each device has an input and output connector, these
are merely wired together -- no re-transmission or amplification is performed by
each device.
Each receiving device typically has a rotary switch which sets the "starting
channel number" that it will respond to. For example, if two 6-channel dimmer
packs are used, the first dimmer pack might be set to start at channel 1 so it
would respond to DMX channels 1 through 6, and the next dimmer pack would be set
to start at channel 7 so it would respond to channels 7 through 12.
The DMX512 communications protocol is very simple and robust. It 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. This is one
of the best features of DMX512 -- if for some reason the data is not interpreted
the first time around, it will be re-sent shortly.
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. It is possible to get DMX512 refreshes at around 1000 times per
second if only 24 channels are being transmitted. If all 512 channels are being
transmitted, the refresh rate is around 44 times per second.
DMX512 pin out and cabling details
Standard DMX connectors use XLR connectors with five pins:
Pin 1: Shield (ground)
Pin 2: Primary Data Complement (-)
Pin 3: Primary Data True (+)
Pin 4: Optional Secondary Data Complement (-)
Pin 5: Optional Secondary Data 2 True (+)
A DMX output connector is always female, and a DMX input connector is always
male. Some manufacturers use 3-pin XLR connectors, eliminating Pins 4 and 5.
The DMX512 signal is transmitted via the industry standard interface EIA485,
more familiarly known as RS485. RS485 is a balanced connection. The standard
wiring is a twisted-pair, shielded, low-capacitance data cable designed for
RS-485 -- never use standard microphone cable. Recommended cables are Belden
8227, Belden 9156, Belden 43906 (European DMX Cable Version).
Data is transmitted in serial format asynchronously with the transmission speed
of 250 Kbps. Voltage on both pins ("+" and "-") should be between +12 volts and
-7 volts (measured to ground). EIA485 defines that the signal voltage between
the two wires should be at least 200 millivolts. Higher voltage on the "+" pin
and lower voltage on the "-" pin results in a digital "1". Higher voltage on the
"-" pin and lower voltage on the "+" pin results in a digital "0". The ground
wire is only a reference point and often used for shielding.
DMX devices such as lights are connected in a daisy-chain fashion: from the
controller to light #1, to light #2, to light #3 and so forth. According to the
standard, a DMX512 controller can only drive up to 32 loads (e.g., one light =
one load). But improvements in technology have reduced the load a single light
puts on the circuit, so you may be able to drive as many as 128 lights (each one
being 1/4 load) from a single controller such as the QM2000 board. To control
additional loads (lights), a DMX splitter is required.
The final device in the daisy-chain must be terminated. Terminating plugs
contain a 120 ohm resistor soldered across pins 2 and 3. The terminator
functions by absorbing signal power which would otherwise be reflected back into
the cable and degrade the data.
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