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he 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 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.
 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.
DMX512 was created in 1986 by the United States Institute for
Theatre Technology (USITT) as a standardized method for connecting
lighting consoles to lighting dimmer modules. It was revised in 1990
to allow more
flexibility.
The
Entertainment Services and Technology Association (ESTA) has assumed
control over the DMX512 standard. At this writing (May 2000), 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. So there is no need to hold off on
implementing the existing DMX512 standard, or on buying existing
DMX512 devices -- these will work fine under any new
revision.
ESTA refers
to the current draft revision as the "BSR 1.11 -- DMX-512/2000"
standard. The draft is being taken through the rigorous process for
establishing consensus so it may become an American National
Standard. The draft may be available for public review sometime
between August and December 2000.
-
DMX512 is a method of connecting a single controlling source to
multiple receivers
-
Serial data can be sent up to 4000 feet over microphone-like
cables
-
Up to 512 devices, or functions on a device, can be controlled
with 8-bit resolution
-
All channels are continually being "refreshed", which increases
safety
-
Although the DMX512 standard is being updated, existing
equipment will still work the same under any new revision.
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