Page 19
Torrent™ Xray Manual Rev. A
(c) 2013 Blizzard Lighting, LLC
5. APPENDIX
A Quick Lesson On DMX
DMX (aka DMX-512) 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 and again in 2000 to allow more flexibility. The
Entertainment Services and Technology Association (ESTA) has since assumed control over
the DMX512 standard. It has also been approved and recognized for ANSI standard clas-
sification.
DMX covers (and is an abbreviation for) Digital MultipleXed signals. It is the most common
communications standard used by lighting and related stage equipment.
DMX provides up to 512 control “channels” per data link. Each of these channels was origi
-
nally intended to control lamp dimmer levels. You can think of it as 512 faders 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.
DMX data is transmitted at 250,000 bits per second using the RS-485 transmission stan-
dard 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 receiv
-
ers. Hence, most of us are most familiar with DMX-512 as being employer over typical
3-pin “mic cables,” although this does not conform to the defined standard.
DMX is connected using a daisy-chain configuration 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.
Each receiving device typically has a means for setting the “starting channel number” that
it will respond to. For example, if two 6-channel fixtures are used, the first fixture might
be set to start at channel 1 so it would respond to DMX channels 1 through 6, and the next
fixture would be set to start at channel 7 so it would respond to channels 7 through 12.
The greatest strength of the DMX communications protocol is that it 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 a great feature of DMX -- 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. The fewer channels are used, the higher the “refresh” rate. It is possible to
get DMX refreshes at around 1000 times per second if only 24 channels are being trans-
mitted. If all 512 channels are being transmitted, the refresh rate is around 44 times per
second.
In summary, since its design and evolution in the 1980’s DMX has become the standard
for lighting control. It is flexible, robust, and scalable, and its ability to control everything
from dimmer packs to moving lights to foggers to lasers makes it an indispensable tool for
any lighting designer or lighting performer.
