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SwitchBlade™ X3 Manual Rev. A
© 2014 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 Associa
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tion (ESTA) has since assumed control over the DMX512 standard. It has also
been approved and recognized for ANSI standard classification.
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 chan-
nels was originally 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 trans-
mission 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 re-
ceiver, 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. Hence, most of us are most famil
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iar with DMX-512 as being employer over typical 3-pin “mic cables,” although
this does not conform to the defined standard.
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 pack-
ets 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.
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.
