Blizzard Lighting Kontrol 6 Skywire(Rev. A) - Manual (Page 17)

Page 17
KONTROL 6 Skywire Manual Rev. A Copyright © 2014 Blizzard Lighting, LLC
4. APPENDIX
4.1 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 light-
ing 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 classification.
DMX covers (and is an abbreviation for) Digital MultipleXed signals. It is the most com-
mon communications standard used by lighting and related stage equipment.
DMX 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 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 cor-
responds 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
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. 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 transmitted. If all 512 channels are being transmitted, the refresh rate is around
44 times per second.
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.