Manual

Page 16
NorthStar™ SPEC Z 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exibility.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-
sication.
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ve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ows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denedstandard.
DMXisconnectedusingadaisy-chaincongurationwherethesourceconnectstotheinput
oftherstdevice,theoutputoftherst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xturesareused,therstxturemight
be set to start at channel 1 so it would respond to DMX channels 1 through 6, and the next
xture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nished,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rst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nd
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exible,robust,andscalable,anditsabilitytocontroleverything
from dimmer packs to moving lights to foggers to lasers makes it an indispensible tool for
any lighting designer or lighting performer.