Blizzard-lighting HotStik User Manual download pdf (Page 17)

Page 17

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 indispensable tool for

any lighting designer or lighting performer.

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