Manual

Page 13

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 pack-

ets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 transmitted. 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.