User guide

Page 15

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 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

indispensible tool for any lighting designer or lighting performer.