Owner manual

Page 15

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.