This is a digital multiplexing system, nothing more.

The idea is only to transmit 512 dimmer levels over a twisted pair cable.  So
they start the frame with a break, send a start code (which is defined as 0x00
until they release the next spec) and the 512 (or less) dimmer levels are sent
in order in 8bit bytes, 1 start bit, 2 stop bits.  A break is sent, and the next
frame starts.  It's possible to send 44 frames/sec at 250Kbps, with each of the
512 values included.  You can send shorter frames by ending the transmission
anywhere after the start byte.  ie, the dongle you can build mentioned on this
thread only controls 64 lights, so it sends the first 64 bytes after the start
byte, and then sends the next frame, etc.  The last 448 dimmer channels are
never sent.  Using this method, it's possible to send the values for all 64
channels over 300 times/second.

You can get a somewhat more detailed description on my site at:
http://www.ubasics.com/adam/electronics/dmx512.shtml


-Adam

Nigel Orr wrote:
>
> At 06:22 31/08/99 -0700, you wrote:
> >        1.  Break
> >        2.  Mark after break
> >        3.  Start Code (0x00 for most lighting devices)
> >        4.  Up to 512 bytes where each byte is a lighting level (0x00 to
> 0xff)
>
> I don't remember the details, but aren't they pairs of address and level?
> Nothing in the above tells you which light is being controlled, and you can
> have lots of lights/dimmers in parallel on the bus.
>
> Nigel