Dear Gabriel:
to generate full package of dmx512 you need to sendout the signal as above:
the first signal call "Break" , a low signal >88usec(in most of the case this value are much bigger then 88usec),after that is a "Mark After Break" signal-- "hi" siganl for 8 usec ,next one you will send is call "Start channel" which is in byte value--"0"(always "0"),before you atarting to send out the first dmx cahnnel data , or between channels there is a "Make Between Frams" signal need to take care,a "hi" signal for MBF value could be 0usec to 1sec,so for more detail about DMX512 check out USITT .
So for the uart eather in 8 bits mode or 9 bits both works find but 8bits mode will be more easy to handle, if you put a singal fram of dmx512 data like "1 start bit ,8 data bits,(2 stop bits + MBF times)",then setup a time base like (44usec+MBF)>=88usce starting the tome base timer first and then enable the uart (like setb tr2)you will get a stable dmx512 siganl.
good luck & have fun
Chris Chang