Actually using the method mentioned before its very easy. DMX-512 (as I understand it) is a synchronous RS-485 bit stream. That makes it easy to clock in the sequential data (Channels 1 - 512). Each channel is an 8-bit number for the brightness level of the output lamp. Just use 512 registers (you'll have to use external memory unless you use a different chip) - each one used for an individual lighting channel. If you are only using 12 or 24 channels you could do it all using the internal register memory of the 8051. Instead of using the timer interupt though, you would generate an external interupt from the 50 or 60 Hz mains supply. So, every 10ms an interupt occurs from the zero-crossing of you mains power sinewave. The interupt starts the timer which counts down. The timer has to be set so that it takes one half of the mains cycle to count to zero. Each decrement of the timer is compared to your PWM registers. If the timer > register value then the out put for that channel is high. If the timer < register value the out put for that channel goes low.

The advantage of using the "software comparator" method (as I call it) is that all the outputs are synchronized together.

I hope this makes cense, but I'm sure it doesn't. If you want some more info please email me and I'll try get back to you with some examples and diagrams.