Hi,
I've never worked with DMX-512 but as far as I understand the first byte is a type byte (called start code) indicating what type of information is being transmitted, this is usually 0 (zero) but there are others. Then there are up to 512 bytes of data, first byte is the data for adress 1, second byte is the data for adress 2 and so on. Each "string" of lights etc are called a universe and each universe can occupy up to 512 adresses. If only adress 1-20 are in use you only need to send 20 bytes (start code + 19 data bytes, one for each adress).

But again, that's just my understanding of it after a short read on Wikipedia and some other places.

/Henrik.