Introduction to DMX
Englandsaurus 
Decoding the Data
The previously discussed DMX packet is then sent down the string of devices, but how should a device know which slot of the packet it needs to listen to? DMX fixtures usually have a DIP switch located somewhere to select the starting data slot to listen to, then the fixture will listen for it's fixed number of slots. For example, a simple RGB dimmer will take up 3 slots, one for each color, so if we set the starting slot to 12, it will listen to the data on slots 12, 13, and 14. DMX fixtures should increment their slot counter on the second stop bit, so when the next frame begins, the microprocessor will know which slot the data should be in. The counter should then reset every time a break and mark are detected. This means that a DMX packet doesn't have to have all 512 data slots, as the counter can reset at variable intervals. However, this limits the amount of fixtures occupying a single Universe depending on how many slots each fixture needs.
Remote Device Management (RDM)
Remote Device Management, or RDM for short is a way to use DMX to get information back about lighting fixtures. The protocol sends out a DMX512 packet with an RDM start code (0xCC) followed by a unique ID of the fixture it is attempting to communicate with. The controller will then release the data lines, and wait for a response. After a certain amount of time, the controller will assume it's failed and may try again. RDM is a great way to find out exactly what each fixture is capable of if you're just plugging into an unknown Universe.