RDM, or Remote Device Management, essentially allows not only communication from your lighting console or control device to your fixtures, but in the reverse as well.
But in order to understand how RDM works, you need to know some basic knowledge behind DMX 512 protocol.
The DMX 512 protocol was first standardized in the mid-nineties but it was originally an idea adopted from a computer protocol called EIA-485. How does it work? Well, first off it speaks in binary (ones and zeros) and divides your light level (0-100%) into 256 steps. This information is sent from your console to your dimmers as a sequence of numbers - first a reset signal called a break, then a start code, then the level for dimmers one, two, three, four, etc.
Why 256 steps? The majority of microprocessors use an 8bit number format which has 256 possible combinations of ones and zeros meaning that if you have to divide your min and max levels into the 256 steps to have usage of the full range within your 8bit constraints. And guess what - 256 + 256 = 512. MATH.
So how the hell does all this 256 step ones and zeros business get sent anywhere? Why, through none other than your 5-pin DMX cable.
Pin 1 - Common or the Shield
Pins 2 + 3 - Data+ (Data True) and Data- (Data Compliment)
Pins 4 + 5 - Data+ and Data-
The above configuration is if you are using RDM and an RDM compatible device. However, Pins 4 and 5 were previously just there for future use (aka: now) and some people don't even connect them to be honest. But us lighting people needed to keep our equipment separate from audio folks anyway by using 5 pins and not 3, even though that's all we needed at the time.
In standard DMX 512 your console sends information to your fixtures or devices using pins 2 and 3 telling it what it needs to do. But in DMX 512 RDM, your devices can tell your console what's up as well. This is beyond awesome in all reality. Now if a device is having an issue it can basically send out an S.O.S. to your console saying "Uhm, excuse me. My lamp just died. Gimme a new one." You can also do some patch work and "discover" devices; but this can take quite a long time if you have a decent sized rig due to the simplicity of how DMX functions - your console would do a tree-sort and that's a tad time consuming.
But still. RDM is pretty exciting, especially since the industry is developing more and more devices that are capable of RDM.
