When Remote Device Management Speaks...Consoles Listen
by Richard Cadena
What does it take to put a man on the moon? A rocket, some fuel, and lots of computer power. What does it take to change the DMX address on a lighting fixture that's flown in a rig? A person on a ladder. Houston, we have a problem.
It's been over 40 years since we landed a man on the moon, so why can't we change the DMX start address remotely from a lighting console? It's not rocket science!
DMX has been used successfully for almost 25 years. It's a uni-directional protocol, meaning that the controller spits out data packets to the receiving devices without regard for whether or not the data was received properly. It works because the data for each of the 512 control channels or "slots" is constantly being refreshed. If one of those packets doesn't reach its intended destination then another one will come along soon enough and the value for the device under control will be corrected in less than the blink of an eye.
But DMX has its limits.
RDM to the Rescue
As a one-way or "uni-directional" language, DMX has no ability to handle seemingly simple tasks that should be as easy as pushing a button on a radio to ask the lighting tech to climb the truss and change the DMX address. And as of December 2006 there is another way to accomplish the same task faster and easier than finding a free stagehand. It's called Remote Device Management or RDM.
RDM is a protocol that is separate from DMX although it works seamlessly with DMX on the same wires. It's intended to replace a person on a ladder with keystrokes on a controller and it allows a lighting operator to perform all of the functions that can be performed through the menu of functions, and possibly more, on a typical automated light or other device, like a dimmer racks.
In most cases RDM doesn't change how we use DMX; it uses the same consoles, the same output ports, the same DMX cables, and the same devices. The difference is that the manufacturer can add a few lines of computer code to the operating systems of the console and devices under control to allow two-way, or "bi-directional" communication on the same DMX network using the RDM protocol. That's because the chip in the console and in the receiving device that outputs DMX has always been capable of bi-directional communication; RDM simply takes advantage of both the transmitting and receiving functions of the transceiver chip. But it can only operate in half duplex mode, meaning that a console or receiving device can transmit or receive data, but never at the same time.
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