DMX (a shortened name for DMX512-A) is a standard for connecting stage lighting. Nowadays most light fixtures support the DMX512 standard, allowing for the logical control of up to 512 things on a single line of cable. A DMX-capable light fixture will have at least two XLR connectors (probably marked DMX IN / OUT) and some way of setting the DMX address (either a row of small switches or a control panel).
Think of DMX addressing like buildings on a street. Each building has its own number (604 Cornerstone Court) and large office blocks can occupy a range of numbers (22-35 Karen Avenue). You never see two houses with the same number, or adjacent office blocks with overlapping ranges of numbers. Each light fixture has a unique DMX start address and occupies a range of addresses (or channels) from that point onward.
DMX Start Address
Whether you are controlling a single dimmer, an LED, a fog machine, or a moving light, you will need to adjust one or more parameters (possibly dozens), and the way the device knows what part of the DMX signal pertains to it is by setting its start address at some number between 1 and 512.
This is the number of DMX512 data addresses, after the start address, that are relevant to the device in question. If the starting address is “9,” and the DMX FootPrint is “4,” the addresses associated with that device would be “9, 10, 11, and 12”. In an LED that has Red, Green, and Blue parameters, for instance, the footprint is 3 addresses (or channels) wide.
RDM (Remote Device Managament)
RDM is an extension to the DMX standard that allows you to make adjustments to user-selectable features in your lights that are not related to the moment-by-moment playing of lighting cues, but which are ideally controlled remotely such as from a light board or a computer attached to the DMX network. One of the big advantages of RDM is that you can avoid messing with those tiny switches to set the DMX Start Address.
This is the phase of gathering information about the lights (and other RDM-enabled devices) which are hooked up together in your rig. Once you initiate a discovery, it’s done automatically by the RDM controller, and when complete you should be able to look at a list of devices, and do things with them.
Many lighting fixtures have a choice the user can make about how they will behave, such as a basic mode or a more advanced mode with greater flexibility that requires more work to control it. These modes, or “personalities” are traditionally chosen by dipswitch settings or by the user navigating through a button-driven menu. They aren’t things you can alter with DMX512 data, you have to physically touch the device to change them. Another step forward taken by the RDM protocol is that this tradition can now be broken by selecting a personality remotely – from a console, computer screen, or even smart phone – making life easier and safer for the end user.
Whichever mode of operation is in use, is defined by the RDM protocol as the current personality.
If there are multiple personalities, this number alerts the user to the fact that they are there, so an active role in choosing between them can be taken.
Sub Device Count
Sometimes an RDM-enabled product can be thought of as both a container and a series of things inside that container. For example, a dimmer pack might have a starting address and a personality or two for the whole device you can set, but also have individual RDM-related things to keep track of for the individual dimmers it contains. You might want to track the current going through each dimmer individually, or the temperature of the whole box, for instance.
The RDM protocol continues to evolve periodically, so knowing which version of the standard the device was designed to conform to is valuable for a troubleshooter to know.
Device Model ID
The manufacturer decides what to call their equipment as reported through RDM, and this is part of that categorization.
There are a limited number of categories, and these capture broader information that applies across manufacturer lines. Categories might include fixture, dimmer, accessory, test equipment, and so forth.
This is a benchmark that the manufacturer can put into the RDM-enabled device to help determine where in the product’s development history the specific unit being polled fits.