Basics of DMX
DMX Informational Article
If you're new to the AV world, it can be a little overwhelming as you learn all the names of the equipment and cabling. One of the most common acronyms you'll hear is "DMX".
DMX cables, DMX connectors, and DMX consoles. DMX this and DMX that. Everything communicates through DMX but what exactly is it?
Simply put, DMX is the standard for controlling lighting and a basic understanding of how it operates is paramount for any AV tech. DMX is the standardized method for controlling stage lighting, linking consoles to dimmers and special effects devices like fog machines and snow blowers.
We'll briefly go over its history, the connectors it uses, signal flow, and projected innovations in the technology.
Fast History
Digital multiplexing or "DMX" was developed in 1986 by USITT. It quickly became the standard for digital communication networks used to control lighting and effects. DMX-512 is the specific name of the protocol. It has been revised a few times and the current, most up to date version is "DMX512-A".
Connectors DMX Uses
The industry standard for DMX connectors is the 5-pin XLR or more recently the RJ-45 connector. Another type that is used is 3-pin XLR, but this is generally only used on cheaper lighting systems.
Signal flow
DMX512 uses unidirectional differential signaling in conjunction with a packet-based communication protocol.
A DMX512 network, known as a universe, employs a multi-drop bus topology with nodes strung together in what is commonly referred to as a "daisy chain". A network consists of one DMX512 controller with one or more slave devices, aka the lights, dimmers, fog machines, etc.
The most common controller for DMX is going to be a lighting console. Each slave device in the network has a DMX "IN" and "OUT" connector. DMX requires a 'terminator' for the last slave on the daisy chain.
A terminator is a stand-alone male connector with an integral 120-ohm resistor. Some DMX slave devices have built-in terminators that can either be manually activated or some are automatic that sense the absence of a connected cable.
The max number of lights in a single run is theoretically 32 lights, but we suggest keeping it to 20 or less as a best practice. This helps minimize any problems and assists with trouble shooting in the event a light fails.
DMX Splitting (Zoning)
This is where a DMX splitter comes in handy. A DMX Splitter allows you to have multiple groups or zones of 20 on the same network. A splitter refreshes the signal across the groups and helps to manage trouble shooting the network.
It is also useful to know that splitters nowadays are optically isolated splitters or opto-splitters. These splitters use optical isolation to prevent the different groups and zones from communicating across each other.
Development (RDM and Network DMX)
Now, while DMX512 is the industry standard, it does have a few limitations: the slot count of 512 per universe, the unidirectional signal, and the lack of inherent error detection.
The most promising development is Remote Device Management or RDM. This protocol is being developed through the definition of Enhanced Functionality. RDM allows for diagnostic feedback from fixtures to the controller and bidirectional communication.
For example, a light would be able to tell the controller its channel setting and DMX address. It can also relay its internal temperature so a lighting tech could have specific data on whether a fixture is going to overheat.
RDM was approved in 2006 and is slowly gaining traction. It has only been within the last 4 years that companies have been making lights and consoles to utilize RDM, but it’s still mostly limited to high-end fixtures currently. RDM minimizes interaction with individual fixtures and allows the tech to control everything from the lighting console.
Conclusion
Hopefully you found this article informative. This is meant to serve as a very brief overview of DMX, intended to introduce you to the concept and provide a little backstory on the technology.