DMX512-A (commonly called simply DMX) is a digital communications and wiring standard that was initially developed to provide control for theatrical and stage lighting. In addition to being used in the theatrical industry, DMX is now also used frequently in architectural applications to provide control of color-changing and dynamic white fixtures.

Physical Characteristics

DMX networks (called "Universes") consist of a single DMX controller and multiple controlled devices. In architectural applications, the controlled devices are typically lighting drivers for color-changing fixtures. Below is a brief explanation of the physical characteristics and data protocol for DMX networks.

DMX data is transmitted over a differential pair using cabling that largely adheres to the EIA-485-A standard. The most commonly used connectors are 5-pin XLR and RJ-45. RJ-45 connectors are only to be used in long-term installations where frequent plugging and unplugging of equipment will not occur, while XLR cables are more common in the theatrical industry.

DMX devices have separate DMX input and output ports, and devices must be daisy-chained together using these in/out ports. Wiring DMX devices in a star ("T-tapping") or ring (class A) topology is not allowed, nor is it allowable to wire multiple DMX cables to a single output at a DMX controller. For devices with 5-pin XLR ports, the male port is the input (from previous device or controller) and the female port is the output (to next device). XLR cables for DMX will typically consist of a female 5-pin XLR connector on one end and a male 5-pin XLR connector on the other. DMX devices intended for use in architectural applications will sometimes have terminal blocks for the DMX cabling instead of XLR or RJ-45 ports. These terminal blocks should be labeled with "DMX in" and "DMX out". 

Each run of DMX should not exceed 400 m (1300 ft) in length and have no more than 32 devices connected. The last device on a run (the end-of-line) should be terminated with a single 120 Ohm resistor across the DMX data output. DMX splitters are available for applications where the number of DMX runs required exceed the number of DMX outputs on the controller. Splitters provide multiple DMX output ports for connection to devices in the field, and will typically have separate DMX in and DMX out ports for connection to the controller or other splitters. Splitters are themselves a DMX device, so each output port can support a maximum of 31 additional devices.

Protocol and Addressing

A DMX universe consists of 512 channels of control, and each channel corresponds to a different DMX address. Drivers are assigned an address, and listen to commands being sent for that address. Color changing drivers will typically occupy multiple addresses, with each address corresponding to a different color value or color temperature (e.g. an intensity/CCT fixture will use one address for intensity and one for color temperature, while an RGB fixture will use one address for red, one for green, and one for blue).

Additionally, the number of addresses that a fixture requires is affected by whether it is has an 8-bit or a 16-bit driver. 8-bit drivers will use a single DMX address for each channel of control, whereas 16-bit drivers will use two DMX addresses per channel. The benefit of using 16-bit over 8-bit is that it allows for fine-tuning fixture color with double the resolution (0-510 instead of 0-255), with the main drawback being the larger address footprint. 

To control fixtures, a DMX controller will broadcast packets of lighting data to all devices on the network. These packets contain 512 bytes of channel data, and DMX devices on the network will adjust their output according to the value of the channel that corresponds to their address. For example, if an 8-bit RGBW driver with a start DMX address of 1 receives a packet that has channel 1 at 3, channel 2 at 223, channel 3 at 252, and channel 4 at 0, then it will adjust the output to the fixture in order to achieve a cyan (green-blue) output. 

DMX fixtures do not need to be uniquely addressed, though fixtures which are addressed-alike will be controlled together. Problems can arise if different types of fixtures (e.g. both intensity/CCT and HSI) have the same start address, or if different fixtures have overlapping addresses (e.g. one RGBW fixture with start address 1 and a second RGBW fixture with start address 3). Most fixtures will come from the factory with a default address, which will usually need to be changed before they will be controlled properly by the ShowRunner™ lighting control system.  

Addressing Drivers

Most DMX drivers must be physically addressed at the driver, but there are some models capable of what is called Remote Device Management (RDM).

For fixtures that are not RDM capable, the methods for addressing will vary by manufacturer. Typically drivers will have dipswitches or dials that correspond to the start address for the driver. Some manufacturers will include an LCD display and buttons to make setting and viewing the current address settings easier. Refer to the manufacturer for driver specific details.

For RDM capable fixtures, there are several remote-addressing possibilities to allow for addressing devices without being physically at the driver. Handheld tools such as the Swisson XMT-350 allow for testing of the DMX cable and remote addressing of fixtures on the network.

Many Pharos DMX controllers, such as the Pharos LPC, also come with remote device management tools that are accessible through the Pharos Designer 2 interface. Once connected to the controller in Designer 2, navigate to the "Patching" page, select the controller and universe, and then click the "Discover on Port #" button to begin. RDM discovery on a port. If any fixtures are discovered, they will populate in the new pop up and indicate their start address and the number of addresses required. Fixtures can also be re-addressed and identified (flash the lights) from this menu.