What are DMX devices

DMX control basics

DMX control basics

Headlights, for example moving heads, are controlled via DMX512. Here you can find out everything about DMX control, how to set up a DMX network, what DMX addressing is all about and what you should pay attention to when wiring your devices.

The DMX basics guide - everything you need to know about controlling lighting technology

By Alexander Cevolani

What is DMX?

DMX512 or simply DMX (»Digital Multiplex«) is the standardized protocol in lighting technology for the transmission of digital control data between a transmitter (e.g. a DMX controller) and one or more spotlights that have a DMX interface.

In the market overview: DMX controller »

The simplest setup for DMX control consists of a transmitter with DMX output and a spotlight with DMX input. Since you need more than just one spotlight to illuminate a stage, you will usually come across a collection of several spotlights and spotlight types (PAR, scanners, floodlights, moving heads, etc.) in everyday event practice. In the following you will find out how to combine these into a functioning DMX network.

Moving heads like the Cameo Auro Spot 400 are controlled via DMX - here via 5, 19 or 27 channels

DMX control: standard & cable

DMX control signals are digital bit signals that are transmitted serially via an RS485 interface. A maximum of 512 channels can be sent via a single DMX line (hence: DMX512), the transmission rate is 250 kBit / s.

You can use different types of cables to transmit DMX signals. The so-called DMX cables with 5-pin XLR plugs are the official standard. These work according to the AES / EBU standard with a characteristic impedance of 110 ohms in order to ensure that the signal transmission is as immune to interference as possible.

In practice you will find DMX cables with 3-pin XLR plugs much more often than the 5-pin version. Although these do not correspond to the official DMX512 standard, they are often used for reasons of cost, as pins 4 and 5 are not used for the actual signal transmission and are not used in simple setups.

DMX cable with 5-pin plug from the Adam Hall Cables 5 Star series - 3-pin connections are more common

Microphone cable for DMX signals?

A microphone cable is not a DMX cable, even if these two cable types are absolutely identical on the outside! And so it is actually not suitable for DMX control. Even so, microphone cables are often used as DMX cables. Note, however, that the basic frequency of a DMX control signal is 125 kHz and microphone cables - according to the human ear - are intended for the transmission of frequencies between 20 Hz and 20 kHz.

In addition, microphone cables do not have a fixed wave impedance, which can lead to signal reflections and thus to disturbances in DMX transmission. Basically, the longer the cable run, the more susceptible to interference are microphone cables.

Construction & cabling

Building a DMX network is comparatively easy as long as you follow a few basic rules. Since DMX512 is a serial protocol, you also have to wire the spotlights serially in your setup. That means: You wire the output of your transmitter (e.g. the DMX controller) with the DMX input of the first spotlight or receiver. Its DMX output continues to the DMX input of the next fixture, etc.

Up to 32 devices can be controlled in the DMX512 universe

You can connect a maximum of 32 devices to a single DMX512 line. If you want to use more than 32 spotlights or the 512 DMX channels are not sufficient, you have to switch to a second DMX universe. A DMX line with 512 channels is called a universe. DMX controllers that can output several universes therefore have several separate DMX outputs.

You should put a so-called »terminator« at the last spotlight in your DMX line. This is an XLR plug with a terminating resistor of 110 or 120 ohms, which prevents part of the digital DMX signal from being reflected at the end of the cable chain and thrown back as interference on the signal path. Since a terminator only costs a few euros, you should literally not save money at the wrong end!

Disadvantages of serial DMX transmission

DMX splitter from the manufacturer Enttec

As simple as serial DMX cabling is, it can thwart your plans. Because if a cable connection or a spotlight fails in the DMX setup, the entire subsequent chain fails because from this point onwards, no more data can be transmitted. By using a DMX splitter, you divide your DMX line into several parallel strands and minimize the devices in your setup that fail at the same time in the event of a technical problem.

In addition, a DMX splitter serves as a booster (comparable to a WLAN repeater) that refreshes the signal quality of your DMX data stream.

DMX addressing

If all headlights are correctly wired via DMX and connected to the lighting console as a DMX controller, you can assign specific DMX channels to the individual devices in your setup. You usually set the respective DMX channel directly using the function buttons on the headlight.

Depending on the selected channel mode, the spotlight occupies a certain number of consecutive DMX channels in your DMX setup. An example: Your first spotlight is a moving head and occupies nine DMX channels in the selected channel mode. Set channel 1 as the start address via the menu of the moving head. Moving Head 1 thus occupies DMX channels 1-9, whereby you can control a separate function of the moving head via each of these DMX channels.

You set the DMX start address via the control panel on the spotlight

A subsequent spotlight would now receive the start address 10 and, for example, occupy six DMX channels (10-16), a third with start address 17 occupy channels 17-32, etc. Of course, you can also give several spotlights the same start address. In the case of headlights of the same model, these then run synchronously.

Here you can find information about the profession of lighting technician »

Number of devices with DMX control

Note that many DMX controllers can only control a certain number of DMX devices (e.g. 16) and often work with fixed channel assignments, whereby the start addresses for the individual spotlights are specified.

Each DMX channel works with a value range of 0-100% and is coded with eight bits, so that 256 bit values ​​(2 to the power of 8) result for the control of a DMX channel. For example, you can control the speed of the PAN movement via a single DMX channel or switch between the colors of an RGBW LED.