DMX Protocol Basics

DMX 512 is communication standard for controlling stage lighting.

DMX 512 is digital (it sends 1 and 0), unidirectional (one way only, from DMX control to DMX device, like moving head), serial (it sends 1 bit (1 or 0) at the time) protocol that was based on RS 485 (now EIA-485) specifications. However, comparing to RS485, DMX’s physical layer is different. DMX does not have any kind of error checking (and auto correcting), and that’s way it is not permitted to use DMX with pyrotechnics and other devices where performers and audience can be at risk.

DMX 512 protocol has been made with idea of controlling only dimmers; however manufactures soon started using DMX for other purposes, like controlling movement of scanner mirrors, fog machines and many other devices.

DMX was developed by USITT – Engineering Commission of United States Institute for Theatre Technology. First version was published in 1986. Major revision was in 1990 that became known as USITT DMX 512/1990.

DMX was revised again in 1998., to become ANSI standard. This revision is now known as “Entertainment Technology – USITT DMX512-A”. In 2004. ANSI – American National Standards Institute officially approved DMX as ANSI standard. Today DMX512-A standard is maintained by ESTA.

To use DMX you need 1 DMX controller and at least 1 DMX device. DMX Controller is first device and has DMX OUT only connector (some consoles have DMX IN too, but for controlling DMX lighting you need only DMX OUT).You should connect DMX OUT (console) to DMX IN (DMX device) connector. In the same fashion (Out -> IN) you can connect up to 32 devices in one DMX line, and control them with one DMX console.

Errors with DMX are very common in practice. However, DMX by itself is robust protocol and in most cases problems are not with DMX as standard, but with irregular implementation of DMX, cables, terminators etc.

Some manufactures use three-pin XLR connectors instead of five-pin XLR. In practice, three-pin XLR are as good as five-pin, however there is good reason for using five-pin XLR. Three-pin can be connected to a sound board by mistake. Theoretically, sound boards are generating much higher voltage, so DMX 512 devices that are connected to the network could potentially be damaged.

DMX cables, theoretically, should use five wires, but in practice only three wires are used, Signal Common, Data -, Data +. Many people use standard microphone cables for DMX. In some cases microphone cable can work, however microphone cables have different electrical characteristics. Standard microphone cables have lower impedance and higher capacitance, so these cables can distort DMX packets and errors can occur. DMX cabling is separate standard today.

At the end of every DMX chain there should be Terminator (120Ω) plugged into DMX OUT. Again, terminating end link is not mandatory, but it’s good practice.