DMX Bus Interface

A balanced differential transmission method is used, in DMX using shielded twisted pair cabling.

Cable Connection

The DMX cable specification states that the cable transmitting data must have 3 wires. One ground, one positive (A) and one negative (B) connection. Logically a digital signal of 1 will be sent when the potential of the positive wire is greater than the potential of the negative wire and a signal of 0 will be sent when the situation is reversed. The ground wire in the cable is usually used as a noise shield offering two benefits: Noise will be of the same phase and magnitude in both wires and voltage degradation will be equal in both wires.

3 Pin XLR Cable and 5 Pin Terminator

In most cases it is recommended that a 5 pin XLR cable is used for DMX applications. Although only 3 lines are normally used. The extra pins are not dedicated to a specific use. The 3-pin XLR connector is also often used. Although this connector is used for audio applications also, the cable for DMX is NOT the same!

Male Female
Pin 1 (screen/GND)--------------------- Pin 1 (screen/GND)
Pin 2 (A -)---------------------------------- Pin 2 (A -)
Pin 3 (A +)--------------------------------- Pin 3 (A +)

EIA/RS-485

The DMX standard uses EIA-485 (formerly RS-485 or RS485). The design of a RS-485 physical layer must set a maximum baud rate appropriate for the link to select a RS-485 driver circuit that is capable of running at the required speed. A driver circuit may be selected that is rated at a speed equal to or greater than the required baud rate. A driver circuit therefore includes an output filter to control the slew-rate of the signal. This shapes the signal to reduce signal harmonics at frequencies above the baud rate. Without this the signal will have spikes in the frequency domain that make it much more likely to induce interference to other signals.

The differential transmission used by RS-485 tries to minimize radiated emissions by using twisted-pair cabling and balanced transmitters. The idea behind this is that the balanced transmitter, as the name implies, will generate two equal but opposite signals that are sent down the two wires in a twisted pair. Because the wires are virtually on top of each other, they will tend to radiate the exact opposite signal that the other wire is transmitting. This has the effect of canceling each other out, and ideally results in no net radiated emissions.

The standard specifies a differential form of signaling. One polarity of voltage indicates a logic 1 level, the reverse polarity indicates a logic 0 level. For transmission of a 0, the signal will be 200mV high on pin 2 and -200mV low on pin 3. For a transmission of 1, the signal will be -200mV on pin 2 and 200mV high on pin 3.

RS-485 offers a half-duplex, multipoint serial connection.

At the receiver, the difference between the voltage on the pair of wires (A,B/A-) is used to detect the data. The difference of potential between A and B must be at least 0.2 volts for valid operation, but any applied voltage between +12V and -7V is allowed at the sender.

A real-world DMX Receiver incorporating a number of additional features.

Pull-up/down resistors (e.g. 50K-100K) - can be incorporated to eliminate stray signals, by ensuring the positive and negative lines carry a bias current. Capacitive coupling to ground may be included (e.g 10 nF, 1kV) to act as a by-pass for RF interference.

Opto-isolated signal should be used, with DC-to-DC conversion when the receiver power supply may be using a different mains phase from that of the sender.

DMX Termination

Cable terminations are also important. Both ends of an RS-485 cable should be properly terminated in the characteristic impedance of the cable to prevent reflections. The termination impedance value should match the cable characteristic impedance. The termination of the cable with this impedance gives the cable the appearance of being infinite length, theoretically allowing no reflections of the transmitted signal. When the cable is cut to any length and terminated, measurements will be identical to values obtained from an infinite length cable.

Resistor and cable tolerances, among other things, can result in mismatches between these two impedances. This will result in reflections that increase the noise and can ultimately lead to corruption of data. Similar to radiated emissions, the higher the frequency components and the longer the cable, the more likely it is that reflections will affect the performance.

The simplest terminator consists of a resistor. A more sophisticated device incorporates a surge suppressor.

Use of a bi-directional typical driver chip, with a termination, Rt.

Multi-Drop Bus

The maximum number of receivers that may be placed across the control bus is given by the receiver input resistance. In DMX-512 the input impedance of 12 k Ohms restricts this to 32 devices on a single bus. (32 parallel receivers have an overall impedance of 376 Ohms) This is the safe value for cable runs up to 300m.

DMX operates at a data rate of 250 k baud. Reducing the number of receivers can allow much longer lengths of cable at the same baud rate. For instance, a cable run of up to 1000m may be possible with some line drivers, with only two devices. Many practical products can not manage to drive a cable of even 300m!

A splitter may be used to divide the bus into multiple cable segments. This allows an increase in the cable length and allows up to 32 devices per cable segment.

DMX Splitters

Since DMX forms a bus, a single cable must pass through all equipment. To split a DMX signal for example to extend the length of a DMX bus a splitter must be used. Simply creating a Y on a link will create data reflection and termination issues.

A splitter used with RDM must be able to deal with the problems of transmitting data on a half-duplex bus and onto the original bus.

DMX Universe

One cable controlling up to 512 channels is called a universe. A universe is a separate bus connecting independent equipment. Multiple universes can exist where more than 512 channels are controlled, however they must be cabled individually. This is usually common practice where one bus controls lighting whilst another controls moving objects. A system information packet (SIP) can be used to provide information to receivers. The SIP contains data on the current universe, number of packets sent and received since last SIP.

Other DMX Cable Types

Shielded twisted pair 100-120 Ohm Cat.5 S/UPT, F/UTP, SF/UTP Cat.6 U/FTP, S/FTP, S/STP Cat.7 S/FTP, S/STP

Asynchronous Character Framing

Packets are formed from a sequence of slots. Slots are grouped into a frame with a size from 25 bytes up to 513 bytes.

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland.