DMX512 (“Digital Multiplex with 512 pieces of information”) is a standard for digital communication networks that are commonly used to control stage lighting and effects. It was originally intended as a standardised method for controlling light dimmers, which, prior to DMX512, had employed various incompatible proprietary protocols. However, it soon became the primary method for linking not only controllers and dimmers, but also more advanced fixtures and special effects devices such as fog machines and moving lights, and has also expanded to uses in non-theatrical interior and architectural lighting.
The first step in understanding DMX is knowing how a DMX Controller works. Modern RGB lighting offers fantastic possibilities for home, commercial and architectural applications, that reach their full potential when combined with a control method such as DMX. A controller is typically split into four control modes, these are:
1 – Zone or Circuit Selection (Selection of individually addressed units or circuits)
2 – Colour Selection
3 – Dimming / Fade Control
4 – Program Selection (Chase, Strobe, etc)
The first step is to select which unit, or selection of units you wish to control, this depends on the DMX controller as to how many individual selections you are able to control.
Then you can select the colour you wish to set, this is covered in more depth in the ‘Data Packet Architecture’ section below but basic controllers use a rotary dial for this selection and the more advance allow you to enter specific values.
The dimming and fade aspects of the control can then be set, which in conjunction with the colour selection can produce the exact colour shade you are trying to achieve.
Lastly, any programmed ‘effects’ you wish to use can then be set. Again, depending on the controller, basic effects such as chase can be set, or on the more advanced models your own effects can be programmed in.
Cable Architecture (RJ45)
The cabling of DMX for use with theatrical or stage setups is usually done with either 3-pin or 5-pin XLR connectors. This was an original specification for the DMX Standard but since then DMX512-A (E1.11-2008) allows the use of eight-pin modular (RJ-45) connectors for fixed installations where regular plugging and unplugging of equipment is not required.
Some DMX512 equipment manufacturers at the dawn of the DMX era employed non-compliant or proprietary connectors and pinouts; eventually, the most common of these became the already common three-pin XLR connector (also called cannon jack in some countries), since the electrical specification currently only defines a purpose for a single wire pair.
The electrical characteristics of DMX512 cable are specified in terms of impedance and capacitance, although there are often mechanical and other considerations that must be considered as well. Cable types that are appropriate for DMX512 usage will have a nominal characteristic impedance of 120 ohms. Cat5 cable, commonly used for networking and telecommunications, has been tested by ESTA for use with DMX512A.
This (right) is the wiring specification of RJ45 for DMX; the avoidance of pins 4 and 5 helps to prevent equipment damage, if the cabling is accidentally plugged into a single-line public switched telephone network phone jack.
Data Packet Architecture
DMX Packet architecture is a fairly in-depth subject in and of itself so it is best to just cover the basics:
- Each packet consists of 512 channels of data
- Each channel has a value of between 0 and 255
- 3 channels are assigned to a unit or circuit (multiple of units) through the use of a DMX Receiver.
- The Receiver is set with an address using dip-switches
- The channel values relate to colour values, the mixture of Red, Blue and Green making up the final shade.
- The values of the channels are transmitted in binary, an example of that is below.
An example can be seen here (right) of how the three Red, Green and Blue channel values result in the final colour produced. As 254 values are available for each primary colour, the actual number of available shades is 255 x 255 x 255, which equals 16,581,375.
Series Wiring & Unit Addressing
Each mono or single-colour unit is assigned a single ‘channel’ of which there are 512, and as mentioned above, that channel is then set a value between 0 and 255.
In the case of the mono unit, this value would dictate the dimming level – 0 would be 0% (hence off) and 255 would be 100%, or maximum brightness.
The receiver is assigned an address based on dip-switches, so as an example address 7.
Within the packet of data that is transmit, whatever value is assigned to channel 7 within that packet will be picked up by that receiver.
The same goes for any and all receivers set up in the circuit, if another is assigned the same address further along, then the same value will be picked up and the same dimming level assigned to that unit.
DMX is always wired in Series for this very purpose, the daisy chain effect forms a half-duplex with one data path that all the devices share.
A standard principal in DMX lighting is that the end of line must be terminated using a 120 Omh resistor, but why?
The simple answer is that it prevents the DMX signal from bouncing back down the data line.
Cable has inductance and capacitance, as well as resistance. The combination of a cable’s inductive reactance, capacitive reactance and resistance is called its “characteristic impedance.” The theory behind it is beyond the scope of this discussion, but a cable’s characteristic impedance is independent of its length. A 10-inch piece of Belden 9841 (a recommended type for DMX-512) will have a characteristic impedance of 120 ohms. A 10-mile piece of Belden 9841 will also have a characteristic impedance of 120 ohms.
Characteristic impedance is very important to the behaviour of pulses travelling down a cable (and DMX-512 signals are high-speed digital pulses). Pulses don’t like to see changes in the characteristic impedance. When the characteristic impedance changes, it causes a part of the pulse to bounce back up the cable in the direction of the source. The bigger the change, the bigger the bounce. If you go from the cable’s 120 ohm impedance to an open circuit (near-infinite impedance), you get a pretty big bounce. The pulses bouncing back from the end of the cable can interfere with the pulses still coming from the controller and cause your lights to misbehave.
Philips Lighting – Introduction to DMX
This is an excellent guide written by Philips Lighting. It outlines most of the above information in greater detail and provides several diagrams to demonstrate the principals.