What is DMX and how does it work?
DMX is an acronym for Digital Multiplex, a communication protocol (a set of rules) used to remotely control lighting dimmers and intelligent fixtures. It is designed to provide a common communications standard between these lighting devices regardless of the manufacturer.
DMX is a communication protocol that is most commonly used in theatrical lighting. If you want to know the entire history of DMX with more technical detail, please click here. The DMX Control Console will broadcast up to 512 channels over one DMX Cable. Some of these channels may not be used, but will still be transmitted, as required by the protocol. The Decoder in this example is built into the Dimmer. It must be set to a desired channel (channel 001, in this example) to control the connected Light Fixture. This is usually accomplished using a DIP switch or LED/LCD display. This desired ‘channel’ is commonly known as the DMX address.
Many DMX devices (such as dimmers and intelligent fixtures) are capable of receiving several control channels at once. If a Dimmer has four channels capable of controlling four Light Fixtures (Figure 3), it must know which four control channels to receive. This is accomplished by setting a ‘base address’, or the DMX address for the first Light Fixture (channel 005, in this example). The remaining Light Fixtures will be controlled by the next three sequential control channels. The DMX Decoder knows it needs only these four control channels, and will ignore the rest.
The DMX 512 protocol is based on the EIA/TIA-485 standard (commonly known as Recommended Standard 485 or RS-485), which uses asynchronous, differential data transmission. This standard supports 32 devices on one network at a distance of
up to 4000 feet. One device functions as the master (the DMX controller) on a network, while the rest function as slaves (dimmers, intelligent fixtures, etc.). Only the master transmits over the network, and all slaves receive the same data.
While 4000 feet may be specified by the standard, most manufacturers recommend DMX runs of no more than 1000 feet (300 feet between devices) before using a repeater to regenerate the signal. Each device should have input and output connectors, but these are usually wired together. No re-transmission or amplification is performed. Devices are connected in a daisy-chain fashion, from the controller to device #1, then device #1 to device #2, and so on. The final device in the daisy-chain must be terminated. The terminator absorbs signal power which would otherwise be reflected back into the cable and degrade the data. A terminator simply places a 110-120 Ohm, 0.5 Watt resistor across the two transmission wires.
NOTE: DMX cannot be split reliably by making Y-cables or T-connectors. DMX splitter/repeaters (such as the ACCLAIM DD-6WM ) typically use optical isolation to protect each segment from electrical faults on other branches. These can be used to increase the number of devices on one network beyond the limit of 32. Each branch of a splitter/repeater can support up to 32 devices.
Standard DMX 512 requires twisted-pair, shielded, low-capacitance data cable (Figure 7) designed for RS-485 (such as ACCU-Cable from Elation). The twisted-pair configuration ensures that any interference affects both signals equally. This practice is common to good data cable, helping the signal driver eliminate any interference. The cable shield also protects against interference. A shield 'drain' wire makes connector installation easier.
There are many cabling characteristics to consider when designing a system. The following list contains a few such characteristics for consideration:
• Impedance (110-120 Ohm recommended)
• Capacitance (< 25 pF cond.-to-cond., < 40 pF cond.-to-shield recommended)
• Number of conductors/pairs (minimum 1 pair)
• Number of twists per foot
• Conductor material/diameter
• Wire gauge (AWG)
• Maximum current and temperature
• Inner and outer jacket material
• Minimum bend radius
• Maximum pull tension
NOTE: Many people often substitute cheaper balanced audio cable (regular microphone cable) with tragic results. Audio cable cannot support the signal rate required by the high speed DMX protocol. While the signal may pass over short distances, it is highly susceptible to interference and degradation, causing unpredictable results (such as blinking lights, confused intelligent fixtures, etc.).
The cable has a male connector on one end and a female connector on the other end. The male connector plugs into the transmitting, female jack and the female connector plugs into the receiving, male jack.
Cabling for DMX512 was removed from the standard and a separate cabling standards project was started in 2003. Two cabling standards have been developed, one for portable DMX512 cables (ANSI E1.27-1 - 2006) and one for permanent installations (draft standard BSR E1.27-2). This resolved issues arising from the differences in requirements for cables used in touring shows versus those used for permanent infrastructure.
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. Also, cables designed for EIA485 typically meet the DMX512 electrical specifications. Conversely, microphone and line level audio cables lack the requisite electrical characteristics and thus are not suitable for DMX512 cabling. The significantly lower impedance and higher capacitance of these cables distort the DMX512 digital waveforms, which in turn can cause irregular operation or intermittent errors that are difficult to identify and correct.
DMX512 data are sent using EIA-485 voltage levels. However, quoting from E1.11, "The electrical specifications of this Standard are those of EIA-485-A, except where specifically stated in this document. Where a conflict between EIA-485-A and this document exists, this document is controlling as far as this Standard is concerned."
DMX512 is a bus network no more than 1,200 metres (3,900 ft) long, with not more than 32 devices on a single bus. If more than 32 devices need to communicate, the network can be expanded across parallel buses using DMX splitters. Network wiring consists of a shielded twisted pair, with a characteristic impedance of 120 Ohms, with a termination resistor at the end of the cable furthest from the controller to absorb signal reflections. DMX-512 has two twisted pair data paths, although specification currently only defines the use of one of the twisted pairs. The second pair is undefined, but required by the electrical specification.
Some equipment manufacturers such as Chauvet and American DJ have disregarded the formal topology rules, and designed their equipment to use nonstandard 3-pin XLR connectors rather than the proper 5-pin DMX connectors, so as to eliminate the need for the unused second pair and allow for the use of regular microphone audio cables. Similarly, end users can create adapter pigtails to convert from the correct 5-pin DMX to a 3-pin XLR microphone cable. For short cable runs of less than about 45 metres (148 ft) with only a few devices, it is sometimes possible to operate without termination. At short distances, cables with higher capacitance and different characteristic impedance such as microphone cable can be used. As the cable length and/or number of devices increases however, following the specification for termination and correct cable impedance becomes of vital importance.
The E1.11 (DMX512 2004) electrical specification addresses the connection of DMX512 signal common to Earth ground. Specifically, the standard recommends that transmitter ports (DMX512 controller OUT port) have a low impedance connection between signal common and ground; such ports are referred to as grounded. It is further recommended that receivers have a high impedance connection between signal common and ground; such ports are referred to as isolated.
The standard also allows for isolated transmitter ports. Systems with the transmitter port and all receiver ports isolated are fairly common. The standard allows for non-isolated receivers.
Good practice dictates that systems ground the signal common at only one point, in order to avoid the formation of disruptive ground loops.
Grounded receivers that have a hard connection between signal common and ground are permitted but their use is strongly discouraged. Several possible grounding configurations which are commonly used with EIA485 are specifically disallowed by E1.11.