DMX 101:A DMX 512 Handbook

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 lighting devices regardless of the manufacturer.Understanding DMX and properly calculating your DMX channels is vitally important to plotting your lighting grid and implementing a successful lighting design.

Our friends at Elation Professional have produced a helpful handbook designed to provide a basic understanding of the DMX 512 protocol,including theory of operation, proper equipment use, and some basic application examples. Following, is an excerpt from that handbook – you can download the PDF free here.

Introduction to DMX

Often those of us plunged into the entertainment lighting industry, through work or circumstance, find ourselves inundated with industry jargon and information overload.

What is a moving head fixture, and how do you control it? What is this DMX I keep hearing about? This handbook is designed to promote a basic understanding of the DMX 512 protocol. With this in mind, it covers the basics in theory of operation, proper equipment use and some very basic sample applications.

DMX 512 is a communication protocol, a set of rules, that are used to remotely control lighting dimmers and intelligent fixtures through a communication standard, a common way of communicating to these lighting devices regardless of the manufacturer. DMX is the acronym for Digital MultipleX, and 512 is the available number of control slots, or channels, for transmission. The 512 channels comprise a DMX ‘universe’. In a simple dimming system, one channel controls the intensity of one dimmer. A single intelligent fixture, however, may require several channels to control its various parameters (one channel each for pan, tilt, color, gobo, etc.), and in many cases, functions or colors are controlled within a given value range on a single channel. A basic dimming control console may support only a few of the 512 available channels, whereas many professional control consoles can support multiple universes, allowing for thousands of control channels.

Industry Standards

Before 1986, most manufacturers used their own proprietary control protocols, forcing system designers to mostly use fixtures and control consoles from the same manufacturer. Although there was no control standard to allow the use of a different manufacturer’s products, a number of companies developed adapters and patches for this purpose, which created control arrangements that were overly complex and somewhat expensive.

Given that a control standard comprises a set of widely agreed-upon guidelines for interoperability at both communications and mechanical level, standardization of protocol and equipment provides many benefits to manufacturers and end users, which include:

Increased product quality and safety

Reduced development time and cost

Sound engineering practices

General cost savings via protection against product obsolescence Theory of Operation

In technobabble, DMX 512 is an asynchronous serial digital data protocol. This section will attempt to explain how DMX operates in a simplified and easy-to understand manner using a Cable TV Analogy and DMX Communications.

The Cable TV Analogy

A central concept of DMX 512 is the ability to transmit data on multiple channels over a single cable. To better understand this concept, imagine a simple cable TV system with four major components: a Cable TV Company, a Cable, a Decoder, and TV - see image 1.

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 image 2 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 (manual electric switch), or LED/LCD display. This desired ‘channel’ is commonly known as the DMX address.

Now imagine a simple DMX system where the Cable TV Company is the DMX Control Console, the Cable is the DMX Cable, the Decoder is the DMX Decoder built into a Dimmer, and the TV is the Light Fixture. 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 image 3 is built into the Dimmer. It must be set to a desired channel, as with the previous example, starting with channel 001 to control the connected light fixture.

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, 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.

DMX Communications

In the world of digital communications, information is sent using precise electrical voltage pulses. A positive voltage pulse represents a 1, on, and a zero-voltage pulse (or no voltage) represents a 0, off. Systems using 1’s and 0’s to encode information are called binary systems (a bicycle has two wheels, a binary star system has two suns).

Each pulse in a digital signal is called a binary digit, or bit. A bit can only have one of two values, 1 or 0. A grouping of eight bits, called a byte, is used to carry one piece of information. This ‘information’ is simply a value ranging from 0 to 255.

The most common method of transmitting digital signals is to send data one bit at a time in one direction over one wire. Since each bit is transmitted in series, this method is known as Serial Communication. In its simplest form, Serial Communication requires one data wire for transmission and one common reference (or ground) wire.

There are two types of serial communication: asynchronous and synchronous. With asynchronous data transmission, data is sent one byte at a time, which means that asynchronous devices do not require perfect synchronization (a “not”, syn “together”, and khronos “time”).

That said, their timing signals (pulses) still need to be somewhat close (at least within a plus or minus 5% range of the data sampling clock circuit, which makes this method relatively simple and inexpensive). [A data sampling clock circuit is data sent/received relative to its clock position, wherein a stream of data has information in the beginning, center, and end to signal the position of the data within a given or known period (data rate)]. Think of a five-second-long data signal, which can be sent at a relatively arbitrary time (plus or minus 5% of the clock circuit used) and know that once the beginning of the signal is received, it signals that it will last exactly 5-seconds, with another signal to mark its end. With synchronous data transmission, data is sent as a group of characters in a single stream of bits known as a bitstream. Synchronous serial digital data protocol requires precise and expensive synchronized devices at both ends.

There are many standards for Serial Communications, each having its own advantages and disadvantages. Communications standards generally fall into two broad categories:

– Single-ended (unbalanced)

– Differential (balanced)

The single-ended specifications allow for data transmission from one transmitter to one receiver at relatively slow data rates and short distances. When communicating at high data rates, or over long distances in real-world environments, single-ended transmission methods are often inadequate.

Differential data transmission offers superior performance in most applications by helping to nullify the effects of interference on the signal. This is achieved by using two wires to transmit the signal (with opposing polarity) instead of just one.

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 so that there’s no retransmission or amplification.

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 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 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.

So how does all this information relate to controlling a light fixture? Think of it in terms of the simple DMX Controller Console. The console may have up to 512 control faders on it (8 in this example). Each fader controls the intensity of one light (using one DMX Channel). The position of the fader represents an 8-bit value (DMX Value) between 0 and 255, where 0 is off and 255 is full on.

Up to 32 devices may be connected in a daisy-chain, with a terminator on the last device. Using a DMX splitter/repeater (opto-isolator) can extend both cable distance, as the signal is regenerated and retransmitted, and the number of devices, up to 32 per branch.