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DMX 101: A DMX 512 Handbook Handbook Revision: May 2008 2 Elation Professional ®
Table of Contents 1. 2. Introduction to DMX ...................................................................................................................................... 4 Industry Standards .......................................................................................................................................... 5 2.1 USITT ..................................................................................................................................................... 5 2.
DMX 101: A DMX 512 Handbook 1. Introduction to DMX This handbook is designed to promote basic understanding of the DMX 512 protocol, including theory of operation, proper equipment use, and some basic application examples. 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.
2. Industry Standards About 20 years ago, most manufacturers used their own, proprietary control protocols. This forced system designers to use only fixtures and control consoles from the same manufacturer. There was no control standard to allow the use of different products. A standard is a set of widely agreed-upon guidelines for interoperability at both communications and mechanical levels. Standardization of protocol and equipment provides many benefits to manufacturers and end users.
DMX 101: A DMX 512 Handbook 2.3 ANSI The American National Standards Institute (ANSI) is an organization composed of representatives from industry and government that collectively determine standards for the electronics industry as well as many other fields, such as chemical and nuclear engineering, health and safety, and construction. ANSI also represents the United States in setting international standards. New electronic equipment and methods must undergo extensive testing to obtain ANSI approval.
3. Theory of Operation DMX 512 is an asynchronous serial digital data protocol. While that might mean something to an engineer, most people are not familiar with how all this techno-gibberish actually accomplishes lighting control. This section will attempt to explain how DMX operates in a simplified and easy-to-understand manner. The topics this section will cover are: • • The Cable TV Analogy DMX Communications 3 www.elationlighting.
DMX 101: A DMX 512 Handbook 3.1 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 (Figure 1) with four major components: • • • • Cable TV Company Cable Decoder TV 3 Figure 1 The Cable TV Company broadcasts many channels at once in a mixed signal over a single cable. The Cable carries the signal to a Decoder, usually a cable TV box or similar device.
Now imagine a simple DMX system (Figure 2), 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) The TV is the Lighting Fixture 3 Figure 2 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.
DMX 101: A DMX 512 Handbook 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.
3.2 DMX Communications In the world of digital communications, information is sent using precise electrical voltage pulses. A positive voltage pulse represents a 1. A zero voltage pulse (or no voltage) represents a 0. Systems using 1’s and 0’s to encode information are called binary systems. 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.
DMX 101: A DMX 512 Handbook 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.
3.3 Summary So how does all this information relate to controlling a light fixture? Think of it in terms of the simple DMX Controller Console (Figure 4). 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.
DMX 101: A DMX 512 Handbook 4. Protocol Specifications Understanding how DMX 512 operates is important, but it’s only part of the picture. The physical equipment, connectors and cabling, play a critical role in the proper functionality of the protocol. The DMX standard specifies exactly what types of connectors and cable may be used. It is also important to understand the logic behind the data that is transmitted.
4.1 Connectors The DMX standard specifies 5-pin XLR connectors (Figure 5). The name comes from the original manufacturer (Cannon X connector, with a Latch and Rubber guard). Only three of these pins have standardized use however, leading many companies to make use of inexpensive and readily available 3-pin XLR connectors (Figure 6).
DMX 101: A DMX 512 Handbook 4.2 Cabling 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.
4.3 Data DMX 512 data is transmitted at 250 kiloHertz (kHz), meaning that 250,000 1’s and 0’s (at a maximum) can be sent each second. Each bit is measured in 4 microsecond (µs) intervals. In order for the receiving device to correctly interpret the data, it must be sent in a particular sequence. A single transmission (DMX Packet) includes synchronizing elements and channel data for up to 512 channels (Figure 8). &KDQQHO %5($.
DMX 101: A DMX 512 Handbook 5. Sample Applications The figures on the following pages illustrate several different applications using various DMX products. Samples include: • • • • Dimmer Control Intelligent Fixture Control Dimmer & Intelligent Fixture Control Distributed Control As mentioned earlier, dimmers normally use one DMX Channel per light. They translate DMX Values in a linear fashion, with 0 being off and 255 full on.
5.1 Dimmer Control 5 www.elationlighting.
DMX 101: A DMX 512 Handbook 5.
5.3 Dimmer & Intelligent Fixture Control 5 www.elationlighting.
DMX 101: A DMX 512 Handbook 5.
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