DMX-512 mini-FAQ

5 09 2009

1.0 Introduction

Welcome to the DMX-512 Mini FAQ. The FAQ is intended to answer some of the common questions relating to the DMX protocol used for theatre lighting control. It is not intended to replace the standard published by the USITT.

1.1 What is DMX.

DMX 512 is a standard protocol by means of which theatre lighting control desks can communicate with lighting equipment. It was designed to allow equipment from different companies to be used together easily.

1.2 Legal Stuff.

Disclaimer
The DMX 512 MiniFaq is compiled in good faith, but no liability whatsoever, ever directly or consequentially, will be accepted for any inaccuracies contained herein, the author accepts no liability for any loss, financial or otherwise arising from the use of the information contained herein.
The DMX-512 MiniFaq is copyright. It may not be sold, but may be posted on any publicly accessible server. It may not be altered or amended without my express permission. It may not be used in any other publication except those only available free in electronic form, without my express permission.
End of legal BS;)

1.3 Who Publishes the standard?

The DMX 512 standard (Actually USITT DMX 512 1990) is published by The U.S.I.T.T. The address is in the useful addresses section. When you order the standard, also get a copy of : 'RECOMMENDED PRACTICE FOR DMX 512 A guide for users and installers'. This book is really useful. The book is by Adam Bennette, it is published by PLASA in the UK and distributed by the USITT.

1.4 How to contribute.

There are large parts of this faq that could use some additions! If you have something to add please send it to

DMXFAQ@abcde.demon.co.uk

Please do not modify this file yourself Send me the mods. and I will do it. This ensures that we don’t end up with multiple versions (all with different data).

1.5 Useful addresses.

USITT Inc.
 Suite 5A,
  10 West 19th st.
   New York,
    NY 10011-4206
     USA
Tel : (212) 924 - 9088
Fax : (212) 924 - 9343
Electronic Industries Association (EIA)
 2001 Eye Street,N.W.
  Washington D.C.20006
   USA.
PLASA Ltd.
 7 Highlight House,
  St Leonards Road,
   Eastbourne
    East Sussex BN21 3UH
     UK.

2.0 Applications.

The DMX protocol was designed to provide a common protocol for communication between controllers and dimmers.

2.1 What it was designed to do.

The protocol was designed to control dimmers.

2.2 What it is used to do.

In addition it is used to control Colour scrollers, Moving lights, Smoke machines and almost any equipment that can be controlled digitally.
Note that as it is an 8 bit protocol many moving lights use 2 channels to provide 16 bit resolution.

2.3 What it should not be used to do.

The DMX protocol does NOT have any form of error correction! Therefore it must not be used to control Pyro’s or any stage equipment or effect that could present a hazard if it was to be triggered unexpectedly. For these applications Midi Show Control (MSC) 2 with 2 phase commit can be used with reasonable safety. I personally feel that this stuff is best done using hardwired switches<g>.

3.0 Electrical details.

DMX-512 is RS485 serial at 250 Kbaud, as such any installation must meet the requirements of EIA485. In particular use of suitable good quality cable is important (Mic cable is NOT suitable) suitable cables include :

  • Belden 9841
  • Belden 9842
  • Alpha 5274
    Amongst others.
    This is not a complete list.
    RS485 is NOT the same thing as RS232 It cannot be made from a standard PC serial port. You can damage a DMX device by attempting to plug RS232 into it!

3.1 Wiring practice.

At the speed DMX works correct wiring practice is important. Always arrange wiring such that the data source is at one end of the cable. NEVER do this :

      *----*-----*------------*---------*
           |                  |
           *                  *
                              |
                              *---*---*
                                  |
                                  |
                                  *---*

Where * is a DMX plug/socket pair. I have seen this done! It worked (sort of, sometimes), but would fail in unpredictable ways when extra devices were plugged in. DMX cannot be split by simply soldering 3 cables together. Dasy-chain everything, but see the section on isolation.

3.2 Line termination.

All DMX lines must be correctly terminated if reliable operation is to be obtained. Some equipment has a switchable line terminating resistor built in. In these cases make sure that only the last item in the chain has its terminating switch set to on. If the last item (Farthest from data source) does not have a terminating switch then a external line terminating network is needed. To make one you will need the following parts :

  • 1 Male 5 Pin XLR
  • 1 110 ohm 0.5W resistor.
    To make the line terminator solder the resistor across pins 2 and 3 of the XLR. You may like to back fill the XLR with potting compound or epoxy resin to provide protection.

3.3 A simple DMX tester.

This tester can be used instead of a line terminator to provide a simple means of checking for cable continuity. Parts required :

  • 1 Male 5 Pin XLR
  • 2 270 ohm 1/2 watt resistor.
  • 1 BiColour LED.
  2>-----------+------------+
               |            |
               -            -
              | |          | |
              | |          | |
              | |          | |
               -            -
               |            +
               |            |
               |           LED
               |            |
               |            |
               |            |
  3>-----------+------------+

The BiColour led has 2 LED’s (one Red and one Green) back to back in the same package. Build this into the back of a 5 pin XLR with the LED showing out of the back.

To use :

  • Insert into the socket that you want to test,
  • Set all channels to 0% (Preheat also = 0)
    The led should glow one colour.
  • Run all channels up to Full power (You may need to turn the dimmers off!)
    The LED should glow the other colour.

If the led fails to light in any one of the 2 above states then there is a short between one of the lines and pin 1.

Its limited but surprisingly useful!

3.4 Connectors.

The DMX standard specifies 5 Pin XLR style connectors, However only 3 pins have standardised uses. The remaining pair (Second data link) are used for anything from Overtemp. indication to supplying power to control desks! If using equipment from more then one manufacturer Check the spec. carefully as to the use that is made of these pins.

Several manufacturers use 3 Pin XLR’s for ‘DMX’ This is not standard. It may be worth having several jumpers to convert between these (and to isolate the second data link).

3.5 Splitting a DMX feed.

You cannot split a DMX feed by simply soldering 3 cables together. Doing this may work some of the time but it cannot be relied upon.

Most DMX compatible equipment has a DMX out socket fitted and this may be used to loop into other equipment.

If you need to split a DMX feed a splitter box can be used. These can also provide isolation such that a fault on one line does not cause a malfunction on the other line. An opto isolator is highly recommended on lines feeding dimmers as a fault could destroy other equipment on the line.

Note that a line from a isolated port is considered to be a new DMX line. These can be obtained from any of the usual suppliers of DMX hardware, or they can be made cheaply by anyone with a electronics background.

The EIA 485 standard supports a maximum of 32 devices on a line. If more are needed a repeater is needed. This will allow another 32 devices to be added for a total of 63 devices (The repeater counts as a device). This can be continued for more devices. However Where large networks are required it is better to use a splitter to produce multiple lines at the source. This means that if one line fails the problem will be limited to that section.

It is also worth noting that not all instruments have “proper” RS-485 receivers in them. Some of the wiggle lights (early I-beams?) simply stuck the LED in an opto-isolator across the DMX line, rather than a real receiver. This is OK, *as long* as you are only driving ONE thing from the line, and the line isn’t too long. I think these things daisy-chained, with each light regenerating the signal for the next instrument.

3.6 Other protocols.

Some short notes on other common protocols.

AMX 192 (USITT) : This analog TDM protocol was used before DMX became common. Mostly found in the US. it suffers from all the problems inherent in any analog protocol (Earth loops, Noise). It is carried on 5 Pin XLR.

D54 (Rank Strand) : This is also a analog TDM protocol. It is single ended and VERY prone to earth loop problems. If you have problems with this one they can usually be solved by making sure that the desk, Monitor and Demux are at the same ground potential. You sometimes get REALLY weird behaviour with this.

PMX (Pulsar) : This protocol is EIA 232. I do not have any more details.

CMX (Colortran) : DMX was based on this (Amongst others). CMX equipment can usually be easily modified for DMX.

SMX (Rank Strand) : Designed to provide fault reporting and other nice features, unfortunately it is so complex that it is seldom seen in practice.

Midi Show Control (MSC) : Not really a lighting control protocol, but so common on large shows that I have decided to put it in. It provides show control functions, like synchronising sound and lighting with pyro. This is used BIG TIME in the theme park industry.

Revision : 1.04
Last changed : 22nd March 96

FAQ Maintained by Dan Mills.

Contributed to by : Loren Wilton.

HTML Version maintained by The Kilowatt Company


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