Contents :
1.1 : What is DMX?
1.2 : Legal stuff and disclamer
1.3 : Who publishes the standard
2.1 : What DMX was designed
to do
2.2 : What it is used to do
(Not the same thing)
2.3 : What it should NOT be
used to do
3.1 : Wiring Practice
3.2 : Line Termination
3.3 : A simple DMX tester
3.4 : Connectors
3.5 : Splitting a DMX feed
3.6 : Other protocols
1.0 Introduction
Welcome to the DMX-512 Mini
FAQ. The FAQ is intended to answer some of he common questions
relating to the DMX protocol used for theater 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 Mini-Faq is
compiled in good faith, but no liability whatsoever, ever directly or
consequentially, will be accepted for anyinaccuracies contained
herein, the author accepts no liability for any loss, financial or
otherwise arrising from the use of the information contained herein.
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 :
'RECOMENDED PRACTICE FOR DMX'.
This book is really useful.
The book is by Adam Bennette, it is published by PLASA
(Professional Lighting and Sound Association) in the UK and
distributed by the USITT.
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 Color scrollers, Moving lights, Smoke machines and almost any
equipment that can be controlled digitally. Note that as it is a 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.
DMX 512 is RS485 serial at
250 Kbaud, as such any installation must meet the requirements of
EIA-485. 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 arange 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. Daisychain 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 Bi-Color LED.
Wiring as follows :
2>-----------+------------+
| |
- -
| | | |
| | | |
| | | |
- -
| +
| |
| LED
| |
| |
| |
3>-----------+------------+
The Bi-Color 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 color. Run all chanels
up to Full power (You may need to turn the dimmers off!) The LED
should glow the other color. 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 suprisingly useful!
3.4 Connectors
The DMX standard specifies 5
Pin XLR style connectors, Howether 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 compatable 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. A opto isolator is
highly recomended 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 usaual 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. Howether 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.
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