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.

Originally developed to control lighting dimmers, it is now used to control Color 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.

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, however it is generally accepted that this sort of work should be accomplished with hardwired switches.

Electrical details

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 (but are not limited to):
* Belden 9841
* Belden 9842
* Alpha 5274

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!

Wiring practice

At the speed DMX works correct wiring practice is important and must be done in a conventional 'daisy-chain' format. Always arrange wiring such that a cable starts from the DMX data source and goes in and out of each item of equipment in turn, never split or 'star' off to additional devices - it won't work!

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 an external line terminating network is needed.

This is achieved by soldering a 120 ohm resistor across pins 2 and 3 of a male XLR plug and connecting to the DMX out socket on the last receiving device..

A simple DMX tester

With any DMX network it is ideal to have some kind of DMX test device to check the installation. 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.