Editing Knowledge Base:DMX512

DMX512
by Mick Martin | Feb 23, 2012 | Knowledge Base

Introduction DMX 512 is the primary protocol for communicating with lighting fixtures. Since its inception in the 80’s it has become ubiquitous in the lighting industry. It is a serial communication that allows for fixtures to be daisy-chained along the transmission line. The fixtures on this transmission line must have an address to identify them.

Details

The serial communication hardware uses RS485 devices which drive a differential transmission line. The noise rejection on this line allows for very long transmission lengths up to 1Km in total. The protocol transmits 512 bytes of channel data in a single packet. Each packet begins with a break signal. The packets are automatically streamed periodically usually about 25ms. Because the fixtures are daisy-chained, all the DMX channels are sent to all the fixtures on the line. Each fixture is then assigned an address which tells the fixture which channels it is to decode for its own use. The number of channels consumed depends on the fixture.

The DMX specification does not require any isolation on the transmission line. We have had much experience of damage to interfaces because of non-isolated DMX transmission lines. This is why all the interfaces we provide now are isolated to protect the interface and the computer it is connected to.

Cabling

An installation should never create a Y junction in the DMX line. The number of fixture daisy-chained on the line should not exceed 32 without buffering. The connections are made with 5 pin XLRs although some devices use 3 pin XLRs.

5pin XLR pinout

gnd/shield
-ve data
+ve data
nc
nc
3pin XLR pinout

gnd/shield
-ve data
+ve data
Termination

The DMX line should be terminated. That is, the last fixture connected on the transmission line has a 100 Ohm resistor connected between the +ve data and the -ve data signals.

Links

The full spec. is available http://www.usitt.org/
An excellent book “Recommended Practice in DMX 512” by Adam Bennette is also available.