DMX 512 is the primary protocol for communicating with lighting fixtures. Since its inception it has become ubiquitous in the lighting industry.

It is a serial protocol transmitted on RS485 hardware devices. These devices use a differential line to cancel noise propagation on a long transmission line which can be up to 1Km in length. The fixtures can be daisy-chained along this transmission line and the last fixture should have a terminating device to balance the transmission line.

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.  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.

An installation should never create a Y junction in the DMX line. Proper connectors should be used. The DMX line should be terminated. The number of fixture daisy-chained on the line should not exceed 32 without buffering.

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.

The full spec. is available from USITT, and some useful links are here and here. An excellent book "Recommended Practice in DMX 512" by Adam Bennette is available through PLASA, USITT and other sources.