Inside the DMX 512

The DMX512 protocol was first developed in 1986 by the USITT (United States Institute of Theater Technology) as a means of controlling dimmers from consoles using a standard digital interface.

DMX512 supercedes but is not entirely replacing analog systems. It's simplicity and reliability (providing it is installed and used correctly!) as well as its flexibility, is fast making it the protocol of choice, funds permitting, Witness the growing list of controlled devices other than just dimmers. It’s a still a new frontier with all sorts of wonderful technogeekisms appearing on a regular basis. Pretty cool.

EIA485 (RS485)

DMX512 is designed around the Industry Standard EIA485 interface. EIA485 pertains to "electrical" side of the interface, voltages, currents etc. We’ll look at this first.

The system is based on a balanced transmission down a twisted pair of screened conductors. The twisted configuration ensures that any interference that occurs does so to both signals equally, thereby ensuring consistent digital phasing. The cable used for this should be a proper data cable consisting of 1 or 2 twisted pairs, foil and braided screens.

Balanced Audio Cable does not do the job.

Normally, as with any network segment, the cable should be terminated at both ends. Since the lighting console is usually at one end only a 120R resistor should terminate the other end.

The IEA485 specification only supports "daisy-chain” or series networking with up to 32 "unit loads" on the segment. Manufacturers claim the segment can be up to 1000m.

Practically speaking however, the use of repeaters should be considered around 700m or 800m to counter any environmental anomalies.

Pins viewed from outside (Click on Image for Full Size)

DMX512 control lines connect to equipment using XLR 5 pin (sometimes 3 pin) connections; female connectors are fitted to transmitters and male connectors to receivers. The specification suggests the use of a 2 pair (4 conductor) cable with shield, although only one pair is required. The second cable pair is reserved for unspecified optional uses.

It should be noted that some dimmers use these lines for fault and status information. If the dimmers use this second channel then specially configured splitters and repeaters will be required.

The safest way to interface the line to logic levels is to use one of the "standard" interface IC's, the SN75176B from Texas Instruments, or for interfacing including isolation, the ISO485P from Burr-Brown are good choices.