DMX512
A Bit of Background
DMX512 is an adaptation of the Industry Standard EIA485 or RS-485 interface. It is based on a balanced-line transmission system using a twisted pair of conductors with an outer screen. The system transmits binary data as an asynchronous transmission at 250kBaud over a maximum distance of 1km and therefore requires a cable designed for high frequency data transmission.
Cable Requirements
PLEASE DO NOT USE AUDIO CABLE - a DMX512 receiver needs to be able to resolve harmonics in excess of 500kHz for correct decoding of the data and most audio cables are only designed for frequencies up to 24kHz (although you may get away with it over a very short distance if it's an emergency!)
Termination
As with any transmission line the cable should be correctly terminated to prevent reflections of data back down the line as they may cause interference and corrupt the data. The EIA485 specification supports daisy-chain networking with up to 32 unit loads on a single line providing a 120ohm termination load is placed at the end across the data lines. Please see diagram for correct wiring of 120ohm load in terminating plugs. Although the DMX512A specification insists that all DMX receivers MUST use the 5pin XLR connector many systems still use a 3pin XLR system so I have included a wiring diagram for XLR3 terminators as well.
Pins 4 & 5
The DMX512A specification doesn't specify a function for pins 4 & 5 on the XLR5 although many system manufacturers now employ these pins for various functions including data feedback, transmission of a second universe, etc. The only requirement made by the DMX512A specification is that these pins MUST NOT be used for power connections, even at low-voltage.
Current Loading
As previously mentioned, the DMX512A specification permits up to 32 unit loads to be daisy-chained together on a single line providing the cable is correctly terminated. To extend a DMX universe to more than 32 receivers use an opto-isolated buffer/distibution amplifier with multiple outputs. Each output from the buffer should then be capable of driving a further 32 receivers.
