DMX512 Protocol

A DMX universe contains 512 channels, which the standard calls slots. Each fixture receives a start address and reads a block of consecutive channels from it, called its footprint. Two fixtures set to the same start address receive the same values and move together. A full frame of 512 slots is sent about 44 times per second, which is enough for smooth fades and sets the upper limit of the refresh rate of one universe.

Two wiring rules keep a DMX line reliable. Fixtures are connected in a single daisy chain of no more than 32 unit loads, the EIA-485 limit, and a powered splitter is used to branch the line or to add fixtures. A 120-ohm terminator is fitted at the end of the chain to absorb the signal and prevent reflections. A missing terminator or a duplicate address can cause flicker and erratic behaviour that resemble a faulty fixture. RDM (Remote Device Management, ANSI E1.20) adds two-way communication on the same pair of wires, which allows fixtures to be discovered, addressed and queried remotely; it requires splitters that support RDM.