DMX512 Protocol
DMX512, standardised as ANSI E1.11 (in its current form, DMX512-A), is the language lighting consoles use to talk to fixtures, and it carries control as a one-way stream of serial data. Electrically it is RS-485, a differential signalling scheme running at 250 kilobaud, which is why it travels on balanced data cable and uses 5-pin XLR connectors — pins one through three carry the data, and the spec reserves pins four and five for an optional second link. That speed is also why ordinary microphone cable is the wrong tool: at 250 kbaud you really do want proper 120-ohm data cable.
One DMX universe is exactly 512 channels, which the standard calls slots. Each fixture is given a unique start address and then reads a block of channels counting up from there — that block is its footprint. The consequence is immediate and physical: two fixtures sharing the same start address will move together whether you wanted them to or not. A full 512-channel frame refreshes about 44 times per second, which is fast enough for smooth fades but also a hard ceiling on how many parameters a single universe can drive in real time.
Two physical rules keep a DMX line reliable. Wire fixtures in a single daisy-chain of no more than around 32 unit loads per run, and use a powered splitter when you need to go wider or branch. And fit a 120-ohm terminator at the very end of the chain to absorb the signal and stop it reflecting back up the cable. Skip the terminator or double up an address and you get exactly the ghosting and flicker that looks like a broken fixture but is really a fault on the data line. There is also a useful extension worth knowing: RDM (ANSI E1.20) adds a back-channel over the very same wire, letting you discover, address, and query fixtures remotely instead of climbing to each one.
One universe, one address per channel — no overlaps.