DMX cables and microphone cables look alike. Both usually have XLR connectors, and a microphone cable will often appear to work for a few lights. But they are built to different specifications, and the substitution is a common cause of flickering and lights that behave strangely.
What is different
DMX512, the control protocol used for stage lighting, sends data as a fast digital signal over a twisted pair, using the RS-485 standard. The cable is designed with a characteristic impedance of 120 ohms. Microphone cable is designed for low-level analog audio, and its impedance is different, typically much lower. When the cable’s impedance does not match what DMX expects, parts of the signal reflect back along the line and corrupt the data.
What goes wrong
- Lights flicker, jump or fail to respond.
- Fixtures at the end of the chain behave differently from those at the start.
- Problems appear only on longer runs or with many fixtures, so a setup that works at home fails at the venue.
On a short run with a few fixtures, a microphone cable can work. It is not guaranteed, and when it fails, the symptoms are intermittent and hard to trace.
Connectors: 3-pin vs 5-pin
The DMX512 standard specifies 5-pin XLR connectors, with pin 1 as the shield, pin 2 as data minus and pin 3 as data plus. Pins 4 and 5 are reserved for an optional second data link and are rarely used. Many fixtures, especially lower-cost ones, use 3-pin XLR instead, so adapters between 3-pin and 5-pin are common. Check each fixture’s manual, because the pin layout, not just the connector shape, has to match.
Terminate the end of the line
The last fixture in a DMX chain should have a terminator, a small XLR plug with a 120 ohm resistor across the data pins. It absorbs the signal at the end so it does not reflect back. If a chain flickers, adding a terminator is the first thing to try.
Chain length and limits
- A single DMX line can drive up to 32 devices before it needs a splitter or buffer.
- One DMX universe carries 512 control channels.
- Run DMX in a daisy chain, from the controller to the first fixture, then onward. Do not use Y-splitters without a proper DMX splitter.
Keep the two cable types apart
Because the connectors match, it is easy to plug a mic cable into a DMX chain, or the reverse. Label them or use different colors, and the mix-up becomes much less likely. For the full picture of what XLR is used for, see XLR vs TRS vs TS.