Choosing DMX Cable: Why Microphone Cable Will Let You Down
The most common mistake when building a DMX line is using whatever cable is at hand — usually the microphone cable there is plenty of on stage. It appears to work over short runs, and then unexplained flicker starts as the system grows. The reason is simple: DMX is not an audio signal, it is a data link running at 250 kbit/s.
In short
- DMX512 is a data link running at 250 kbit/s over RS-485.
- At that speed the cable’s characteristic impedance (120 Ω) becomes decisive.
- Microphone cable is 40–90 Ω, which causes reflections and random flicker.
- Cat5/Cat6 (100 Ω) is a reasonable alternative; mic cable is not.
- Always fit a 120 Ω terminator at the end of the line.
DMX is a data link, not audio
DMX512 is built on RS-485, an industrial serial communication standard. It carries 250,000 bits per second over a two-wire balanced line, which means every single bit lasts only 4 microseconds. At that speed a cable stops being “wire that joins two ends” and starts behaving like a transmission line.
The practical consequence is this: if the cable’s characteristic impedance does not match what the driver and the terminator expect, part of the signal reflects back from the end of the line. That reflection rides on top of the outgoing signal, and the receiver misreads whether a bit was a 0 or a 1.
Why microphone cable does not fit
Microphone cable is designed for analogue audio between 20 Hz and 20 kHz. In that band the characteristic impedance simply does not matter; what matters is flexibility, low handling noise and good shielding. A typical mic cable sits somewhere around 40–90 Ω and has relatively high capacitance per unit length.
| Property | Microphone cable | DMX / RS-485 cable |
|---|---|---|
| Characteristic impedance | ~40–90 Ω (uncontrolled) | 120 Ω (controlled) |
| Design band | 20 Hz – 20 kHz | 250 kbit/s digital |
| Capacitance | High | Low |
| Twisted pair | Often, but uncontrolled lay | Controlled twist rate |
| Behaviour on long runs | Reflections and signal corruption | Stable |
“But it works for me”
Over a short run with a handful of fixtures, mic cable really can work — the reflected signal never grows strong enough to fool the receiver. The trouble appears when the system scales: the run gets longer, the fixture count rises, stage power raises the electrical noise floor, and one day during rehearsal the fixtures start flickering on their own. At that point blaming the cable is hard, because “it worked yesterday”.
What to look for in the right cable
- 120 Ω characteristic impedance. It is stated on the jacket or in the datasheet. Look for cable sold specifically for RS-485 or DMX512.
- Twisted pair with a shield. The twist makes external noise appear equally on both conductors so the balanced receiver can cancel it.
- Low capacitance. High capacitance rounds off the signal edges and blurs bit boundaries over long runs.
- AWG 24 or thicker conductors. Thin conductors mean both resistance and mechanical fragility over distance.
- A flexible jacket for touring use. Fixed-installation cable fails quickly on a line that is coiled and uncoiled every day.
Distance and device limits
Even with the right cable the line has physical limits. The commonly accepted figures:
- A maximum of 32 receivers on one segment. Beyond that, use an opto-isolated splitter.
- Around 300 metres of total run. This is not a guarantee but a sensible starting limit; cable quality and the electrical environment decide the rest.
- A 120 Ω terminator on the last device is mandatory. Without one, the reflection problem shows up even on correct cable.
Can you use Cat5/Cat6?
Yes, and it is common — but it is a compromise. Ethernet cable is 100 Ω, so it does not match 120 Ω exactly, though it is far closer than mic cable and its twist rate is controlled. It works fine over short and medium runs. The things to watch: put one DMX universe on one pair and leave the remaining pairs unused or reserved for a second universe, and never run power or audio in the same cable. Solid-core Cat cable is good for fixed installations; choose stranded for anything that moves.
A quick way to diagnose it
If you have flicker and suspect the cable, shrink the system: connect the console to a single fixture with a short, known-good DMX cable and fit the terminator. If the problem disappears, grow the cable run and fixture count step by step until it returns. In most cases the culprit is a single microphone cable somewhere in the middle of the chain.
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