DMX termination: when the 120 ohm resistor matters

A DMX terminator is a single 120 ohm resistor connected across the two data wires (data+ on XLR pin 3, data- on pin 2) at the last fixture in the daisy chain, and nowhere else on the line. It absorbs the signal when it reaches the end of the cable instead of letting it reflect back toward the source: on a short, lightly loaded line you may never notice its absence, but on a long or heavily loaded line a missing terminator is the classic cause of random, intermittent flicker.

What a reflection actually is

At DMX data rates a cable stops behaving like a simple wire and starts behaving like a transmission line. DMX runs at 250 kbit/s, so each bit lasts 4 microseconds, but the voltage edges between bits switch in a small fraction of that, and those fast edges travel down the cable as a wave at roughly two thirds the speed of light (about 5 ns per metre). When the wave reaches an open end (an empty DMX thru socket) there is nothing to absorb its energy, so part of it bounces back up the line onto the bits that follow; a receiver sampling at the wrong moment reads a corrupted bit. ANSI E1.11 (DMX512-A) specifies cable with a characteristic impedance of 120 ohms, inherited from the EIA-485 electrical standard DMX is built on. A 120 ohm resistor across the far end looks, to the arriving wave, exactly like more cable that never ends: the energy is absorbed, and nothing bounces.

When it matters, and when you get away without it

Three things stack the odds against you, and they compound:

• Cable length. On a 20 m run the reflection returns in about 0.2 microseconds and settles long before the receiver samples the middle of the bit, which is why short unterminated lines usually work. On a 100 m+ run the round trip approaches a quarter of a bit and the bounce lands right where receivers are reading: gear that ran fine in a bedroom flickers across a venue. See how long a DMX cable can be.
• Fixture count. Every daisy-chained receiver is a small stub and every connector an impedance bump. DMX allows up to 32 unit loads per line; the closer you get to that, the less margin you have for reflection noise on top. See how many DMX fixtures fit on one line.
• Data rate. DMX is fixed at 250 kbit/s, which makes it more forgiving than faster RS-485 systems. Forgiving, not immune: at about 44 packets per second on a full 512-channel universe, corrupted packets quickly show up as visible glitches.

Wrong cable makes everything worse: microphone cable has a characteristic impedance around 45 to 70 ohms, so the line is mismatched along its whole length and a terminator cannot fully fix it. Termination becomes mandatory as lines get longer and busier; "it worked without one at the last gig" is survivorship, not evidence.

How to terminate

Two options, identical electrically:

• Buy a terminator plug. A male XLR shell (3-pin or 5-pin to match your fixtures) with the resistor already inside, a few dollars from any lighting supplier. Plug it into the DMX thru/out socket of the last fixture.
• Make one. Solder a 120 ohm, 1/4 W resistor across pins 2 and 3 inside a male XLR connector, leaving pin 1 (ground) unconnected. 5% tolerance is fine. The resistor sees the line's differential voltage continuously, so it dissipates a few tens of milliwatts in practice, and about 0.2 W at the 5 V theoretical maximum: hence the 1/4 W rating.

Either way it goes at the last fixture's output socket: never mid-line, and never at the console.

Devices with built-in termination

Some fixtures and rack devices include a TERM switch (or an internal jumper) that connects an onboard 120 ohm resistor for you. If your last fixture has one, switch it on and skip the plug. Two cautions: enable it only on the physically last device (mid-line termination loads the bus and weakens the signal for everything after it), and do not assume a switch exists. Many fixtures have none. When in doubt, use the plug.

Common myths

• "Terminate both ends, like RS-485." Wrong for DMX. Generic multidrop RS-485 buses are often terminated at both physical ends, but a DMX line has exactly one transmitter, fixed at one end, and its low output impedance already controls that end. A second 120 ohm resistor at the console halves the load impedance the driver sees and drops the signal level for every fixture. One terminator, far end only.
• "Modern fixtures don't need it." The DMX512 standard has specified far-end termination since 1986. Modern receiver chips are more tolerant, not immune; the cable physics has not changed.
• "Termination fixes flicker." It fixes reflection-induced flicker only. It does nothing for Y-cable splits (never passively split DMX), missing ground continuity, damaged cable, or overloaded lines: if a properly terminated line still misbehaves, work through DMX Basics: Troubleshooting.

Splitters: every output is its own line

A DMX splitter such as the ENTTEC D-Split or DIN RDS4 re-drives the signal on each output, so each branch is an electrically independent line needing its own terminator at its own last fixture; one terminator does not cover the whole system. Splitting is also the correct fix when a single line is too long or too loaded to stabilise with termination alone: see optical isolation and DMX splitting for large installations.

Symptom checklist

Missing termination rarely kills a line outright; it degrades it. The signatures:

Termination costs a few cents of resistor and rules out an entire failure class: fit it from day one. If flicker persists with a terminator fitted, work through DMX Basics: Signal Loss, and for the underlying protocol start at What is DMX512?.