Basics 8. Cables and the line: connectors and the terminator

The most underrated lesson of the course. The software is configured, the addresses are set, the profiles are right — and the light flickers, heads twitch, distant fixtures occasionally freeze. Nine times out of ten the cables are to blame: the wrong type, a star connection or a missing terminator. Let’s look at how a DMX line is built and what makes it reliable.

Fixtures are wired in a chain

DMX is a bus: one cable passes through every fixture in turn. Each fixture has two sockets: DMX IN (input, facing the desk) and DMX OUT (output, going on to the next fixture).

The layout is always the same: interface → IN of the first fixture, OUT of the first → IN of the second, OUT of the second → IN of the third, and so on. The English name for this is a daisy chain.

Fig. 1 — A correct chain with a terminator, and an incorrect star connection

⚠️ Careful: you must not wire fixtures in a «star». A star is when several cables run out from the interface to different fixtures (through a Y-splitter, a passive box or a home-made adapter). The signal starts reflecting off every branch, fixtures receive corrupted data and behave unpredictably: flickering, twitching, freezing. If you need to branch, you need a splitter — more on that below.

The limits of a single line:

Parameter	Limit
Fixtures per line	up to 32
Line length	up to 300 metres total
Channels	512 (that is a universe)

In practice, with good cable and a small rig, things work beyond these numbers too — but it is beyond them that inexplicable problems begin. Treat 32 fixtures and 300 metres as the border past which you need a splitter/booster.

Connectors: 3-pin and 5-pin XLR

The standard specifies a 5-pin XLR for DMX. In reality most budget fixtures — and nearly all Chinese models — come with a 3-pin one, the same connector as on a microphone cable. Both are in use, and adapters exist between them.

The pinout (identical in both cases, the 5-pin simply has two extra):

Pin	Purpose
1	Shield (common)
2	Data − («cold»)
3	Data + («hot»)
4, 5	A second data pair — unused on the vast majority of fixtures

Fig. 2 — Pinout of 3-pin and 5-pin DMX XLR connectors

💡 Tip: if your rig has both 3-pin and 5-pin fixtures, buy a couple of adapters in advance and keep them in the case. Hunting for them on site an hour before the event is a bad plan.

A microphone cable is not a DMX cable

The connector is the same, so the temptation is strong: grab a mic lead and link the fixtures. Sometimes it even works — and that is the real trap.

The difference is a property called characteristic impedance. DMX cable is around 110 Ω, microphone cable roughly 50–75 Ω. DMX data runs at 250,000 bits per second, and a signal that fast is distorted and partly reflected in the «wrong» cable.

What you will see on site:

on a short cable (2–3 metres) with a single fixture — probably nothing, it all works;
on a long line or with several fixtures — random flashes, twitching heads, distant fixtures freezing;
in the nastiest version — «it worked at the rehearsal and fell apart at the event», because a couple more fixtures were added.

💡 Tip: the right cable is identified by the marking «DMX 110 Ω» or «AES/EBU 110 Ω». It costs barely more than microphone cable and saves incomparably more nerves.

The terminator: the plug that kills the echo

A terminator is an XLR plug with a 120 Ω resistor between pins 2 and 3. It goes into the DMX OUT of the last fixture in the chain.

Why: when the signal reaches the end of the cable it reflects back, like an echo in an empty corridor. The reflected wave overlaps the incoming data and corrupts it. The resistor absorbs the signal at the end of the line, so no echo occurs.

The most commonly reported symptoms of a line without a terminator:

the fixtures work, but every so often blink or twitch for no reason;
distant fixtures respond late or miss commands;
the problem gets worse as the line grows longer or gains fixtures;
everything «fixes itself» when cables are moved around — and comes back half an hour later.

⚠️ Careful: a line without a terminator may well work fine — a short one especially. But it is a lottery: a short line forgives it, a long one does not. A terminator costs pennies, takes a second to fit and rules out an entire class of problems. Keep two in the case: one in use, one spare.

💡 Tip: some fixtures and splitters have a built-in terminator — a switch on the housing marked TERM, 120R or Termination. If the last fixture has one, no separate plug is needed; just flip the switch.

The splitter: how to branch a line properly

When you have many fixtures, they are spread around the venue, or the line runs longer than 300 metres, you need a DMX splitter (also called a splitter/booster). The device takes one incoming DMX feed and produces several independent outputs, each of which is a full new line with its own terminator.

What that gives you:

Legitimate branching. You can run separate lines to the stage, to the ceiling and to the bar.
Signal boosting. Each output starts a line afresh, so the 300 metres are counted from the splitter.
Isolation. A break or a short on one branch does not kill the others.
Protection. Models with optical isolation (isolated) separate the fixtures from the computer electrically — that saves your interface and laptop when the venue has different phases or a floating earth.

💡 Tip: an opto-isolated splitter is one of the best-value purchases in a lighting rig. It also solves the classic headache of touring venues: the potential difference between the stage sockets and the control position.

Rigging rules that save your evening

Keep DMX away from power cables. Running tight alongside mains is a sure way to pick up interference. If you cannot avoid crossing, cross at a right angle.
Do not pull cables tight. Leave a little slack at each fixture — a yank on the cable breaks the connector, and sometimes brings the fixture down.
Label the ends. Masking tape reading «stage left» or «truss» saves time at the get-out.
Test cables in advance. The most common hardware fault on site is not a fixture but a single bad cable. Carry two spares.
Patch before powering up. Both fixtures and interface are better connected cold.
Do not block ventilation. Cable runs must not cover the fixtures’ vents.

What to fix by symptom

Symptom	Most likely cause
Flicker and random flashes across all fixtures	No terminator, or unsuitable cable
Fixtures past a certain point do not respond	A broken cable, a bad connector or an over-long line
One fixture twitches, its neighbours are fine	A bad cable to that fixture, or its IN connector
Everything works, but commands arrive late	The line is too long; you need a splitter/booster
Problems started after adding fixtures	The line is overloaded — 32 fixtures is the limit
Works until the fog machine kicks in	Interference from a heavy load; separate the cables

The full troubleshooting routine is in lesson twelve.

📍 How this looks in SvetoMaster: the program cannot see what is happening inside the cable — DMX is one-way, remember lesson one. But it honestly reports its own end of the link: whether the interface is connected and whether the stream is running. If the adapter is pulled out of the USB port the status changes, and the program restores the connection by itself once it is plugged back in. Everything that happens after the interface is diagnosed by hand — with cable, terminator and swapping links.

Key points

Fixtures are wired in a chain, IN → OUT; a star is forbidden.
Limits of one line: 32 fixtures and ~300 metres.
Connectors are 3-pin and 5-pin XLR; the signal pins are 2 (data −) and 3 (data +), 1 is the shield.
Microphone cable will not do: you need 110 Ω DMX cable, otherwise distortion and «floating» problems.
A 120 Ω terminator in the OUT of the last fixture removes signal reflection — always fit one.
Branching and extending happen only through a splitter, ideally opto-isolated.
Most «mysterious» lighting problems are a cable, a connector or a missing terminator.

Coming next: we will look at how the computer connects to this line. The next lesson is about interfaces, universes and network output: what a USB dongle is, when 512 channels stop being enough, and what those mysterious Art-Net and sACN names mean.

Next lessonInterfaces, universes and network output (Art-Net and sACN)