This is one of those questions that pops up on social media frequently. These posts receive lots of attention. Some responders try to provide useful information. Others flame newbie level questions and whoever posts them. Here’s my attempt to provide useful information.
Vocabulary
The first problem with the question is the problem of sloppy vocabulary. What is an XLR cable? In my book, any cable with XLR connectors on it is an XLR cable. When I need to be more specific, I’ll say, “XLR mic cable” or “Three pin XLR DMX cable.”
Some time in the 1950’s, James Cannon of Cannon Electric created the Cannon XL connector. They invented versions of the connector with three, four, five, six and as many as seven pins. When they listed it in the catalog, it was called XLR. 1
The audio industry adopted the three pin version and it became the industry standard for balanced analog audio. In entertainment technology, an XLR cable frequently refers to an audio grade cable with male and female three pin XLR connectors on each end.
DMX
The DMX protocol is based upon RS-485 which is a three wire protocol comprising of ground, +data and -data. The protocol only needs three wires. Legend has it that the standards bodies chose 5-pin XLR connectors for DMX with pins 4 and 5 left unused because they didn’t want folks getting too sloppy and mixing microphone cables with DMX cables. So what’s the big deal if they mix?
Cabling
All cables are not created equal. Audio cables are designed to provide optimal quality for 600-Ω circuits. DMX cables are designed to provide optimal quality for 120-Ω circuits. This is really the crux of the answer when people ask about mixing DMX and audio cables.
You can break the rules. But breaking the rules degrade performance of the circuit. Every time you break a rule, the peformance diminishes a little bit. At some point, the performance drops to the point the signal gets corrupted then misinterpreted. If you’re in a small club with four LED PARs and everything is under 50 feet, you can probably get away with 600-Ω microphone cable without termination. If you need your signal to reach the other end of a stadium, use the right wire.
Follow the rules, you’ll get the best quality over the longest distance.
- Use 120-Ω cable designed for RS-485 applications like Belden 9729.
- Terminate the end with a 120-Ω terminator.
- Never split the signal path (Y-Cable).
- Opto-isolate devices.
Maxim Integrated makes the MAX485 chip used in many DMX devices. They have a great article with charts showing how the signal degrades if the rules aren’t followed. Guidelines for Proper Wiring of an RS-485 Network
Connectors
“No plan survives contact with the enemy.” - attributed to Helmuth von Moltke.
The plan for DMX is defined by ESTA and ratified as ANSI publication E1.11-2008 and raffirmed in 2013. Section 7.1.1 of the Standard reads, “7.1.1 Required connector Equipment in this category shall use 5-pin XLR connectors with the physical pinout of the 5-pin XLR in accordance with Table 3”
Table 3 then enumerates that
- Pin 1 Data Common
- Pin 2 Data 1-
- Pin 3 Data 1+
- Pin 4 Data 2- (optional)
- Pin 5 Data 2+ (optional)
Most first class equipment manufacturers take great pride in adherance to the Standard.
In the past several years, pressures to create cheaper equipment have lead to cutting corners with the stanards. Ecconmies of scale mean that 3-pin connectors are substantially cheaper than 5-pin connectors. Some manufacturers have used pins 4 and 5 for power or they’ve used those pins for manufactuere specific purposes like dimmer-rack status.
Doctor DMX
Doug Fleenor started Doug Fleenor Designs in 1990 and sort of accidentally became an industry expert on DMX. His products have a reputation for being solid and reliable. While his products are seriously awesome, he has a reputation for having seriously good humor about him.
Doug Fleenor has created two videos talking about what makes a high quality DMX cable.
Part 1:
Part 2:
Footnotes
[1] From XLR History