DMX fails at installation but works on the bench

Symptoms/Issue

If you have narrowed down a DMX issue to a failing device, but are confused when you take it down/out for testing and find that it actually works, this article is for you.

Description/Explanation of Issue

DMX is a serial protocol composed of 3 lines, plus, minus, and common.  This protocol is quite robust if used only for dimming, and you can have the plus or minus entirely disconnected with no loss of communication or operation... IF it's a short enough run of cable.  But, if you have a long run of cable with one line disconnected or intermittent, it can introduce issues with flickering, jitter, or outright loss.

This is why you may have a failure at the installation, but then it works fine when you take it down or out of that environment to test on the bench or with/at a source/fixture close to the device.

A better way of testing.

There are 3 better ways to test DMX if you run into this rather than running simple dimming commands directly on a short run.

• The most precise test is to use an oscilliscope to look at the data on both the plus and minus lines referencing common.  Both lines should look like nice, sharp square waves.  If it is round or missing, this indicates a broken line, missing/blown component, bad DMX chip, or some other electronic problem.  But few people have access to or know how to operate an oscilliscope...
• If you happen to have a 1000ft of DMX cable lying around, this will usually cause the dimming to be jittery or flicker when a smooth trasnsition is made.  But again, this is not ideal.
• The best way to test is with RDM which is extremely sensitive to even slightly bad connections on the line.  If you run DMX through a gateway or other DMX device with RDM, it will not pass if there's a line missing or a circuit problem.

Further troubleshooting...

If the device is now confirmed to have failed, ETC product that was previously working fine should probably be returned for repair, and a new product should be replaced if it failed out of the box.  Check for a broken pin on the male DMX input connectors.  If someone recently swapped a DMX chip, check for a pin that didn't make it into the socket.  If there was a recent power event, current may have been inducted into DMX and blew something inside (usually the DMX chip).

If the device still works over RDM at your bench, obviously there's a problem at the installation, usually a cable/connector issue.  Missing/corroded terminals, connectors, or splices are the most likely culprits.