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DMX (why it does strange things sometimes)


There is one simple rule, 99% of ALL DMX problems are caused by cabling issues. This can be anything from using the wrong type of cable, to incorrect earthing, lack of termination, or using Y-splits. Any or all of these can cause DMX to do strange things.


Unfortunately the results of such cabling errors do not always show up straight away. It may be 12 months later when the connectors start to corrode, or when they install a Cell phone tower just up the road. Also in touring systems it may work for the first 5 shows, then on the sixth night it stops working, only to work again on the next night.


So how do solve these problems?


1) Are you using RS-485 computer data cable?

This is VERY important.

You CANNOT use microphone cable !


Sure it will work most of the time, but sometimes you will get flickering of lights, or random movements of mirrors, etc. Many users first discover this problem when upgrading to an LSC console, and therefore wrongly blame the new console. The problem is that the LSC consoles update the DMX at the maximum refresh rate (as defined in the DMX1990 standard). This is to ensure the fastest response from the lighting rig. However it also shows up bad cabling, that may have worked with a slower console, but does not work with full speed DMX.


2) Do you have a DMX terminator installed?

Once again this is VERY important. The system may work without a terminator, but it will not be reliable. The terminator stops data reflections, which can cause standing waves in the line, often stopping one light in the middle of the cable from working, whilst those on either side work perfectly. Sometimes replacing a cable in the system will move the fault to another light, seemingly at random.

To make a DMX terminator simply solder a 120R resistor across pins 2 & 3 of the XLR connector.


3) Remove any Y-split leads.

If you want to split your DMX cabling into 2 separate runs (eg front and rear truss) then use a DMX splitter (such as the LSC Delta units shown). If you try to use a Y-split lead (2 leads into one plug) then it will not work.


As with all things DMX the Y-lead may work sometimes, but you can be sure that it will cause problems when you least expect them.


4) Check your earthing.

Always connect the earthing shield of the cable to the Pin 1 of the XLR plug.

NEVER connect the body of the XLR plug to the earth of the cable.

Otherwise you will usually get problems connecting different brands of equipment or connecting equipment with different power sources.




The above 4 items highlight the biggest problem with DMX. It will very often work with incorrect cabling. This makes us all a little lazy, so we start to take shortcuts with our cables and then cannot work out what is wrong when something does actually go wrong.


In the last 30 days I have fixed 5 systems with strange DMX problems. ALL OF THEM were due to bad cabling. One client had been using the wrong cable type and no terminators for 3 years without any problems, and then suddenly in one venue nothing worked properly. As they had gone so long without any problems they were convinced that it could not be cabling, but after replacing the cables and installing terminators the system worked perfectly. They have since noticed a lack of flicker, something that they had assumed was faulty dimmers for the last 3 years........

So next time you have ANY sort of DMX problem start at the basics and check ALL the cabling in the venue, both installed and portable. We recently solved a random flickering problem at a venue where we knew the cabling was correct. As part of the fault finding we discovered that one of the patch leads, connecting two of the dimmer racks, had been repaired. The repair company did not understand DMX and so had used a normal shielded audio cable. One small 30cm cable, a tiny part of the 800m of total cabling was the cause of the problem.


Last Updated ( Friday, 16 April 2010 16:01 )


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