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dmx_basics:dmx_fault_finding

DMX Fault finding


This is a small guide of how to find the most common errors when a setup doesn’t behave.

First of all is to be systematic in the fault finding even in the panic phase.

Nothing happens at all.
No lights are reacting on any fader control.
This means no communication to the fixture.
See that FreeStyler actually has an output to the fixture. This can be seen in the “Show Output” Panel.
Isolate the system by removing the output cable from the first fixture.
The setup now consists of the PC and only the first fixture.
If the there is an output from FreeStyler and it still don’t work then it is most likely a cable issue or the fixture that fails.
Shift the cable and if this don’t help then take the next fixture in the line and connect that one to the setup. But remember to remove all other fixtures so it if only fixture 2 that are on the system.

If the system works when only one fixture are on the chain then the error are on one of the next fixtures.
So connect the rest of the chain and check that the error is still there.
The fast way is then to split the chain in the middle and see if this part is working.
If it is working then the error are in the last part of the chain and if it fails then the error are in the first part of the chain.
The failing part can then be divided again in the middle and so on until the failing part is isolated.
It is most likely a cable but can also be a fixture.
If it is the fixture then verify it by unplug the input, output DMX cable from the fixture and connect the two connectors direct. If the system is working then see that the error appears again when the fixture are inserted.
This is to verify that the fixture actually has an error and not a faulty cable / connector.

But if you'r realy sure that the address is Set correct on the Fixture and the Software, But you have still no Reaction.
Then the Problem can be Caused by The fixture them Self, Sometimes they Only work on DMX channel 1 to 256.


Only the first part of the chain works.
The point where the chain stops working is worth to look at. Try bypassing the fixture by unplugging the input, output DMX cable from the fixture and connect the two connectors direct.
If it works then verify the error by inserting the fixture again and see that the error reappears.
If the error moves to the next fixture when the fixture is bypassed, then have a look on the cable.
The most common error is cable failure.
Not some fixtures can have a build-in line driver or protection circuit that may course the chain to fail if there no power on the fixture.


One or More fixture don’t react as expected.
It is most likely that the fixture has been set to a wrong address.
Select the fixture in FreeStyler and use a know function like shutter. If the fixture does something else than turning on or off the light then it is most likely a wrong address on the fixture.
Compare fixture address in FreeStyler and the address set on the hardware fixture.


More fixtures react on one control.
If a fixture set to a wrong address there may come an overlap in the addressing. This gives some strange behavior of the fixture. One symptom could be when example the movement are adjusted then the wrong addressed fixture shift colors or make other strange things.
Select the fixture that behave strange in FreeStyler and use a know function like shutter. If the fixture do something else than turning on or off the light then it is most likely a wrong address on the fixture.
Compare fixture address in FreeStyler and the address set on the hardware fixture.

Strange behavior.
The are more possibilities for strange behavior but remember there are only a few items in the chain. It is fixture and cables and most fails are cable fails.
Set all fixture of same type to do the same thing like move Pan / Tilt, parcans to dimme. Have a look that they actually also are doing as expected.
If they don’t then have a look on addressing and faulty chain as described above.

If the error type is a bit more complex such as all fixture are moving in the same pattern but one fixture shift colors, flash or what so ever then there are more possibilities.
One could be the controller from time to time sends a wrong frame.
This is very difficult to detect because all frames somehow has to be recorded and checked.
Another and more likely possibility are reflections on the cable coursed by missing DMX termination or a cable that are not the 120 ohm DMX type.
This will course the DMX signal bouncing forward and backward on the cables. It just take one bit to interfere with and then color shifts or shutter opens and so on.
See eventually Measurement on the DMX cable.

One funny thing with the DMX is that it is verry robust and will work even if one of the wire are broken if the terminator is not fitted.
So if the setup works without the DMX terminator but fails with the DMX terminator added, then one of the wire may be broken.

Also interfere from external sources can be a problem. HID Lamps, Strobes and other thing that can generate a spike. Transmitter, mobile phones. Mains, spike on mains that influence on the fixtures or even worse through the fixture out on the DMX cable.
There are many possibilities.


Reproduce the error.
A failure can be so tricky that a work-around are necessary.
Before doing a workaround then it may be a good idea to decide if this problem are to be investigated later.
If it is to be investigated later then record everything. Fixture order, addresses, other setting, cables.
It is important to use the exact same cable in the same position.




Mark the faulty items.
Mark the faulty items seems like a logical thing but we are in a panic phase and there is just two of this type so it is easy to remember.
NO, NO, NO……

MARK THE ITEM! “DEFECT” “ FAULTY” …….


Finally it is difficult to find errors and some simple equipment can be extreme helpful.
See: DMX Test Equipment.





dmx_basics/dmx_fault_finding.txt · Last modified: 2013/03/12 22:03 (external edit)