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DMX Wiring Problem?

#1 User is offline   wirralmatt 

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Posted 07 March 2014 - 11:10 AM

Hey,

I'm looking for a little bit of help with a problem I can't get my head around.

Since an (in-house) repair on some lighting units I have developed a rather strange fault.

I have two Showtec dimmer bars and four LEDJ Omni Tri 9 all on the same universe.

If I feed DMX to the LEDs first and then the dimmer bars everything is all ok.

If I plug the LEDs in on their own, everything is fine.

If I plug the dimmer bars in on their own, everything is fine and the little signal light blinks away quite merrily.

If I then plug the LEDs in further down the DMX chain (after the dimmer bars) the LEDs work fine but the little signal light disappears from the dimmer bar after about a second and the bar becomes unresponsive.

When I first had the problem I worked out pretty quickly that the fault was a wiring one - the pins had been crossed between the input and output in the rewire. I've since rectified that and the problem still presents.

Any help!? It's driving me round the loop!

Thanks

Matt

This post has been edited by wirralmatt: 07 March 2014 - 11:12 AM


#2 User is offline   timsabre 

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Posted 07 March 2014 - 11:15 AM

Sounds like you have a break in one of the DMX wires somewhere, either pin 2 or pin 3.

Link all the units up without power or a desk, and do a continuity test on each pin of the DMX wiring from one end to the other.
Tim Mitchell
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#3 User is offline   wirralmatt 

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Posted 07 March 2014 - 11:20 AM

View Posttimsabre, on 07 March 2014 - 11:15 AM, said:

Sounds like you have a break in one of the DMX wires somewhere, either pin 2 or pin 3.

Link all the units up without power or a desk, and do a continuity test on each pin of the DMX wiring from one end to the other.


I've tried combinations of just one dimmer bar and just one Omni Tri 9 (I've tried it with all four in isolation) so would I be right in thinking that I can eliminate that?




#4 User is offline   ceecrb1 

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Posted 07 March 2014 - 11:34 AM

I have seen similar faults when the DMX receiving chip within a fixture fails.
Just a few weeks ago this poped up while I was visiting someone else's rig.
He told me I had to wire DMX "this (strange) way" because those robots dont work well with those others..

The fault was in that the last robot had a bad DMX receiver... Inside the chip there was a short which took pin 3 to ground..... so it was doing this for the entire chain from the splitter. Took me secconds to swap the chip with a spare I had in my tool kit. There are variety of brands and models but in the end are pretty much the same thing.

Simple test is use a volt meter from pin 1 to 2 and 1 to 3 and see if there is any wild diffrences in voltage. Or make one of those xlr-led testers (or buy on ebay).. theres lots of videos on youtube.

#5 User is offline   jbrown96 

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Posted 07 March 2014 - 11:37 AM

View Postwirralmatt, on 07 March 2014 - 11:20 AM, said:

View Posttimsabre, on 07 March 2014 - 11:15 AM, said:

Sounds like you have a break in one of the DMX wires somewhere, either pin 2 or pin 3.

Link all the units up without power or a desk, and do a continuity test on each pin of the DMX wiring from one end to the other.


I've tried combinations of just one dimmer bar and just one Omni Tri 9 (I've tried it with all four in isolation) so would I be right in thinking that I can eliminate that?





It would add value to the argument that the cable was not the problem but I have had experience with breaks in DMX were the problem with cable was intermittent and simply moving the cable has solved the problem.

#6 User is offline   timsabre 

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Posted 07 March 2014 - 11:40 AM

View Postwirralmatt, on 07 March 2014 - 11:20 AM, said:

View Posttimsabre, on 07 March 2014 - 11:15 AM, said:

Sounds like you have a break in one of the DMX wires somewhere, either pin 2 or pin 3.
Link all the units up without power or a desk, and do a continuity test on each pin of the DMX wiring from one end to the other.

I've tried combinations of just one dimmer bar and just one Omni Tri 9 (I've tried it with all four in isolation) so would I be right in thinking that I can eliminate that?



Not really, DMX can work in strange intermittent ways when one of the balanced lines is faulty. Could be a physical break or a faulty chip as ceecrb suggests. I once had a rig where someone had wired a 5 pin cable as 1-2-3 at one end and 3-4-5 at the other end, so only pin 3 was connected. It worked fine in some configurations but not at all in others, took ages to find the problem.

If you can get an oscilloscope on the DMX line that will straight away show you what is going on. If you don't have one of those then you will need to mess around with a meter.
Tim Mitchell
www.sabretechnology.co.uk
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#7 User is offline   PaulDF 

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Posted 09 March 2014 - 06:33 PM

I know it sounds more like a break but some back to basic questions first, have you got an end of line resistor installed? Are you using DMX cable? Do you have a lead checker?, I found some XLR socket pins had opened up on my rig causing havoc.

#8 User is offline   timsabre 

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Posted 09 March 2014 - 09:14 PM

View PostPaulDF, on 09 March 2014 - 06:33 PM, said:

have you got an end of line resistor installed? Are you using DMX cable?


Although these are both good practice, neither should be necessary to obtain a functioning system. There is a fault here.
Tim Mitchell
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#9 User is offline   Jivemaster 

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Posted 09 March 2014 - 09:24 PM

Ultimately if you are really stuck then try a real isolated DMX splitter and run bits on their own leg.

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