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Author Topic: Trying to understand why - DMX.  (Read 3014 times)

Debbie Dunkley

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Trying to understand why - DMX.
« on: November 10, 2014, 04:54:00 pm »

Ran sound for a small bar band at the weekend. They asked me to run a few lights also so I took a tree of 3 LED lamps along with me. However, I normally run everything on 2 different iPads wirelessly through routers. However, I forgot my art net node so I was stuck with letting the lights run on auto - no big deal as venue was very small and no big production necessary. It took me a while to get them set to auto - one on master and 2 on slave as I had never run them like that before and Blizzard menus are I think are a  bit odd but I got them programmed OK.
This is the thing though - out of the 3 LED fixtures only one (the one set to master) seemed to do as it should. The other 2 were running through some auto program but were doing some strange flashing at times. I didn't have time to mess with them so once I had them working independently, I had no choice but to concentrate on the PA.
The band was very happy with everything so gig went very well.

When I got home I set everything back up. I tried them wirelessly and they worked fine. I then decided to use them with my old Chauvet Obey 10 because I figured if I get into the habit of throwing it in the van, it might help me out if my wifi lets me down in the future and I wanted to make sure everything would work well together. When I used the controller, the same 2 lights that were acting strangely did not work at  all. I switched back to wi-fi and they all worked properly again.
I just checked the DMX cable that is connected from the first fixture to the second one and it is faulty. The ground is touching the neutral. This would normally explain why lamps 1 and 2 were not working but ……. my question is this…why would this faulty cable affect the internal auto programs and the Chauvet controlled DMX but not be affected when connected to wifi art net node.???
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Josh Daws

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Re: Trying to understand why - DMX.
« Reply #1 on: November 10, 2014, 08:35:28 pm »

in short (pardon the pun) because dmx is just this way...its super unpredictable. i have had problems with my last fixture and the cable linking fixture 1+2 was the problem. its messed up, im sure there is a reason, but welcome to DMX
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duane massey

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Re: Trying to understand why - DMX.
« Reply #2 on: November 10, 2014, 09:09:46 pm »

I'm just guessing here but:  when you set them to WiFi, did you set all three to WiFi? If so, the dmx cabling issue becomes moot, as the fixtures would be recieving their signal thru the wireless. A short from shield to either pin 2 or 3 would cause problems with any fixture set to slave or dmx EXCEPT the master, which would unaffected by a cable issue.
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Duane Massey
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Debbie Dunkley

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Re: Trying to understand why - DMX.
« Reply #3 on: November 10, 2014, 10:23:45 pm »

I'm just guessing here but:  when you set them to WiFi, did you set all three to WiFi? If so, the dmx cabling issue becomes moot, as the fixtures would be recieving their signal thru the wireless. A short from shield to either pin 2 or 3 would cause problems with any fixture set to slave or dmx EXCEPT the master, which would unaffected by a cable issue.

Not sure what you mean by setting all three to wifi. I think maybe I have been unclear and I misled you into thinking I use wifi antennas on the fixtures . I actually use a router and an artnet node connected to the first fixture by DMX cable. Then DMX cable to fixture 2 and then DMX cable to fixture 3. Then I set each fixture to its own DMX address. This is why I don't understand because the only way fixtures 2 and 3 get info is through the DMX cable coming from the first fixture - no?
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Tom Bourke

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Re: Trying to understand why - DMX.
« Reply #4 on: November 11, 2014, 12:35:07 am »

in short (pardon the pun) because dmx is just this way...its super unpredictable. i have had problems with my last fixture and the cable linking fixture 1+2 was the problem. its messed up, im sure there is a reason, but welcome to DMX
DMX is very robust and well understood.  Other than equipment failure it only gets wonky when you don't follow the rules.

Debbie, a few thoughts.  DMX is based on RS485 with a couple of additions.  RS485 uses 3 wire, 120 ohm impedance cable, signal +, signal -, and a common reference feeding differential receivers.  This is similar to how balanced audio works as far as noise immunity.  The end of the line should be terminated as well.  The full DMX specification also adds shielded cable, 5 pin XLR, and opto-isolated drivers to the mix.  Note, standard audio mic cables are NOT the proper cable for DMX.  On top of this physical connection is a specification for data packet layout and timings.

Just as not all users follow the rules, some manufactures ignore some of them as well.  Most commonly they use 3 pin XLR and skip the opto-isolation and the timings may be off.  Also keep in mind there are different grades of interface chips and circuits, just like some mic inputs are better with noise than others.  Improper cables or termination can cause degraded signal and reflections on the line.

With your setup the driving interface is different between the various setups.  The artnet nodes may have isolated outputs or some other difference.  The ground/neutral connection may be causing interference, reflections, or pulling the signal voltages too far off spec. It may be close enough to work with one setup and not the other.
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Debbie Dunkley

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Re: Trying to understand why - DMX.
« Reply #5 on: November 11, 2014, 01:00:48 am »

DMX is very robust and well understood.  Other than equipment failure it only gets wonky when you don't follow the rules.

Debbie, a few thoughts.  DMX is based on RS485 with a couple of additions.  RS485 uses 3 wire, 120 ohm impedance cable, signal +, signal -, and a common reference feeding differential receivers.  This is similar to how balanced audio works as far as noise immunity.  The end of the line should be terminated as well.  The full DMX specification also adds shielded cable, 5 pin XLR, and opto-isolated drivers to the mix.  Note, standard audio mic cables are NOT the proper cable for DMX.  On top of this physical connection is a specification for data packet layout and timings.

Just as not all users follow the rules, some manufactures ignore some of them as well.  Most commonly they use 3 pin XLR and skip the opto-isolation and the timings may be off.  Also keep in mind there are different grades of interface chips and circuits, just like some mic inputs are better with noise than others.  Improper cables or termination can cause degraded signal and reflections on the line.

With your setup the driving interface is different between the various setups.  The artnet nodes may have isolated outputs or some other difference.  The ground/neutral connection may be causing interference, reflections, or pulling the signal voltages too far off spec. It may be close enough to work with one setup and not the other.

Sounds fair to me Tom - thank you. It was driving me crazy because I didn't see how one method could detect the cable fault but the other could not.
FYI…I am using DMX cables -not mic cables and I use a terminator so no issues there. 3 pin though - not 5. Lamps are Blizzard Q 12's. The wireless method has been 100% reliable so far - …...I'll never forget any piece of the system again for sure !!
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Jerome Malsack

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Re: Trying to understand why - DMX.
« Reply #6 on: November 11, 2014, 09:31:48 pm »

Add in some fair warning that even with DMX cables this problem most likely would be the same if the cable in question had the same failure.   The cables getting crushed by a guitar amp or a food rack rolled over them can cause these errors. 

The mic cable would see this as a loss in the signal and become unbalanced. 

Using a cable tester routinely between gigs can help find these problems.  Our group usually does a cables test twice a year minimum.  I keep one in the truck for onsite questions. 
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Debbie Dunkley

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Re: Trying to understand why - DMX.
« Reply #7 on: November 11, 2014, 10:07:29 pm »

Add in some fair warning that even with DMX cables this problem most likely would be the same if the cable in question had the same failure.   The cables getting crushed by a guitar amp or a food rack rolled over them can cause these errors. 

The mic cable would see this as a loss in the signal and become unbalanced. 

Using a cable tester routinely between gigs can help find these problems.  Our group usually does a cables test twice a year minimum.  I keep one in the truck for onsite questions.

I am pretty good at testing and even cleaning down my cables quite regularly but this one bad cable is a 30" DMX cables which stays attached to the light tree bar.
It gets no strain and also doesn't get used anywhere near as much as the PA does so I didn't even think to test it.  Again….I took things for granted and got caught out.

However, like i said, it wasn't the worse thing in the world as the lights were functioning in auto - just not the same auto program as the first fixture.
I must admit I thought it was my programming to be quite honest because I find the Blizzards to be not very user friendly in the menu section. My Chauvets are very simple to program. I had never had to program these other than DMX out of the box so I got caught out really….
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Steve M Smith

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Re: Trying to understand why - DMX.
« Reply #8 on: November 12, 2014, 04:21:18 am »

I am pretty good at testing and even cleaning down my cables quite regularly but this one bad cable is a 30" DMX cables which stays attached to the light tree bar.

Sounds like you should build that cable tester I sent you the circuit for!


Steve.
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Debbie Dunkley

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Re: Trying to understand why - DMX.
« Reply #9 on: November 14, 2014, 11:03:38 pm »

 :-[

Sounds like you should build that cable tester I sent you the circuit for!


Steve.
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ProSoundWeb Community

Re: Trying to understand why - DMX.
« Reply #9 on: November 14, 2014, 11:03:38 pm »


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