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Distance vs null pixel

Discussion in 'RGB Lights - Intelligent Pixels and 3-Channel RGB' started by scamper, Dec 16, 2015.

  1. scamper

    scamper Senior Elf

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    This is only my first year dealing with pixels and I have read everything I can and I have my display running at this point with no problems.
    I am running the show from a raspberry Pi, from there through a network switch to 2 separate f16v2 controllers.
    The first one sits near the controller, the second one is out the front under the mega tree.
    I then use the dmx outputs on these 2 controllers to run all the dumb strips.
    and the pixels are all run from these obviously.
    Now I read somewhere that cat5 cable would give you a longer run before requiring null pixels, so that is what I used.
    I have the longest run of about 6meters and the display all works fine.
    My question is....
    What is the normal run length before requiring null pixels?
    And what would the symptom be if the length was too far and I did require a null pixel?
     
  2. AAH

    AAH I love blinky lights :) Community Project Designer

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    There are a couple of factors that govern how far between pixel elements.
    -The 1st factor is the series resistance between pixel devices. For most elements this isn't an issue but with J1 Sys pixel controllers and maybe others it may be. The J1 Sys P2 for instance uses 300 ohm resistors in series with the line driver IC's and this dramatically reduces the distance you can get to the 1st pixel. Most other pixel boards use a much lower resistance in line with the recommended resistance from the pixel specs which is more in the range of 30-50 ohm. For this reason a null pixel right at a J1 Sys controller will give you much greater distance than what the controller by itself will give. As the resistance in the wires used is negligible their resistance typically won't be a factor.
    -The 2nd factor is cable capacitance. The causes rounding of the signal and the greater the capacitance, in conjunction with series resistance, the worse the signal looks when it gets to a pixel. Low capacitance cable reduces the effect. Heavier insulation and thicker conductors reduces the effect.
    -The 3rd factor is voltage drop and it is primarily due to using cable that isn't especially suited to the task. Using security cable is a huge reason for voltage drop. The conductors are not suited to carrying any appreciable current so when the data appears to be going bad people stick in a null pixel when in most cases the issue will be lack of copper.

    The WS2811 specifications say that 11m is possible between pixel elements. I personally am using 6m and haven't had an issue. Using heavy cable for the power portion of the pixels is the key. Heavy duty figure 8 speaker cable is a must imho.

    As to what you would see if the length was too far then what will happen typically is that the display will flicker at certain brightness level due to the way the rounding of the signal effects the shape of the digital waveform. If it's a voltage issue the higher the intensity of the entire pixel run the more likely it is that it will show up. Pinkness when displaying white is the 1st symptom followed by pixels that become unresponsive and require a power cycling to fix the issue.

    /rant complete/
     
  3. OP
    OP
    scamper

    scamper Senior Elf

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    Thanks Alan. And that wasn't a rant, that was a concise answer to a question.
     
  4. BundyRoy

    BundyRoy Dedicated Elf

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    This is an issue that I had. I have 3 arches with 96 pixels in total across the three. They are 10m from the controller. The power runs on a separate cable to the data signal (common earth). Last year I had a null pixel at 5m in the data cable (security cable) and all worked fine. This year I set it up (same position) and got flickering in the pixels on all colours. I tried swapping out the cables and null pixel and no difference. I removed the null pixel and it works fine. Seems odd to me but I'm going with the flow. They are all ws2811 and the 96 pixels are the only lights on the line. I have three arches beside these (separate power and data line again) and they are still going strong with their null pixel. Go figure.
     
  5. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    It could be a flaky pixel due to being unplugged and plugged back in with the power on, this can cause some weird pixel issues with the 2811.

    Are you using the same pixel for a null pixel as what is used in your string because different pixels mixed together can also be an issue as not all 2811 are the exact same.

    Cable positioning can also make a world of difference due to the amount of potential electrical interference we can produce with our shows.
     
  6. BundyRoy

    BundyRoy Dedicated Elf

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    I have been guilty of this in the past when I first started (last year). Very conscious of it now and don't do it anymore.

    Exactly the same pixel. Have tried several different pixels for the null pixel as well. Nothing changes.

    Could be this as my cable layout this year is not as tidy as last year. No new cables but there's a few there in a bit of a mess.
     

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