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Thread: Help with signal integrity

  1. #11
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    Default Re: Help with signal integrity

    Quote Originally Posted by pakeonoahu View Post
    I used three pairs on a single CAT5 ~ 50' without any issue on F16v3, controller only provide data, not power.
    How were they connected? Using the screw terminals on the pluggable connectors on the F16v3? Did you use both lines in each pair for the (same) signal, or with a ground in each pair? What kind of pixels are you driving?

    Thanks.

  2. #12
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    Default Re: Help with signal integrity

    Quote Originally Posted by ags0000 View Post
    How were they connected? Using the screw terminals on the pluggable connectors on the F16v3? Did you use both lines in each pair for the (same) signal, or with a ground in each pair? What kind of pixels are you driving?

    Thanks.
    they are connected with the screw terminals on the F16v3 end, I connected each pair per terminal with GND/DATA since I extended the cable on the other end to different location for different elements. All my pixels are WS2811.

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  4. #13
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    Default Re: Help with signal integrity

    Quote Originally Posted by Barnabybear View Post
    Hi, some thoughts.

    My first, according to the data sheet the 74ACT574 has tristate outputs which I've had problems with in the past. Whilst tristate outputs are fine when enabled, if disabled the pin floats and picks up noise which I found to be enough to imply a logic level. Given that the resting state for WS281x IC’s is data line low I cured this with a 10K pulldown resistor at the controller end.

    Are you using an output resistor in series with the data line? You can still get ringing on twisted pair cables. 50 ohms seems to generally considered optimum but given that the 74ACT574 can only supply 24mA you might want to consider something like 220 ohms. This would prevent more than 24mA being drawn if the output ever became shorted to ground.

    Note: If you fit both pulldown and output resistors the pulldown goes first and the output goes second.

    Let us know how you get on.
    So experimenting has shown that a 100Ω series resistor reduces the signal/pixel data line ringing, and all is well. Looking at it with a scope shows just how bad the ringing was without. I will see if I can get some screen shots just for fun.

    Further experimentation and study also shows that I can run 4 twisted pairs in Cat5 cable (driving 4 independent pixel strands) without crosstalk causing signal integrity issues.

    The only additional point is exactly what you already mentioned: the tri-state lines float when not driven, and the pixels see that noise as a false signal and the first few pixels end up with strange colors. Originally I used the 574 FF/driver because it has D (input) on one side, Q (output) on the other, making the PCB layout clean. After thinking about it a bit, I decided I would put the OE line to good use. Whenever the controller is not actively driving the pixels, the output is disabled. The controller is always on; I only turn on/off the power supplies when the show is active. Having tri-state outputs seemed like a nice additional saftey feature, isolating the driver (and controller by proxy) from damage if something odd should happen to the pixel signal lines - like a squirrel, or me. I often work on the display without powering down the controller... but I don't generally do things when a show is running.

    But I have functions for testing that send a frame and then stop -- and the pixel lines are in hi-Z. So the test patterns then are corrupted by the noise on the undriven lines, making the testing not optimal. So I have to use pulldown resistors.

    Bottom line is that this is all simple and fixable -- but now I have 32 channels needing 100Ω series resistors and 32 channels requiring 10kΩ pulldown resistors. I'll use resistor networks/arrays to simplify, but it is adding to the part count. I could always just tie the OEn line low and skip the pulldowns, but then I also skip the added protection for swapping connections (as long as the show isn't actually playing).

    Even with SMD resistor networks, that's going to put more pressure on my board size. Other suggestions welcomed...
    Last edited by ags0000; 04-03-2018 at 02:00 PM.

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