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How does voltage drop affect the data line?

Discussion in '101 Display Basics' started by aplant92, Nov 5, 2018.

  1. aplant92

    aplant92 Full Time Elf

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    As per the title, I always see voltage drop being talked about around the power going to the pixels, but no real mention of the data line.

    I'm assuming eventually the length of cable leading to pixels will become an issue with the data line, but at what point is this? And is there any way to inject power into the data line to be able to run a larger distance from the controller?
     
  2. darylc

    darylc 404 darylc not found Global Moderator Generous Elf

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    It's not as simple as injecting power into the data line - you would corrupt the data. You'd need a null pixel to repeat the data or any of the various products the vendors sell to repeat or amplify the data. Remember the other leg of the data is the ground wire, so make sure you aren't getting much voltage drop along the ground wire. This is why people recommend using 2 wires in a 4 core cable (for example) for ground when using 3 wire pixels.
     
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    aplant92

    aplant92 Full Time Elf

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    Ok great that makes sense - is there a general recommendation for how far away the first pixel can be from a controller in terms of ensuring the data is fed through properly?
     
  4. darylc

    darylc 404 darylc not found Global Moderator Generous Elf

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    too many variables (controller model, size of cables, power draw through those cables, interference, etc etc etc) to give general rules of thumb for distance to first pixel.
     
  5. AAH

    AAH I love blinky lights :) Community Project Designer Generous Elf

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    It actually depends on the controller as well as the cable. Most controllers should do 10m+ these days. J1Sys controllers which are virtually obsolete now were more like 3-5m. Pixel to pixel distance is typically maxed out around the 5m mark. There's a video on the bottom of http://www.hansonelectronics.com.au/product/null/ that shows the difference between what you can get with pixel to pixel distance with and without a buffer. The same applies when/if you have distance issues from a controller. Unlike the need for power injection like occurs with pixels with data it's actually the rounding of the signal due to cable capacitance that is the limiting factor. The further the data has to travel the less like the original square wave it started as it looks like. The voltage does go down but it's not due to lack of voltage. The pixel cable is both resistive (which is why you need power injection) and capacitive. The effect of the 2 changes the shape and voltage. See the attached pic. The data signal starts off like a) and progressively gets more like c) but unlike in the picture the voltage reduces.
     

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    aplant92

    aplant92 Full Time Elf

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    That's a great explanation - thanks, and the video is also great for illustrating the effects! It's good to know things like that buffer exist too - I take it they just allow you to run longer distances between the controller and first pixel/pixel to pixel?
     
  7. AAH

    AAH I love blinky lights :) Community Project Designer Generous Elf

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    They allow the distance between any 2 pixel "things" to be extended. If you need to go further than what 1 will go you can add a 2nd, 3rd, 4th. I had someone tell me that they went to 170m the other day before they ran out of cables on hand. I thought 33m that I got was a good effort. Not sure how many nulls were added to get 170m but it's a BIG number for non differential pixel transmission.
     
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    aplant92

    aplant92 Full Time Elf

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    Ok cool!

    Out of curiosity - how do the null pixels allow the distance to be boosted so drastically? Are they amplifying the data signal or something similar?
     
  9. David_AVD

    David_AVD Bite my shiny metal ass! Community Project Designer Generous Elf

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    The null pixels / signal buffers accept the slightly degraded signal in and regenerate a fresh clean signal out. So as long as they are put inline before the signal gets too badly affected they will work well.
     
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  10. AAH

    AAH I love blinky lights :) Community Project Designer Generous Elf

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    A standard pixel inserted into the line will regenerate the signal and go an extra x metres. The difference between a pixel and a buffer is the ability to drive a square wave signal into a capacitive cable. Referring back the that square wave picture what you are seeing is the effect of an RC (resistor capacitor) curve. The capacitance is from the length of cable and increases the longer the cable is. The resistance comes from 2 sources. It comes from the very low resistance of the cable (it has to be low to allow amps of current to power pixels) and the internal resistance of the IC that is driving the cable. Pixel chips have a fairly high source resistance / low drive current which is 1 of the major limiting factors in how far they can be separated. The chips that I chose for the buffer were specifically chosen because they have a high drive current. This means that the shape of the square wave data signal remains squarer for longer. Other sneaky stuff in the design is there to reduce the ringing as seen as in d) and e) of the picture.
     

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