Data line resistors - do we really need them?

Proffieboards
1

This is kind of a back-to-basics thing, but working on a tight install with lots of pixel accents, it’s occurred to me to ask the question - are data line resistors really necessary?

My understanding is that they are there to clean up noise on the data line, which presumably prevents data being corrupted or misinterpreted by the pixels. In an appliance with long wire runs, this makes sense to me, but wire runs in a hilt are very short, which begs the question, can noise really be induced in such a short wire run to the extent that it materially affects functionality?

I guess these days this mostly applies to accents and not main blades, as many neopixel blade connectors have built-in data resistors. And of course Proffie Data 1 is pre-resistored in any case (hence I often use data 1 for accents and data 2, 3 or 4 for main blades).

So does anyone here routinely not bother with resistors on accent pixel data lines using pads 2, 3 and 4? If so, do you suffer any ill effects? Is there any particular thing you have to watch for when omitting them? I did a quick search but couldn’t find anything about the science of it all.

Just curious really. :slight_smile:

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tl;dr; yes

longer answer: maybe, maybe not

There are three reasons for having resistors.

  1. Reducing noise
  2. circuit protection
  3. prevent power leeching

So let’s go through them:

  1. Noise: I’m not sure if this is a real thing or not. I think the idea stems from something called impedance matching. Impedance matching is important at high frequencies, but 800kHz is not a very high frequency.

  2. Circuit protection: This one is important if there is any chance of getting battery voltage back through the data pin. Data2 and Data3 are 5v tolerant, so they should be able to handle battery voltages, but Data1 and Data4 are not, and so if you hook them up to a pogo pin connector (where shorts are common) then a resistor is a MUST. (Of course Data1 already has one, so it’s already covered.) For data2 and data3, I would still recommend resistors if you’re hooking it up to a pogo pin connector.

  3. power leeching: Some pixels (but not all) will go into low-impedance mode and draw power from the data line when they don’t get power from the LED pad. Modern versions of ProffieOS will put the pin in input mode when it powers down the pixels, which reduces the scope of the problem, but there are still cases where this happens. Basically; if you see a LED that stays on when it’s supposed to be off, a resistor on the data line might be needed. There is also a possibility of reverse power leeching where the CPU is getting power from the pixels when the CPU is supposed to be off.

Summary:

  • If your data line goes through a connector that could possibly cause a short, make sure you have a resistor.
  • If your neopixels stay on when they aren’t supposed to, make sure you have a resistor
  • if your off switch is on BATT+ and your saber is still drawing some power when off, make sure you have a resistor
  • If none of the above applies, then you don’t need a resistor
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This is amazing! :smiley:
Thank you so much Prof.
Really good info - and even learning the difference between datas 2/3 and data 4 (I had no idea about the 5 volt thing!) is useful when laying out an install to minimize the risk of things going wrong and causing damage in the future.

I am in your debt as always!
Thanks again.
:slight_smile: :+1:

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@profezzorn For full clarity in your reply wouldn’t it be when it’s supposed to be OFF, not ON?

re: Power Leeching…Basically; if you see a LED that stays on when it’s supposed to be (OFF), a resistor on the data line might be needed.

Given the resistor will halt any leeched voltage/amperage so the LED stays off as intended.

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Edited.

To be clear, resistors won’t completely halt leeched voltage, but it can reduce it by a factor of 1000, which is enough. :slight_smile:

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Anecdotally, I’d get a little bit of green in my first few pixels when off (black).

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@LyleStyle Yup. Even I see that sort of blip at times when building a pixel plug. Luckily there’s small resistors that can be added as need be.

@profezzorn I know, lol, my industry is proof of that w the EV systems. #GremlinSlowDrains Still a good idea to run a resistor unless that becomes moot w the new board once it releases.