I just noticed these two components while setting up an APA-102C test rig. They don’t seem to be placed very often – just after the manufacturer’s solder joints. The markings read “270” on each.
Do you know what they might be?
It looks like they are resistors on the data and clock lines. They stabilize the data signals at the beginning of every strip.
270 = 27 Ohm resistor.
@Jon_Burroughs
That sounds like it would make sense if the data and clock lines were shared between every pixel, but they’re not – the data is reshaped and delayed by half a clock cycle for every APA102 in a strip.
@Luminous_Elements Why don’t you do some destructive snooping? Scratch off the mask to see where the pads go. Or perhaps, remove the resistors and short them to see if you get any difference. Lastly, the manufacturer would give you the best answer on this I would think.
Generally those resistors are there to prevent or suppress “ringing”, or signal reflection in the lines. They are only placed at specific distances along a strip. Most I’ve seen have then every 50cm. However, if they are pull-downs, it may be to keep the voltage difference clean over distance. Regardless, the manufacturer of the internal chip would be able to answer that question with more details.
No need to destroy your strip. Just use a multimeter !
Those resistors are likely place between the CI pad of the strip and the clock-in of the APA102. Same thing for DI pad and the APA102’s Data input.
The marking would suggest a 27 Ohms resistor. They would be used to limit the in-rush current on the APA device.
I find 27 Ohms to be quite low for that purpose but I do not know the innards of that APA102 !
One would imagine a chip like the APA102 to already have a high-impedance input on both the CLK and DATA lines. I’ll give the DMM a whirl on them though.
Not necessarily. Something about making them dirt cheap. 
Yes it has relatively high impedance inputs. That is why a small resistor like 27 Ohms or even a few hundred ohms have little effect on the DC level of the input signal.
But if you consider that even a small capacitance acts as a short circuit until it charges up, you effectively get a surprisingly high current for a very short amount of time (in the nanoseconds…) thus the 27 ohms resistor would limit that ‘in-rush’ current to no more than 185 ma peak and I guess that the strip manufacturer determined that this is insufficient to cause any damage to the APA102 inputs.
Note that this is only really useful on the 1st device of a strip as those inputs stand a chance of getting some voltage applied on them before the strip gets any power !!!
That is why you do not see these resistors on each device. Only in places that could be the first device on a strip !
Now considering that you should be able to cut that strip anywhere, the designers did not plan carefully for that and I guess you should add these resistors if there is any chance you apply a live signal to that cut-up strip !!
@JP_Roy , they’re not always at the beginning. I’ve seen them at the end as well, a lot actually. The only explanation I’ve ever received from a manufacturer is “to prevent signal reflection or ringing”.
Well… what can I say…
I am an experienced digital circuit designer and I think that’s a difficult answer to accept !
That being said, you should always listen to the actual product designers.
Yeah, I found it odd when they told me that, I just follow what they tell me. I never know whether to truly trust them or if things get lost in translation.
@Ashley_M_Kirchner_No I have never got my hands on any APA102 strips or device but I am curious to know the spread of those resistors in a typical APA102 strip ?
For sure you won’t find them at all in the 144LED/Meter style. Does that mean they have no ringing/reflection problems ?
Do they appear where there is no strip splicing just ahead of them ?
@JP_Roy
I’ve only seen them immediately after a splice.
I’m now thinking it may be a good idea to throw some 27Ω resistors at the start of data/clock lines that are connected by wire.
In my humble opinion, do not bother with them, if you are careful never to plug/unplug a strip from a live controller they are useless.
But… if there is even a small chance that you would do that, these resistors may offer some protection to your 1st device on that strip!
FWIW, examining an APA102c strip on a scope it appeared to me that the clock signal is not reshaped, only the data signal is. I have not seen these resistors on my 60 LED/m strips.
What part? From what I saw the clock signal is indeed inverted, but it is not reshaped. I don’t think that contradicts the article.
@Gabriel_Schine
The clock signal is inverted, essentially delaying it by half a cycle: https://cpldcpu.wordpress.com/2014/11/30/understanding-the-apa102-superled/