Pixracer cannot see SBUS recevier via RCin pin


Sometime ago about 2 years or so I bought an aliexpress version of Pixracer. Upon initial flash I figured out that Ardupilot (at that moment it was 3.4/3.5) is not able to see the SBUS receiver (xsr/rxsr/x8r). So at that moment I though it is some configuration issue, thus I reflashed it to PX4 and it works fine. Later I tried again to flash AP however the only NuttX+AP was able to see the receiver, but not ChibiOS+AP. So I jumped back to PX4. Now I need that controller again, but even with newest AP 4.0.6 receiver is still not visible. PX4 still works fine.

Could you please advise how to debug that? What can I do in order to get this issue solved?

P.S. Just for reference, it loops like following one: http://www.banggood.com/Pixracer-Autopilot-Xracer-V1_0-Flight-Controller-Mini-PX4-Built-in-Wifi-For-FPV-Racing-RC-Multirotor-p-1056428.html

If you have Arducopter flashed on it now post your parameters file, let’s take a look. I have 3 Pixracers, never a problem with Ardupilot. Which Rx do you have? Also, can you post a screenshot of the HW ID screen from Mission planner after connecting?

RX is R-X6R. Both SBUS and Sport are connected, “custom” wiring is verified by px4.
pixracer.param (17.9 KB)

Hard to see how it’s wired but the green wire from the RX6R (Sbus out) is connected to pin 2/RC Input (looks like a white wire) on the RC connector? Nothing in the parameter file out of the ordinary, compared it to one of mine with a Frsky Rx. With the HW ID I just wanted to see if that clone version has 2 IMU’s. Unrelated to this issue just curious. Not much to go wrong here.

Yes, you’re right about sbus-out connection. As I said before it is even working with PX4 in current configuration with no wiring modifications. Yes it has two IMUs, one is 9250 another is… hm… let me check

I don’t see a reason why it doesn’t work.

well… I checked it against original mRo schematic, and at least sensors set looks legit.
Another question: do you have following pins shortened?:

And do you know what is this electronic component? it is marked as 1F4 A40, is it resistors array?

I don’t know. The 3 that I have are in housings in models. One is an original AUAV Pixracer and the other 2 are clones. All perform the same. I don’t see a component designation like that on the BOM doc.

Ok, I managed to understand :).
First of all it seems to be R14 based clone, thus it does have mentioned pins shortened by design. The element I’m pointing to seems to be some unknown clone/version for 74LVC2G86 part (logical inverter).

I suspect that support for R14 was eventually “voided”, and I checked today, and with a fresh version of PX4 it does not work anymore as well. So I’m going to do some soldering and will see what logic analyzer will show on PC7 pin…

That’s the 1st I have heard of that after years of the Clones working but OK. PX4 was the “Native” Firmware anyway. Still is actually for the Mro FC’s. No problem with this just the way it is.

Yes, you’re right. The point is that during investigation of how pixracer schematics was changing R13->14->15 I see that there are some schematic changes that requires software changes as well. So in my particular case it is either pure hw problem, or unfortunate pixracer clone that has more flaws then yours one.
By checking with logic analizer SBUS input pin at MCU, I will clearly see if it is schematics problem, so the signal cannot reach MCU, or it is something else :smiley:

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Yes, there are definitely different clone version around. The latest ones only have 1 IMU and I recall seeing one w/o the ESP8266 header. The 2 clones I have are several years old. I have switched to F/H 7 boards from Matek and Holybro on the latest builds.

Ok, after some HW debug it seems XOR gate is dead :slight_smile: There is no inverted SBUS output from it, while input pins are properly configured. By overriding (with breadboarded inverter) XOR gate directly, everything start working and channels are visible. Ordered a new gate and going to replace it.Screenshot_20210217_212646