I wanted to read some data from an external microcontroller via the UART, but couldn't get any communication working.

After checking the RXD and TXD pins, I saw that there is also a signal on the RXD-pin of the Raspberrypi (actually its the same signal as on TXD / same level).

Screenshot of Oscilloscope connected to pin 8 (yellow) and pin 10 (blue) of the GPIO: http://i.imagebanana.com/img/oy96inb9/SCR04.PNG

Any Ideas why there is a signal on the RXD pin of the RPi? Or is my RPi just broken?

  • I have a question regarding reasons above: 1) What is the expected resistance between GRD and UART TXD (pin 8), GND and RXD (pin 10), GRD and GPIO 12? assuming my Rpi is in Power off mode 2) How to verify which pins is UART configured in my Rpi system? I cannot receive any data on UART RXD and cannot figure out the reason!
    – user10649
    Nov 18, 2013 at 0:14

1 Answer 1


Possible reasons:

  1. an electric connection (short-circuit) between the pins. -> check the board for damage. You can measure the resistance between the pins in Power off mode.

  2. A miss configuration of the Pins. SoC's like the CPU on the Pi can be configured to have the UART on different pins. -> check the Software(Operating System) that you use on your Pi. Test if the effect also occurs with the standard/ different Image.

  3. there is a fault ion the chip that causes a short-circuit or misconfiguration. -> use different Pi board, as this one is broken.

  • Thank you for your answer. I have tested the things you suggested and looks like I have a fault on (in) the PCB or in the chip. I can measure a resistance of 2Ω between the pins...
    – rogger
    Sep 1, 2013 at 15:23

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