Screen keep Flashing and getting slower while connecting FCU telem2 port to TX2 TX/RX ports.

The monitor screen keeps flashing, and it gets slower, and sometimes even dead while I connect the telem2 port of FCU/FX4 to the J21 TX, RX, power and ground (Pin 4, 6,7, 8, 10). The connection is correct I believe. When I pull the wires out from J21 pins, the GPU and screen then runs very well. So is there anything wrong with FX4, or register memory of TX/RX ports. or power, or GPU memory, or wire connection problems? I am so confusing. Anyone who has some idea?

And the board cannot boot up while connecting the RX/TX to FCU. I could only boot first and then connect those wires. I am not sure whether it is normal.

FYI, I know nothing of “telem2 port of FCU/FX4”.

For reference on J21:

  • Pin 4: 5V DC Out
  • Pin 6: GND
  • Pin 7: AUDIO_MCLK
  • Pin 8: Serial Console UART TX
  • Pin 10: Serial Console UART RX

So far as serial console operation goes, any ground plus pins 8 and 10 provide the minimal console. This is a 3.3V TTY level interface. Are you using pins 8 and 10 for serial console? If not, then you will have very odd operation.

If voltages other than 3.3V are used on TX/RX, consider lower voltage signals will simply fail. Higher voltage signals will probably cause damage.

If RX receives data at the correct 115200 8N1 setting, then everything it sees is being piped directly in to the command line console for user “nvidia”. Everything coming from TX is a result of the nvidia shell’s output. If the settings are incorrect, then binary gibberish is being sent to the console as user nvidia.

Unless you’ve set it up, I doubt anything would come from the audio master clock, so if you are using this as a clock source you should double check to see if there is an actual expected waveform.

If you are powering something from the 5V rail, and if the power consumption is too high, I could see the possibility of the system either becoming unstable or powering down.

I did look up an image of a telem2 port, looks like a pixhaw connector. I don’t know what voltage level it is set for, but it does look like a serial UART since it has RX, TX, RTS, and CTS. If this is the case, then you shouldn’t be touching the AUDIO_MCLK.

Unless such a port is requesting to receive power, and is not supplying power, then the 5V should not be connected. If the 5V of the telem2 port requires 5V power, then you might check if the current available on J21 pin 4 is sufficient to supply that power.

You found RX and TX of the serial console. I would guess that your port is not expecting a serial console, and is instead probably expecting a regular serial UART for its own communications. In that case you would be advised to instead use J17 since this is free to do anything you want without removing other software. J17 corresponds to “/dev/ttyTHS2”.

The picture I saw of telem2 was this:
…even if this is a different device than yours I would suspect the wiring is the same. I would start by removing anything connected to the 5V if and only if the telem2 is not using remote power through this (versus providing power). I would completely cut the AUDIO_MCLK wire and not use it. You can use CTS and RTS if desired, but that is usually optional. You can connect CTS/RTS without issue even if you don’t use it, and it will simply be used if software enables CTS/RTS flow control.

Do you have a program needing to talk to the TX/RX? If so, go to J17 instead. If speed is anywhere up to 115200, then it will be ok without much effort. For higher speeds there may be other recommendations.