pawelsky

Just for reference, if someone finds this thread in the future, the reason of the issue was not the mixup of the LPUARTx registers, but incorrect values of the LPUART3_TX_SELECT_INPUT and LPUART4_TX_SELECT_INPUT DAISY registers that my library...

You got instructions in the thread dedicated to the library, let's not spam this forum here anymore.

After another look the HW Serial inits in the core seem to be OK, but the annotations in the reference manual seems to be swapped. @PaulStoffregen this seems mixed up compared to the code

Rather than changing the core files try using Serial 2 instead of Serial 4 in your code (and vice vera) but keep the physical connections unchanged. Also use the latest version of the FrskySportTelemetry library, the one you linked to on...

Could be wrong, but after having a very quick look into Teensyduino code, I'm afraid it also has the LPUART registers swapped for Serial 2 and 4. See here for HardwareSerial2.cpp...