thebigg
Senior Member

Yes. I needed to connect every GND pin on the 40 pin connector to Teensy GND to get reliable comms

Try reducing the SPI speed, perhaps the 10" long wires are too long.

Does it work if you add lib_ldf_mode = chain+ to your platformio.ini?

It seems your rx_arr is too short and overflows. There are more than 90 characters in the last line printed in the serial monitor

Did you try just running teensy_serialmon from a cmd prompt?

See this link for details on setting build optimizations in PlatformIO

If you scroll a little further down that same page you will find this Teensy 4.1 manufactured after March 2022 has TLV75733P (U4) replaced by NCV8186AMN330TAG, and BAT54C (D1,D2) replaced by BAS40-05V. and Teensy 4.1...