Hello @gbrunner Note: I don't have the UHF RFID unit, so my conclusion could be incorrect. My guess is the error stems from the GPIO conflict regarding GPIO16 and GPIO17. In the M5Stack Fire those two GPIOs are already used internally for the PSRAM. Note from M5Stack Fire documentation´╝Ü GPIO 16 / 17 of FIRE are connected to PSRAM by default, so when you connect or stack other modules, you need to pay attention to avoid conflicts with these two pins to prevent the device from not working properly and generating instability.. Unfortunately in the early days (before M5Stack Fire existed) M5Stack engineers have chosen GPIO16 and GPIO17 to be used for the UART port C (blue). In other words port C (blue) is inoperable in this case I am afraid. M5Stack remedied this situation in some modules by using dip switches to allow for different GPIOs being used for UART modules, but I don't see dip switches in the M5GO base. I guess you could try a different port (and adapt the program), use an M5Stack Base (where GPIO16 and GPIO17 are not blocked by PSRAM) or modify the M5GO to use different GPIOs for UART. Thanks Felix