I have no trouble with my communicating program - it's fine and it got a checksum control and it runs smooth.
I cant' test the electrical signals because I don't have an oscilloscope.
Basically, 115200 works fine on either platform.
The maximum board-specific Arduino UART clock has to be quite close to one of the following (it's limited to this list because the corresponding Raspi supports just this subset > 115200):
230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000 and 4000000
the different Arduino boards are the following:
16MHz AVR (e.g., Nano, Uno or Mega2560) <=> Raspi,
Zero (M0, 48 MHz cpu) <=> Raspi,
Due (M3, 84 MHz cpu) <=> Raspi,
and esp8266 (80 MHz cpu) <=> Raspi
Which max UART clock values exactly for either Arduino board has to be chosen?