Robin2:
Can you send alphabetic characters for testing? It will make debugging easier.
Yes, I can. I have now sent a buffer with all uppercase letters {65,...90} and the result was this:
Ready to receive data
113 195 255 127 255 255 111 234 75 234 223 255 255 239 127 216 213 242 255 223 255 255 95 126 123 122 255
69 123 127 247 127 187 255 222 95 123 127 255 255 127 95 83 245 246 251 254 223 118 223 248 221 255 255
Robin2:
Have you a GND connection to the sending device?
No, grounds are not connected.
Robin2:
Is the sending device using TTL voltages or RS232 voltages which are not compatible with an Arduino?
sending device is using 3V3 TTL voltage, not RS232.
Robin2:
Post a link to the datasheet for the sending device?