A colleague of mine has the B+B converter but it is the version with TTL high level of 5V and uses the power supply from the PC (via serial port). The USB TTL converter in the image above has a TTL high level of 3.3V for TXC and RXC using the USB power supply and costs 4,5 €. I will try with two AA batteries but, in case of bad result, which one of the solution above has to be preferred? Thank you for your support!