Robin2 I am not asking for help with my Python code, as I said:
Have in mind that even if I connect the Arduino Nano on my PC using the same RS485 converters and use the Serial Monitor of the Arduino IDE I get the same result
so its not the Python code fault... This parts are just a small part of my home automation system and the code that I posted here are just the part that is connected with the issue I have, the full Python script is more than 300 lines of code and the Arduino code is more than 100 lines of code (in near future I plan to switch to pure C++ code in Atmel Studio because the Arduino code is way too heavy)... Thanks for your suggestion and thanks for trying to help.
Riva thanks for your suggestion, I will test your solution and report if it works... You are right I am switching the "rs485CtrlPin" pin for TX and RX since the Arduino Nano is the slave in my setup and MAX485 requires a control pin to be switched HIGH when in transmitting mode... The USB to RS485 does this automatically but for the Serial to RS485 converter you need to do this manually...
By the way, the USB to RS485 converter is made using CH340 (which is USB to Serial chip) and MAX485 (which is Serial to RS485 chip), I suspect that it might be a problem with the ability of the CH340 but I cant be sure until I get another Serial to RS485 converter and test this...