Hi Istvan,
The serial port is generally used for doing Serial to IP communication — it can pass data between the serial port and a TCP/UDP socket that you configure.
To access the radio from a PC, there is a feature labeled “Telnet Radio Access”. If you turn this on, you can make a telnet/socket connection to the MTR and it will allow you to communicate directly with the radio.
If you do need to use the serial port to access the radio, you can combine the two features and configure the Serial-IP feature to connect to the IP and port for the Telnet Radio Access feature.
Jesse