What is a “serial port tunnel“?

A serial port, accessible at the 3-pin terminal block, can be bound to a specific TCP port in the settings. After that, it is possible to open e.g. a Telnet connection to IP:Port on a PC and the characters that you type in the terminal will be relayed to the physical serial port. RS-232 baud rate and other parameters can be set in the NETIO 4C web configuration interface.