From OpenSCADAWiki
In the normal mode of the serial interface, multiple use of one and the same port in input and output transports is not allowed. A global blocking of the serial device is not performed due to the ambiguity of this process at the system level, and multiple use can lead to unpredictable and difficult-to-understand issues. If you need to organize a local serial channel with a couple of linked ports, it is recommended to use the command socat -d -d pty,raw,echo=0,perm=0666 pty,raw,echo=0,perm=0666.