I had the same problem: I have a serial GW on the PI that outputs data on /tty/USB20, and is used by Domoticz.
I want to test MQTT via node-red, so I need to read this serial port from node-red.
sudo socat PTY,link=/dev/ttyS81,mode=666,group=dialout,raw PTY,link=/dev/ttyUSB20,mode=666,group=dialout,raw
But nothing appears on ttyS81: I tried with
sudo screen /dev/ttyS81 115200
and of course on node-red, I set up a node that reads from ttyS81 and outputs to debug: the serial port is opened successfully, but nothing appears on debug.
If I use
sudo screen /dev/ttyUSB20 115200
I can see some data coming
I'm no expert with socat (I learned its existence via this forum post), so any help is appreciated.