To test your serial cable, loop the far end (the generator end), ensure Genmon is no longer running, then run the serialtest.py program. My example specifically tests ttyUSB0 - if you don't specify a device, the genmon default will be used.
/opt/genmon/startgenmon.sh stop
python3 /opt/genmon/OtherApps/serialtest.py /dev/ttyUSB0