Use TinyFPGA to test.
See echo of slightly modified input.
cd ../usb_cdc/examples/TinyFPGA-BX/OSS_CAD_Suite/ tinyprog -l make PROJ=soc clean all make PROJ=soc prog tinyprog -b minicom -D /dev/ttyACM0
Attach another USB2TTL to pins 4,5 and see characters being passed between.
tinyprog -l make clean all make prog tinyprog -b minicom -D /dev/ttyACM0 minicom -D /dev/ttyUSB0