arduino~ish usb-serial lib for the ch32v203 (only tested on 203k8) modified the files from WCH found here: https://github.com/openwch/ch32v20x/tree/main/EVT/EXAM/USB/USBD/SimulateCDC removed the UART references, addes some read/write functions. modifications made to usb_endp.c, hw_config.c, usb_prop.c. Not tested on mounriver, but might work if the files are placed in the right folders.