Here you have Modbus RTU working on CH32V307VCT6.
Connect an USB-TTL adapter to A9/A10 and use RMMS (RADZIO MODBUS) 19200, 8N1 to test.
More info in the GitHub:
https://github.com/NedelcuBogdanSebastian/CH32V307ModbusRTU
If someone would want to help me port also Modbus TCP will be very usefull!
臉/笑脸