There are 2 serial ports,which are separately located on 2 expansion sockets (P16, P17) ,on BiBoard.
The serial port 1 on the P16 can be connected to the USB downloader and the external serial device. Please do not use the downloader and the external serial device at the same time. The serial port voltage division will lead to communication errors.
In the Arduino demo, Serial represents the serial port 0, Serial1 represents the serial port 1.Serial and Serial1 send to each other.