The debug port is a special serial port used by the RomBOOT firmware burned inside the CPU ROM, the boot loader and the Linux Kernel loaded from the SD memory card to send the systems messages during their execution. The debug port is used also to have a low level access to the Linux system console.

The asyncronous serial signals are available on these pins:

Pin # CPU Pin Primary Description --- Pin define on USB Seraial Cable   1 VDD5V   5V0 power in/out <---  Red line   2 PA9 DRXD Debug serial port <--- Blue Line   3 PA10 DTXD Debug serial port ---> White Line   4 GND   Ground line <--- Black line

The VDD5V is the power in for the board, so you can use the USB serial cable to connect the board for debug and power supply the board.