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:
|S13||Debug port TXD|
|S14||Debug port RXD|
This are 3.3 volt level lines so an adapter is required to access them using a PC USB or RS232 serial port and have access to this system console using a terminal emulator.
This scheme shows how to wire the Acme Systems DPI interface but any other 3.3 volt to USB os RS232 serial converter can be used.