D2 Network:jtag serial
Once disassembled you can see the JTAG 8 pin connector, pin 1 is toward the hard disk, pin 8 is toward the power switch.
This is a 3.3 volt serial port, to connect it to a standard one we need an adapter, like those based on the MAX232 chip.
The U-Boot boot loader will write to the serial port at 115200,8,N,1, so the kernel during bootstrap. Once started the system will spawn a getty on the same serial port, with the same baud rate. On the original system, only root can login, but we don't know its password.