serial Nullmodem

You need a serial nullmodem for most of the Dbox modifications. And in cases where you do not need it, it is at least very helpful.

You can buy one, or make one yourself.
This is the wiring; RxD, TxD and GND are sufficient for use with the Dbox.

9pol (female)                           9pol (female)
 5 ---GND---------------------GND-------  5

 2 ---RxD--------. ,----------RxD-------  2
                  X
 3 ---TxD--------' `----------TxD-------  3

 7 ---RTS--------. ,----------RTS-------  7
                  X
 8 ---CTS--------' `----------CTS-------  8

 4 ---DTR--------. ,----------DTR-------  4
                  X
 6 ---DSR--o-----' `-------o--DSR-------  6
           |               |
 1 ---DCD--'               `--DCD-------  1

Also make sure you use the correct settings for the serial interface in the Windows device manager:

Bits per second	57600
Databits	8
Parity	none
Stopbits	1
Flow control	none