If you are using a VID and PID which is different from the default FTDI ones (VID = 0x0403 and PID = 0x6001 and 0x6010), you must edit the FTDIPORT.INF file using a text editor such as NotePad. You will need to add an extra line into the [FtdiHw] section containing the new VID and PID. You should also add a description of your product into the [Strings] section as shown.
Do not change any other parts of the supplied INF files unless you are an expert and capable of debugging the results for yourself.
The following example shows a new device with a VID of 0x1234 and a PID of 0x5678 which we shall call "My New USB Device". In this example, the additional lines are shown in red to highlight them. If the device you are adding is an FT2232C or FT2232D, an entry must be created for each of the ports.
DriversDisk="FTDI USB Drivers Disk"
PortsClassName = "Ports (COM & LPT)"
VID_0403&PID_8372.DeviceDesc="USB Serial Port"
VID_0403&PID_6001.DeviceDesc="USB Serial Port"
VID_1234&PID_5678.DeviceDesc="My New USB Device"
FtdiPort.SvcDesc="USB Serial Port Driver"
SerEnum.SvcDesc="Serenum Filter Driver"