Some software applications require asynchronous vendor commands to be enabled, such as Siemens Step 7 software.
An entry in the registry called 'ConfigData' has to be modified to enable asynchronous vendor commands. It can be found under the 'Enum\FTDIBUS' branch. There is an entry for each device.
For Windows XP, it looks like this:
where VID is the VID used, PID is the PID used and Serial_Number is the device serial number.
The registry key should look like
01 00 3f 3f .....
To enable asynchronous vendor commands, change the first byte to be 05 so it looks like
05 00 3f 3f ......
This change can be made in the *.INF files for future installations by searching for ConfigData and making the change to the first byte. You will need to uninstall and re-install for it to take effect.