Hi RPS,
You said "I dont understand well about FTDI DLL"
I will reconmend that you go to FTDI website They have a lot of information about the Chip and the DLL and How to install it on your system.If you search this forum for how to use DLL and LabVIEW you will also find lot of information.
Hope This is Helpfull?? Good luck on your Project.
CLAD
Certified LabVIEW Architect
Certified TestStand Architect