Menhwa,
How are you going about calling the dll? Below is an example of a way to define the the open unit and close unit calls for the PS5000 just to give you an idea on how to implement the dll.:
private const string _DRIVER_FILENAME = "ps5000.dll";
[DllImport(_DRIVER_FILENAME, EntryPoint = "ps5000OpenUnit")]
public static extern short OpenUnit(out short handle);
[DllImport(_DRIVER_FILENAME, EntryPoint = "ps5000CloseUnit")]
public static extern short CloseUnit(short handle);
Regards,
Richard Boyd
Crag Technologies, Inc
http://www.pc-oscilloscopes.com