failure of serial communication

Hi, I have a problem about the serial communication between LabVIEW and a FTDI chip (UART to USB). I cannot read from this serial port. Even no response when I sent *IDN?\n in MAX.
 
Here is some other information:
 
I can detect the port (COM4).
I have installed the driver of FTDI chip.
I have called the DLL file.
I can write and read through another software made by the chip company.
When highlighting the data flow and setting an pin point in the output of “VISA read”, it reads zero bytes. It’s like the data I wrote did not enter into VISA.
 
 
Could you give me some advice?  

Thanks!