Unable to Acces GSM modem via usb to Serial cable

I am using GSM modem for my collage project. In my project I try to access that GSM modem by using USB-Serial cable. My Os is Windows 7 (32 Bit) that ask the prolific usb to serial adapter Driver need to install but I install so many drivers, but I am not able to access my GSM modem via X-CTU (Application that I used to access my modem by AT-commands). My modem will return the text which I gave through AT-commands.

Eg; >AT (Command by me)

 开发者_如何学运维;     >OK (Modem’s reply)

X-CTU app show that “prolific usb to serial adapter(Com9)” but I used to click ‘Test/Query’ button it will pop-up the window that shows “Unable to Connect.” Please help me...

I hope you've not connected the RS232 interface (using +/-12V to signal) of the USB-to-serial adaptor directly to the logic-level interface of the GSM modem (using 0 and 5V or even 0 and 3.3V). That might have broken it...

Assuming you have the right voltage levels, can you use a simpler terminal (like Teraterm) to establish a connection, to at least narrow down where the problem might be?

Can you get an oscilloscope and put a probe on the Tx and Rx lines of the serial port?