Using MAX232 to fix prolific usb to serial voltage levels
Have you tried a different usb to serial converter?  Prolific marketed
products are famous for being counterfeited with fake prolific chips
in them.


  Warning Notice: Please be warned that counterfeit (fake) PL-2303HX
   (Chip Rev A) USB to Serial Controller ICs using enter link description here trademark
   logo, br
  
  andname, and device drivers, were being sold in the China market.
  Counterfeit IC products show exactly the same outside chip markings
  but generally are of poor quality and causes Windows driver
  compatibility issues (Yellow Mark Error Code 10). We issue this
  warning to all our customers and consumers to avoid confusion and
  false purchase.