Problems with the connection of computers to the modems or Field Controllers have been reported. There are several “cheap” USB Serial cables made in China coming recently to the market that are not compatible and wont talk to the authentic “Prolific drivers.
We recommend the use of authentic prolific cables, like PLUGABLE. More information in http://plugable.com/drivers/prolific
http://www.prolific.com.tw/US/ShowProduct.aspx?pcid=41&showlevel=0017-0037-0041
If you have updated the drivers or for a different cable this is the cause of you problem.
Type is PL2303 HXD from Plugable. 