I use this one.  Transparent to the PC software....
     http://www.usdigital.com/products/ad2a/


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     (2)  Somebody on the list  recently indicated that it was  easy to convert
     from RS232 to RS485, I have no problem at the PIC level but without
     purchasing an expensive RS485 Card for my PC which is  intended  to be the
     master controller in a multi-drop system I am having difficulty in enabling
     the RS485 driver from a standard PC RS232 port while transmitting  and then
     disabling the driver and still be able to listen  for a reply on the PC
     port. Thanks in anticipation Pat Hanson