I use an insideout networks edgport 8.

This is a little box which gives you 8 serial kports from one usb.
each port may be configured as dte or dce, or as rs-422.  A very flexible 
and useful little box.
It draws its power  from the usb port, and, so far, I haven't found an 
application which can tell it from a standard hardware serial port.
Hope this helps.