[GPSNavX]

You need two serial ports. This can be achieved with two keyspan adapters or one. Once the Keyspan OS X driver software is installed then each serial port will have it's own name (i.e. /dev/USA-XXX1, /dev/USA-XXX2)

You can use (2) of these..

http://www.keyspan.com/products/usb/usa19hs/

Or (1) of these..

http://www.keyspan.com/products/usb/USA49w/

The advantage of the four port model is you could drive an autopilot with the third serial port.