well as I understood it... The pcmcia serial ports are true serial ports, the usb ones are just simulating them. On most serial devices you can get away with unregulated speeds. But with audi diagnostics your laptop must dial into the bus and talk at a certain baud rate. The USB being unregulated tends to screw the whole process up. When I tried using the usb to serial dongle I could talk to like 2 modules but most wouln't work.