more than 2 usb-serial adapters

i'm running parallels 4, under mac os 10.5.6.

i have 4 usb to serial adapters. i use a usb hub. only 2 work at a time. no matter what comination i use, only 2 work. when i plug in a 3rd, parallels puts up an error message saying in can't install the usb controller.



as i rotate combinations of these 4, 2 at a time, visa finds them and my apps use them just fine. native vista runs all 4.

how do i get > 2 recognized?