как обозначается порт USB?

Зависит от устройства! # ls /dev/usb/ dabusb0 dabusb2 dc2xx0 dc2xx2 lp0 lp2 mdc8000 mdc8002 rio500 scanner15 scanner9 ttyUSB15 ttyUSB9 dabusb1 dabusb3 dc2xx1 dc2xx3 lp1 lp3 mdc8001 mdc8003 scanner0 scanner2 ttyUSB0 ttyUSB2 dabusb10 dabusb4 dc2xx10 dc2xx4 lp10 lp4 mdc80010 mdc8004 scanner1 scanner3 ttyUSB1 ttyUSB3 dabusb11 dabusb5 dc2xx11 dc2xx5 lp11 lp5 mdc80011 mdc8005 scanner10 scanner4 ttyUSB10 ttyUSB4 dabusb12 dabusb6 dc2xx12 dc2xx6 lp12 lp6 mdc80012 mdc8006 scanner11 scanner5 ttyUSB11 ttyUSB5 dabusb13 dabusb7 dc2xx13 dc2xx7 lp13 lp7 mdc80013 mdc8007 scanner12 scanner6 ttyUSB12 ttyUSB6 dabusb14 dabusb8 dc2xx14 dc2xx8 lp14 lp8 mdc80014 mdc8008 scanner13 scanner7 ttyUSB13 ttyUSB7 dabusb15 dabusb9 dc2xx15 dc2xx9 lp15 lp9 mdc80015 mdc8009 scanner14 scanner8 ttyUSB14 ttyUSB8