Because USB is a complicated serial device, it uses USB communication protocol to switch messages with a host (PC),which includes version , vendor ,and operation modes (stream or bulk) information. The OS of a host makes use of these to search for a new driver for the USB device. All of these require the host can detect any new USB devices inserted automatically.So,when an USB connects into the USB host controllers or USB host hubs, the host will find that the D+ signal of controllers or hubs is high and protocol acts on. Finally, one word is to need the printer.