Home : USB : Physical interface
A cable connnects:
The cable contains four wires:
Data is transmitted asynchronously, using differential signalling, at either 12 Mbaud (Full Speed) or 1.5 Mbaud (Low Speed). States:
The data states depend upon the connection speed:
When nothing is plugged into the hub connector, the data lines float to the SE0 state.
When a device is plugged into the hub connector, and the data lines are idle (all drivers high impedance), they float to the 'J' state.
SOP (Start Of Packet) is indicated by a transition to the 'K' state. A packet always starts with a SYNC sequence, of KJKJKJKK (equivalent to a data byte of 80 hex).
Each data byte is transmitted least significant bit first. The bit stream is subjected to bit stuffing, where a '0' bit is inserted after every six consecutive '1' bits. The bit stuffed bit stream is then transmitted using NRZI (Non Return to Zero Inverted) encoding, where:
EOP (End Of Packet) is indicated by two bit periods at the SE0 state, followed by one bit at the 'J' state.
A reset is indicated by an extended period at the SE0 state; a hub must maintain this state for at least 10ms to ensure the device sees the reset.
A suspend is indicated by more then 3.0ms at the idle state.
A resume is indicated by any data traffic.