Physical interface

Electrical specification

A cable connnects:

A "master" device (a hub), using an "A" type connector.
A "slave" device, using a "B" type connector. The device may be another hub, allowing a tree structure of connected devices, which are all controlled by the root hub.

The cable contains four wires:

VBUS and GND (+5V power from the hub to the device, up to 500mA).
D+ and D– (bidirectional half-duplex data; in Full Speed cables the wires are shielded, and form a twisted pair, with 90ohm impedance).

Data is transmitted asynchronously, using differential signalling, at either 12 Mbaud (Full Speed) or 1.5 Mbaud (Low Speed). States:

Differential '1': D+ at nominal +5V, D– at nominal 0V.
Differential '0': D+ at nominal 0V, D– at nominal +5V.
SE0 (Single Ended 0): both D+ and D– at nominal 0V.

Signalling

The data states depend upon the connection speed:

Full Speed: 'J' state = logic 1, 'K' state = logic 0.
Low Speed: 'J' state = logic 0, 'K' state = logic 1.

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:

'1' is transmitted as no state change.
'0' is transmitted as a state change ('J' / 'K').

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.