The USB [Universal
Serial Bus] spec defines the Mechanical, Electrical and Protocol layers of the
interface. Cables and connectors are fully defined. USB defines 2 types of
hardware, Hubs and Functions. Up to 127 devices may be connected together in a
tiered Star topology. The limiting factor being 7 address bits. The physical
wire segments are point-to-point between a Host, Hub, or Function. The system
may only have one Host.
The USB bus is a [Differential] Bi-directional serial interface cable bus.
Differential NRZI data is transmitted Isochronous or Asynchronous between
devices. Data is transferred at three different rates over a maximum cable
length of 4 meters ~ over 4 wires, 2 of which carry data on a balanced twisted
pair.
A Slow-Speed mode of 1.5Mbps is used for devices such as mice. Full-Speed mode
is used by most devices and allows a transfer rate of 12Mbps. High-Speed mode
[defined by USB 2.0] allows rates of 480Mbps. Transmission at the High-Speed
mode requires the addition of 45 ohm termination resistors between each data
line and ground. Operation at Full-Speed mode is 2.8 volts [High] to 0.3 volts
[Low]. Operation at High-Speed mode is at 400mV +/-10% [High] to 0V +/- 10mV
{Low]. Cable impedance for both modes is 90 ohms +/- 15% (differential).
Four different (packet) protocols are used; Control, Interrupt, Isochronous and
Bulk. Each exchange contains 3 packets; A token packet which holds the address,
a data packet which holds the data, and a handshake packet which terminate the
exchange.
NRZI produces a change in the signal indicating a logic zero, no change
indicates a logic one. Bit stuffing is used with NRZI to stop the signal
remaining in the steady state condition; if more then 6 ones are transmitted
(no change in the signal) a zero is inserted to produce a transition. NRZI,
with bit stuffing is self clocking, allowing the receiver to synchronize with
the transmitter.
Quelle: