Usb Data Transfer - Canon FAX-L360 Series Service Reference Manual

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b) USB interface

USB is a serial interface which connects up to 127 peripheral devices to a host computer, and transmits data at a high-

speed rate of 12Mbps. Hot plugging, in which connecting/disconnecting devices while the host or the printer is in use, is

supported. Each device is connected to a hub's port, where each port's detection/disconnection status is returned to the

host.

Data transfer

The data transfer in USB is executed in terms of the transfer unit called a frame, a time frame of approximately 1ms, into

which the data is divided. Data is transferred by piling up these frames.

All packets begin with a SYNC (synchronizing) field to synchronize with the local clock, and are separated with an

EOP (End of Packet) field.

Frame lines begin with an SOF (Start of Frame) packet. An SOF is composed of a PID (Packet Identification Field) that

represents the type of the packet and the direction, frame number, and a CRC (Cyclic Redundancy Check) used for

error-check.

Inside a frame is a packet line containing a token packet, data packet and a handshake packet, which indicates the status

of the flow control.

A token packet is composed of a PID, an address field which can specify up to 128 addresses, an ENDP (endpoint) field,

and a CRC.

Inside a data packet are a PID, data field, CRC, and EOP.

Only a PID is present inside the handshake packet.

Frame Line

Frame Structure

Packet Structure

Frame

SYNC

Number CRC

Encoding/Decoding the Data

In USB, data transfer lines are ultimately encoded with NRZI (Non Return to Zero Invert) method. When the original

data bit is 0, sent data bits are inverted; when the original data bit is 1, the value is retained.

However, if the level of the transferred data remain unchanged for a certain period of time, the receiving side may not be

able to synchronize with the data sample position, which will result in data bits being out of phase. This is prevented by

a method called bit stuffing; when data bit 1 is repeated 6 times, one 0 bit is added to the original data before encoded

with NRZI.

Data

Idle

NRZI

Idle

1Frame (approx. 1ms)

SOF

Printer

Packet Line

Token

EOP

SYNC

PID ADDR ENDP CRC