<Technical explanation>
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.
1 frame (Approx.1ms)Time passage
Frame lineSOFSOF
Frame insideSOFPrinterOther functions
Packet lineTokenDataHandshake`
Packet inside
Frame
SYNCNunberCRCEOPSYNCPID ADDR ENDP CRC EOPSYNC PIDDATACRC EOPSYNC PID EOP
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 encoding
with NRZI.
0110101000100 110
Data
Idle
NRZI
Idle
5 |
ACEC
