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COPYRIGHT © 2000 CANON INC. CANOSCAN D660U REV.0 JULY 2000 PRINTED IN JAPAN (IMPRIME AU JAPON)
USB cable has A plug for connecting to upper layer and B plug for connecting to lower layer.
Figure 2-21
3. USB data transfer
USB data is transferred in the following four data structures called "packet".
* Token packet
: Used to start a data transfer
* Handshake packet
: Used to report the status of a data transfer
* Data packet
: Used to send and receive data
* Special packet
: Used for other transfer
USB device may support multiple data transfer endpoints, so there are four types of data
transfer protocols.
* Isochronous transfer : Allocates a data transfer time to a device. Highest priority is given but
no error is corrected.
* Interrupt transfer
: Periodically transfers data within a specified waiting time. Second
priority is given.
* Control transfer
: Used to configure the host computer when USB device is attached/
removed.
* Bulk transfer
: Lowest priority is given but larger amounts of data is sequentially
transferred to a free bus.
A Plug
B Plug