CHAPTER 2
2 - 22
COPYRIGHT
2000 CANON INC. CANOSCAN N650U/N656U/N1220U REV.0 JUNE 2000 PRINTED IN JAPAN (IMPRIME AU JAPON)
3. USB Data Transfer
USB data is transferred in the following four data structures called "packet".
1) Token packet
: Used to start a data transfer
2) Handshake packet
: Used to report the status of a data transfer
3) Data packet
: Used to send and receive data
4) Special packet
: Used for other transfer
USB device may support multiple data transfer endpoints, so there are four types of data
transfer protocols.
1) Isochronous transfer
: Allocates a data transfer time to a device. Highest priority is give
but any error is not corrected.
2) Interrupt transfer
: Periodically transfers data within a specified waiting time. Second
priority is given.
3) Control transfer
: Used to configure the host computer when USB device is attached/
removed.
4) Bulk transfer
: Lowest priority is given but larger amounts of data is sequentially
transferred to a free bus.
This scanner uses control transfer and bulk transfer.
Figure 2-23
A Plug
B Plug
USB connector has A plug for connecting to upper stream and B plug for connecting to
lower stream.