Using USB terminology, a device may send or receive a transaction every USB
frame (1 millisecond). A transaction may be made up of multiple packets (token,
data, handshake) but is limited in size to 8 bytes for low-speed devices and 64
bytes for high-speed devices. A transfer is one or more transactions creating a set
of data that is meaningful to the device—for example, Input, Output, and
Feature reports. In this document, a transfer is synonymous with a report.
转载于:https://www.cnblogs.com/fanzi2009/archive/2009/07/17/1525777.html