USB标准全(图)

HCD

The hardware that contains the host controller and the root hub has an

interface toward the programmer which is called Host Controller Device (HCD)

and is defined by the hardware implementer. In practice, these are hardware

registers (ports) in the computer.

At version 1.0 and 1.1 there were two competing HCD implementations.

Compaq's Open Host Controller Interface (OHCI) was adopted as the standard

by the USB-IF. However, Intel subsequently created a specification they called

the Universal Host Controller Interface (UHCI) and insisted other implementers

pay to license and implement UHCI. VIA Technologies licensed the UHCI

standard from Intel; all other chipset implementers use OHCI. The main

difference between OHCI and UHCI is the fact that UHCI is more

software-driven than OHCI is, making UHCI slightly more processor- intensive

but cheaper to implement (excluding the license fees). The dueling

implementations forced operating system vendors and hardware vendors to

develop and test on both implementations which increased cost. During the

design phase of USB 2.0 the USB-IF insisted on only one implementation. The

USB 2.0 HCD implementation is called the Extended Host Controller Interface

(EHCI). Only EHCI can support high-speed transfers. Each EHCI controller

contains four virtual HCD implementations to support Full Speed and Low

Speed devices. The virtual HCD on Intel and Via EHCI controllers are UHCI.

All other vendors use virtual OHCI controllers.

某些版本的

Microsoft W indows

上，

打开设备管理器，

如 果设备说明中是否有

―

增

强

（

），就能够确认它是

2.0

版的。而在

Linux

系统中，命令

lspci

能够列出所有的

PCI

设备，而

USB

会分别命名为

OHCI

、

UHCI< /p>

或者

EHCI

。

设备分类

Devices that attach to the bus can be full-custom devices requiring a

full- custom device driver to be used, or may belong to a device class. These

classes define an expected behaviour in terms of device and interface

descriptors so that the same device driver may be used for any device that

claims to be a member of a certain class. An operating system is supposed to

implement all device classes so as to provide generic drivers for any USB

device. The most used device classes are:

USB

人接口设备类键盘、鼠标等

.

?

USB

大容量存储设备类用来做

U

盘

,

便携式硬盘

,

多媒体读卡器

,

数码相

机

, MP3

播放器等

.

此设备类都把设备当一个文件系统对待

.

?

USB

通信设备类

(

用于调制解调器

(

及

winmodems),

网卡

(

及交

叉电缆

), ISDN

连接

, Fax

?

USB

打印设备类打印机一类的设备

?

USB

音频设备类

,

声卡一类的设备

?

USB

视频设备类

, webcam< /p>

之类的设备

,

可以捕捉动态图像

?

设备的分类取决于

USB

构建论坛的设备工作组。< /p>

USB

连接器

The connectors which the USB committee specified were designed to support

a number of USB's underlying goals, and to reflect lessons learned from the

varied menagerie of connectors then in service. In particular:

?

?

?

?

?

?

The connectors are designed to be robust. Many previous connector

designs were fragile, with pins or other delicate components prone to

bending or breaking, even with the application of only very modest force.

The electrical contacts in a USB connector are protected by an adjacent

plastic tongue, and the entire connecting assembly is further protected

by an enclosing metal sheath. As a result USB connectors can safely be

handled, inserted, and removed, even by a small child. The encasing

sheath and the tough moulded plug body mean that a connector can be

dropped, stepped upon, even crushed or struck, all without damage; a

considerable degree of force is needed to significantly damage a USB

connector.

不可能将< /p>

USB

连接器插错，连接器不能上下插反，从外观和

kinesthetic

sensation

可以 非常清楚地进行连接

when the plug and socket are

correctly mated.

连接器的生产非常的便宜

The connectors enforce the directed topology of a USB network. USB

does not support cyclical networks, so the connectors from incompatible

USB devices are themselves incompatible. Unlike other

communications systems (e.g. RJ-45 cabling) gender-changers are

never used, making it difficult to create a cyclic USB network.

A moderate insertion/removal force is specified. USB cables and small

USB devices are held in place by the gripping force from the receptacle

(without the need for the screws, clips, or thumbturns other connectors

require). The force needed to make or break a connection is modest,

allowing connections to be made in awkward circumstances or by those

with motor disabilities.

The connector construction always ensures that the external sheath on

the plug contacts with its counterpart in the receptacle before the four

connectors within are connected. This sheath is typically connected to