Digicom Tintoretto LAN Plus ISDN LAN Modem Instrukcja Użytkownika Pobierz pdf (Strona 2)

and keyboard. As before, knowledge of the Univer-

sal Serial Bus and the Linux USB driver architecture is

necessary when operating scanners, printers and

multimedia devices, but as a user of the one of the

abovementioned distributions you do at least save

yourself the kernel installation procedure described

in the box “USB installation and configuration” as

all the necessary driver modules will have been “fac-

tory compiled”.

Kernel modules

The structure and dependencies of the device dri-

vers for the Linux USB system are shown in Figure 2.

The basis for all USB drivers is the combination of

the USB core system (usbcore) with at least one host

controller driver (usb-uhci or usb-ohci). Apart from

the integrated hub driver, the core driver offers the

option of creating an information structure in the

/proc directory (“USB device file system”.) It is with

this in user mode that drivers can also access USB

devices. The usbdevfs also enables tools such as

lsusb to output a list of connected devices (analo-

gous to lspci for plug-in cards in a PCI bus).

After compilation and installation of the mo-

dules the drivers can be integrated as usual into the

current kernel using insmod:

insmod usbcore

insmod usb-uhci # or: insmod usb-ohci

You can check the success of this action with

dmesg. If no USB device is connected, at least the

host controller should signal and output the num-

ber of USB ports detected. Further hubs are also

KNOWHOW USB

50 LINUX MAGAZINE 2 · 2000

USB Principles

The “Universial Serial Bus” has a

strictly hierarchical structure and is

managed by a host controller. The

host uses a master/slave protocol to

communicate with the USB devices

that are connected: in other words all

data transfers are initiated by the

host. USB devices cannot communi-

cate with each other, which is of no

great significance to peripheral

equipment and saves on costs, as the

hardware and software need no

great intelligence. Also, this means

that problems like collision detection

or bus arbitration don't arise. At the

moment, a USB enables a maximum

of 127 devices to be connected, pro-

viding a bandwidth of 12MBit/s via

the four-pole connecting cable (+5V,

ground, data+ and data-), but this

can only be used to 90 per cent

capacity. In the case of the USB 2.0

specification recently shown for the

first time, up to 480MBit/s should be

possible. To enable the synchronisa-

tion of multimedia dataflow, such as

audio or video, USB transactions are

embedded in a frame structure. One

frame lasts exactly 1 ms (12,000 bits).

Normally, the required USB host

controller is integrated into the

motherboard. Older circuit boards

can be retrofitted with suitable PCI

cards. In spite of the multitude of

USB chip sets in existence, the manu-

facturers fortunately stick to two

standards only: the “Universal Host

Controller Interface” (UHCI) devel-

oped by Intel and the “Open Host

Controller Interface” (OHCI) from

Compaq and Microsoft. This makes no

difference for USB device drivers (rather

as in the case of SCSI host adapters.)

What are known as “class specifica-

tions” exist for frequently used USB

devices such as modems, bulk storage,

keyboards, mice, joysticks, monitors,

audio devices, printers and USB-to-IrDA

converters. Therefore, it should be the

case that as many devices as possible with

the same functions will work with the

same driver. Unfortunately, there are no

open class specifications in existence for

webcams, digital cameras, scanners and

USB/RS232 converters and a special driver

is needed as a rule. In the case of Linux

this causes familiar problems.

Although the programming models of

typical USB devices certainly don't display

any technical peculiarities some manufac-

turers are trying not to give out specifica-

tions or will do so only if a non disclosure

agreement is signed. Therefore, the

development of drivers for these devices

is difficult or even downright impossible

and at best only possible with time-con-

suming reverse engineering (with the

help of tools like “USB-Snoopy” and

“Playback” – see “Info” box.)

The functions of USB devices are logi-

cally subdivided into so-called “inter-

faces”. An interface covers all communi-

cation with a particular part of the

device. An interface can have different

operating modes, known as “Alternate

Settings”. Only one alternate setting can

be active at one time. Audio input and

output interfaces, for instance, use alter-

nate settings in order to distinguish dif-

ferent scanning formats. An interface

can include several end points. An end

point is to a certain extent comparable

with a TCP/IP port. End points are, how-

ever, unidirectional, which means that

data can only be transferred in one direc-

tion.

Fig. 1: The world turned

upside down. In reality,

the universal serial bus

has a star structure and

the hubs work physical-

ly like switches. The

drivers communicate

with one another using

the end points of the

interfaces of a device.

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