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Chapter 5 Input/Output Interfaces
5-22 Compaq Deskpro EXS and Workstation 300 Personal Computers
Featuring the Intel Pentium 4 Processor
First Edition – December 2000
5.7 UNIVERSAL SERIAL BUS INTERFACE
The Universal Serial Bus (USB) interface provides asynchronous/isochronous data transfers of up
to 12 Mb/s with compatible peripherals such as keyboards, printers, or modems. This high-speed
interface supports hot-plugging of compatible devices, making possible system configuration
changes without powering down or even rebooting systems.
NOTE: It is recommended to run the Windows 98 (or later) operating system when
using USB peripherals, especially a USB keyboard and USB mouse. Problems may be
encountered when using USB devices with a system running Windows 95, although
some peripherals may operate satisfactorily.
As shown in Figure 5-7, the USB interface is provided by the 82801 ICH2 component and a USB
hub component. All models provide four series-A USB ports on the rear panel. For more
information on the USB interface refer to the following web site:
http://www.usb.org
Figure 5-6. USB I/F, Block Diagram
5.7.1 USB DATA FORMATS
The USB I/F uses non-return-to-zero inverted (NRZI) encoding for data transmissions, in which a
1 is represented by no change (between bit times) in signal level and a 0 is represented by a change
in signal level. Bit stuffing is employed prior to NRZ1 encoding so that in the event a string of 1’s
is transmitted (normally resulting in a steady signal level) a 0 is inserted after every six consecutive
1’s to ensure adequate signal transitions in the data stream.
82801
ICH2
Rear Panel
Tx/Rx Data
USB
I/F #2
USB Port C
USB Port D
Tx/Rx Data
Tx/Rx Data
USB
I/F #1
USB Port A
USB Port B
Tx/Rx Data