USB basic

USB Basic
USB bus has lots of advantages in comparison to ISA and PCI bus:
Add-in slots are limited on internal PC buses while USB allows up to 127 devices.
Add-ins require opening PC, inviting new problems while USB has standard external connector.
External peripheral connectors (printer,keyboard,mouse,etc.) vary, while USB has one type of connection.
USB connector has just 4 signals - cost saving on board design and cables. For example parallel port uses 25 pins connector.
Peripheral interrupts are limited - sharing required. USB is using polling mechanizm instead.
USB has power line. Peripherals can be powered directly from USB cable in the range of 100 - 500mA.
USB supports hot auto configuration while PCI devices can only get identified during boot time.
USB supports data rates upto 12Mb/s, new standard is going to extend it to 480 Mb/s.