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The PCI bus is fine for attaching high-speed peripherals to a computer, but it is far too expensive to have a PCI interface for each low-speed I/O device was connected to the computer in a special way, with some free ISA and PCI slots for adding new devices. Unfortunately, this scheme has been fraught with problems from the beginning. For example, each new I/O device often responsible for setting switches and jumpers on the card and making sure the settings do not conflict with other cards. Then the user must open up the case, carefully insert the card, close the case ,and reboot the computer . For many user ,this process is difficult and error prone. In addition, the number of ISA and PCI slots is very limited (two and three typically). Plug’n Play cards eliminate the jumper settings, but the user still has to open the computer to insert the card and bus slots are still limited.
To deal with this problem, in the mid 1990s,representatives from seven companies (Compaq, DEC, IBM, Intel, Microsoft, NEC, and Northern Telecom) got togethet to design a better way to attach low-speed I/O devices to a computer. Since then, hundreds of other companies have joined them. The resulting standard is called USB and it is being widely implemented in personal computers.
Some of the goals of the companies that originally conceived of the USB and started the project were as follows:
1. Users must not have to set switches or jumpers on boards or devices.
2. Users must not have to open the case to install new I/O devices.
3. There should be only one kind of cable.
4. I/O devices should get their power from the cable.
5. Up to 127 devices should be attachable to a single computer.
6. The system should support real-time devices (e.g., sound, telephone.).
7. Devices should be installable while the computer is running.
8. No reboot should be needed after installing a new device.
9. The new bus and its I/O devices should be inexpensive to manufacture.
USB meets all these goals. It is designed for low-speed devices such as keyboards, mice, still cameras, snapshot scanners, digital telephones, and so on. The total USB bandwidth is 1.5 MB/sec, which is enough for a substantial number of these devices. This low limit was chosen to keep the cost down.