The bus is the pathway -- on the motherboard and beyond -- that allows data to flow from one part of your computer system to another.

The data bus is important because it's one of the three components that most affect the overall performance of any system (the others being the CPU and the amount of RAM).

Two factors determine how quickly a data bus can move information: bit-width and the speed of transmission. Bit-width refers to how many bits of data can be moved at the same time. A 32-bit bus moves, no surprise, 32 bits of data at once. Transmission speed is measured in megabits per second (or Mbps), which refers to the amount of data that can be moved in a second. The higher the bit width and transmission speed, the better a computer will perform.

Driving a real bus, of course, is only safe when there's agreement on which side of the road to use. In the same way, information moving between computer components on the data bus can only occur if a common protocol is used.

Types of data buses

As any student of computer history knows, agreeing on computer standards has been a problem. The result has been a profusion of buses still in use. The different data bus standards include:

ISA (Industry Standard Architecture). This is one of the oldest buses, dating back to the dawn of the PC. Despite being relatively slow, ISA peripherals were available for just about all uses. ISA is no longer the industry standard.

PCI (Peripheral Component Interface). PCI is the most common data bus. It is much faster than earlier buses, and it is the current industry standard. The PCI bus can be found on both Macintosh and IBM-compatible computers.

PCI boasts a feature called "Plug & Play" that many other buses do not support. This feature gives your computer the power to recognize and configure any new devices you plug into your computer, automatically.

In other words, you don't have to fool around, sometimes for hours, with switches and hard-to-find drivers. That's the theory, anyway. Plug & Play works most of the time, but it still has some bugs.

SCSI (Small Computer System Interface). This is an internal/external bus protocol.

SCSI (and SCSI-2) are high-speed buses useful for connecting a number of devices to a computer. You can use an SCSI external expansion port to link several external components together in a daisy chain that's connected to your computer. So you can plug the scanner into the hard drive, which is plugged into the printer, which is then plugged into the back of the computer.

One computer system will often support several different types of data buses.