A bus is an electronic path along which bits (signals) are transmitted. The different computer components communicate along buses. Buses carry data, address, and control signals. They are composed of a complex collection of electronic circuits, a number of chips, and expansion slots into which we plug expansion boards that connect input, output, and secondary storage devices. A bus speed is the number of transmission operations it can handle per second and is measured in hertz with 1 hertz = 1 cycle (operation) per second. A bus width is the number of bits it can carry. A bus with speed 8 MHz and with width 16 bits can deliver 8 x 106 x 16 = 128000000 bps (bit per second) = 128 Mbps (megabit per second).
A typical microcomputer has three data buses:
- System bus that connects the microprocessor and memory
- Expansion bus that connects low-speed devices as mice and keyboards to memory. The typical standards for this bus are the industry standard architecture (ISA) and the extended ISA (EISA). Also most computers today have a universal serial bus (USB). A USB can support up to 127 input/output (peripheral) devices. USB devices are hot-swappable meaning that they can be plugged or unplugged at any time with no need for rebooting or shutting down the computer.
- Local bus that connects high-speed devices as hard-disk drives and monitors directly to the CPU. This bus looks as an extension to the CPU's local (internal) bus and that is where its name comes from. The typical standards for this bus are the Video Electronics Standards Association local bus (VL-bus) and the peripheral component interconnect (PCI). The PCI is faster, can connect more devices, and supports the plug and play (PnP) feature. PnP gives the computer the ability to automatically detect and configure new hardware devices. See comparison. | 
