The bus in a PC is the common hardware interface between the CPU and peripheral devices. Parallel buses with multiple lines (wires) were superseded by serial versions. Serial buses use one line for data. Following are the various buses used in the PC.
Today there are two primary data buses in a PC: PCI Express and USB.
PCI Express - (Parallel/Serial)
PCI Express (PCIe) is the current bus interface, superseding PCI. All new desktop PCs and Macs have PCIe slots. See PCI Express
USB - (Serial)
Permanently or temporarily attach almost anything (flash drives, hard drives, printers, phones, cameras, etc.). See USB
Prior to PCI Express and USB, there were seven data buses used from time to time, as follows:
PCI - (Parallel)
PCI was popular in all hardware platforms but was superseded by PCI Express (PCIe). Transition motherboards may have one PCI slot. See PCI
FireWire - (Serial)
Mostly used for digital camera connections. Popularized by Apple, adapters were required to use FireWire on new Macs. See FireWire
AGP - (Parallel)
The graphics interface between PCI and PCI Express. AGP was faster than PCI and freed up a PCI slot. See AGP
and display adapter
ISA - (Parallel)
-suh" and debuting on the IBM PC AT, ISA was the evolution of the first PC bus in 1981. See ISA
Micro Channel - (Parallel)
IBM introduced Micro Channel with its PS/2 line in 1987, then later supported ISA and eventually gave up Micro Channel for PCI. See Micro Channel
EISA - (Parallel)
-suh," this ISA extension was created to counter Micro Channel, and EISA slots accepted ISA cards. Later abandoned for PCI. See EISA
VL-bus - (Parallel)
The VL-bus was introduced during the 486 era and offered more speed than ISA. It too gave way to PCI. See VL-bus
Types of Expansion Cards
Except for PCI Express and PCI, all the rest of these interfaces have been discontinued.