A bus is a computer component, generally a slot attached to the motherboard, that enables the flow of information between two or more devices. The local bus, also referred to as an internal bus, is defined as the particular bus that allows for connections to and communication with the motherboard from devices inside the computer. Local devices would include such items as the video card, sound card, and modem. There are multiple architectures employed for a local bus that have gradually evolved as computer technology advances. Most computers contain a number of buses to control input and output (I/O).
Legacy equipment and possibly even some contemporary computers contain an industry standard architecture (ISA) bus, which may or may not have actual slots on the motherboard. ISA was the first standard architecture and is still sometimes used for compatibility with older or slower devices, such as mice and modems. Extended industry standard architecture (EISA) buses were around briefly, but were quickly usurped by the following standards discussed.
As computer performance needs increased and the industry shifted from a character-based to a graphic-based system, new bus architectures were developed. The video electronic standards association (VESA) local bus, also known as VLB, came about in 1992. This standard increased the speed of communications and dramatically improved video performance.
Developed and introduced in 1992, the peripheral component interconnect (PCI) local bus quickly became the popular choice for internal buses. The PCI bus provided improved information transfer using a burst mode and improved performance through bus mastering. The PCI standard also facilitated high bandwidth usage. The speed of this type of local bus can be set either synchronously or asynchronously, which gives a user the ability to overclock the system to increase processes.
PCI buses are unable to handle the extreme graphics demands of the modern computing era. Accelerated graphics port (AGP) slots were intended to replace PCI slots. Some consider the AGP slot to be a port rather than a bus, as it connects only two devices, a video card and the motherboard. AGP did not hold the top slot for long, however, as peripheral component interconnect express (PCI-E) entered the ring to become the new standard.
The PCI-E local bus utilizes serial connections that are less susceptible interference. In addition, this standard allows for true bidirectional communication. These factors make PCI-E faster than its predecessors. The scalability and speed of this standard make it the principal local bus architecture for I/O control as of 2011.