Bus Architectures

The bus architecture of the PC is made upof the wires, connectors, and devices that move data and instructions around the PC. The bus structure, which got its name from the fact that it resembles the lines on a city bus map, connects the controllers on the motherboard, the CPU, memory, I/O ports, and expansion slots.

The PC’s bus architecture becomes very important when you add additional device controller cards to the motherboard’s expansion slots. Most of the latest motherboard designs include expansion slots for multiple bus structures, including PCI (Peripheral Component Interconnect) and AT Bus, and possibly SCSI. Each of the bus architectures supported on a motherboard requires a bus controller chip.

While not technically a bus architecture, another interface type you will see listed as a major feature of some, especially the newer chipsets, is support for AGP (Accelerated Graphics Port). AGP is a 66MHz bus that is usually combined with a 32-bit 33MHz PCI bus to provide advanced support and faster data transfers from main memory for video and graphics adapters.

AT Bus

The AT expansion bus is included on current PC motherboards primarily for backward compatibility to expansion cards from older systems, such as network adapters. The AT bus, which runs at 8MHz and uses a 16-bit data path, is commonly referred to as ISA (Industry Standard Architecture). However, the ISA bus standard also includes the 8-bit PC XT bus, which is rarely used on any current PC.

Another bus related to the AT bus is the Extended Industry Standard Architecture, or EISA, bus. EISA bus expansion slots have been included on some motherboards since the time of the 386 processor. It is a 32-bit bus but is also backward compatible to the AT and ISA buses.

Local Bus

AT and ISA bus structures are unable to keepupwith the speeds required for high-resolution graphics and faster processors, so many manufacturers have moved to what are called local bus architectures. A local bus architecture is more directly connected to the microprocessor than nonlocal buses by communicating directly to the processor through its dedicated controller and bypassing the standard bus controller. Although they provide for faster data movement, local buses do not support many devices, which is why most motherboards also include AT or ISA expansion slots as well. The most common of the local bus architectures are the PCI and the VESA (Video Electronics Standards Association) local bus, or VL-bus. Of these two, the PCI, promoted by Intel, is becoming the de facto standard for virtually all Pentium class computers.

SCSI Bus

The Small Computer System Interface, or SCSI (pronounced “skuzzy”) is a bus architecture that attaches peripheral devices to a PC through a dedicated controller card. SCSI supports very fast data transfer and multiple devices over the same I/O bus structure. Very few PCs, outside of the Macintosh, feature a SCSI interface as a standard, and if this bus is desired, it must be added to the PC through an expansion slot, typically a PCI slot.

USB

The Universal Serial Bus, or USB, is an emerging standard for device connectors and interface. USB is a plug-and-play architecture that allows users to add a wide range of peripheral devices to the PC without the need of an expansion card. It is considered a low-speed interface and works best for a keyboard, mouse, scanner, or printer.