Bus types

bus

In a computer or on a network, a bus is a transmission path on which signal are dropped off or picked up at every device attached to the line. Only devices addressed by the signals pay attention to them; the others discard the signals. According to Winn L. Rosch, the term derives from its similarity to autobuses that stop at every town or block to drop off or take on riders

In general, the term is used in two somewhat different contexts:

(1) A bus is a network topology or circuit arrangement in which all devices are attached to a line directly and all signals pass through each of the devices. Each device has a unique identity and can recognize those signals intended for it.

(2) In a computer, a bus is the data path on the computer's motherboard that interconnects the microprocessor with attachments to the motherboard in expansion slots (such as hard disk drives, CD-ROM drives, and graphics adapters).

VESA Local Bus

VESA Local Bus (sometimes called the VESA VL bus) is a standard interface between your computer and its expansion slot that provides faster data flow between the devices controlled by the expansion cards and your computer's microprocessor. A "local bus" is a physical path on which data flows at almost the speed of the microprocessor, increasing total system performance. VESA Local Bus is particularly effective in systems with advanced video cards and supports 32-bit data flow at 50 MHz. A VESA Local Bus is implemented by adding a supplemental slot and card that aligns with and augments an Industry Standard Architecture expansion card. (ISA is the most common expansion slot in today's computers).

Industry Standard Architecture

ISA is a standard bus (computer interconnection) architecture that is associated with the IBM AT motherboard. It allows 16 bits at a time to flow between the motherboard circuitry and an expansion slot card and its associated device(s).

Also see Extended Industry Standard Architecture and Micro Channel Architecture.

Extended Industry Standard Architecture

EISA is a standard bus (computer interconnection) architecture that extends the Industry Standard Architecture standard to a 32-bit interface. It was developed in part as an open alternative to the proprietary Micro Channel Architecture (Micro Channel Architecture) that IBM introduced in its PS/2 computers.

EISA data transfer can reach a peak of 33 megabytes per second.

Also see Industry Standard Architecture, Micro Channel Architecture, VESA Local Bus, and  Peripheral Component Interconnect.

Peripheral Component Interconnect

PCI is an interconnection system between a microprocessor and attached devices in which expansion slot are spaced closely for high speed operation. Using PCI, a computer can support both new PCI cards while continuing to support Industry Standard Architecture expansion cards, currently the most common kind of expansion card. Designed by Intel, the original PCI was similar to the VESA Local Bus. However, PCI 2.0 is no longer a local bus and is designed to be independent of microprocessor design. PCI is designed to be synchronized with the clock speed of the microprocessor, in the range of 20 to 33 MHz.

PCI is now installed on most new desktop computers, not only those based on Intel's Pentium processor but also those based on the PowerPC. PCI transmits 32 bits at a time in a 124-pin connection (the extra pins are for power supply and grounding) and 64 bits in a 188-pin connection in an expanded implementation. PCI uses all active paths to transmit both address and data signals, sending the address on one clock cycle and data on the next. burst data can be sent starting with an address on the first cycle and a sequence of data transmissions on a certain number of successive cycles.

Also see Extended Industry Standard Architecture and Micro Channel Architecture.

Micro Channel Architecture

Developed by IBM for its line of PS/2 desktop computers, Micro Channel Architecture is an interface between a computer (or multiple computers) and its expansion cards and their associated devices. MCA was a distinct break from previous bus architectures such as Industry Standard Architecture. The pin connections in MCA are smaller than other bus interfaces. For this and other reasons, MCA does not support other bus architectures. Although MCA offers a number of improvements over other bus architectures, its proprietary, nonstandard aspects did not encourage other manufacturers to adopt it. It has influenced other bus designs and it is still in use in PS/2s and in some minicomputer systems.

Also see Extended Industry Standard Architecture and Peripheral Component Interconnect.

SCSI

Abbreviation of Small Computer System Interface. Pronounced "scuzzy," SCSI is a parallel interface standard used by Apple Macintosh computers, PCs, and many UNIX systems for attaching peripheral devices to computers. Nearly all Apple Macintosh computers, excluding only the earliest Macs and the recent iMac, come with a SCSI port for attaching devices such as disk drives and printers.

SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.

Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.

While SCSI has been the standard interface for Macintoshes, the iMac comes with IDE, a less expensive interface, in which the controller is integrated into the disk or CD-ROM drive. Other interfaces supported by PCs include enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end new PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.