Bus ABCs
ISA
MCA
EISA
VLB
PCI
AGP
USB
Mini PCI
PCI-X
AMR
CNR
Bus specifications
Troubleshooting

BUS ABCs

A computer bus is a method of transmitting data from one part of the computer to another part of the computer. The computer bus will connect all devices to the computer CPU and main memory. The computer bus consists of two parts, the address bus and a data bus. The data bus transfers actual data, whereas the address bus transfers information about where the data should go.

This page contains a basic overview of each of the computer buses as well as related links to each bus.

ISA

Introduced by IBM, ISA or Industry Standard Architecture was originally an 8-bit bus that was later expanded to a 16-bit bus in 1984. When this bus was originally released it was a proprietary bus, which allowed only IBM to create peripherals and the actual interface. However, in the early 1980's other manufacturers were creating the bus.

In 1993, Intel and Microsoft introduced a PnP ISA bus that allowed the computer to automatically detect and setup computer ISA peripherals, such as a modem or sound card. Using the PnP technology, an end-user would have the capability of connecting a device and not having to configure the device using jumpers or dipswitches.

To determine if an ISA card is an 8-bit or 16-bit card, physically look at the card. You will notice that the first portion of the slot closest to the back of the card is used if the card is an 8-bit card. However, if both sections of the card are being utilized, the card is a 16-bit card.

Many manufacturers are trying to eliminate the usage of the ISA slots; however, for backwards compatibility you may find 1 or 2 ISA slots with additional PCI slots, AGP slots, etc. However, today you may also have a motherboard that has no ISA slots. We highly recommend, when purchasing any new internal expansion card, that you stay away from ISA as it has, for the most part, disappeared.

MCA

Short for Micro Channel Architecture, MCA was introduced by IBM in 1987. MCA, or the Micro Channel bus, was a competition for ISA bus. The MCA bus offered several additional features over the ISA such as a 32-bit bus, automatically configure cards (similar to what Plug and Play is today), and bus mastering for greater efficiency.

One of the major downfalls of the MCA bus was it being a proprietary bus and because of competing bus designs. The MCA bus never became widely used and has since been fazed out of the desktop computers.

EISA

Short for Extended Industry Standard Architecture, EISA was announced September of 1988. EISA is a computer bus designed by 9 competitors to compete with IBM's MCA bus. These competitors were AST Research, Compaq, Epson, Hewlett Packard, NEC, Olivetti, Tandy, WYSE, and Zenith Data Systems.

The EISA bus provided 32-bit slots at  an 8.33 MHz cycle rate for the use with 386DX or higher processors. In addition, the EISA can accommodate a 16-bit ISA card in the first row.

Unfortunately, while the EISA bus is backwards compatible and is not a proprietary bus, the EISA bus never became widely used and is no longer found in computers today.

The VESA (Video Electronics Standards Association) is a nonprofit organization founded by NEC. The VLB, or VESA Local Bus, 1.0 was released in 1992. The VLB is a 32-bit bus that had direct access to the system memory at the speed of the processor, commonly the 486 CPU (33 / 40 MHz). VLB 2.0 was later released in 1994 and had a 64-bit bus and a bus speed of 50 MHz. Unfortunately, because the VLB heavily relied on the 486 processor, when the Pentium Processor arose in the Market place, manufacturers began switching to PCI.

PCI

Introduced by Intel in 1992, revised in 1993 to version 2.0, and later revised in 1995 to PCI 2.1. PCI is short for Peripheral Component Interconnect and is a 32-bit computer bus that is also available as a 64-bit bus today. 

The PCI bus is the most commonly used and found bus in computers today.

AGP

Introduced by Intel  in 1997, AGP, or Advanced Graphic Port, is a 32-bit bus designed for the high demands of 3-D graphics. AGP has a direct line to the computer's memory, which allows 3-D elements to be stored in the system memory instead of the video memory.

For AGP to work in a computer, it must have the AGP slot which comes with most Pentium II and Pentium III machines. The computer also needs to be running Windows 95 OSR2.1, Windows 98, Windows 98 SE, Windows 2000, Windows ME or higher.

USB

Universal Serial Bus is an external bus that supports transfer rates of 12 Mbps, can support 127 devices and supports hot plugging. Additional information on USB can be found on our USB page.

MINI PCI

Mini PCI is a new standard which measures at 2.75-inch x 1.81-inch x 0.22-inch is a new standard developed by leading notebook manufacturers. This technology could allow manufacturers to lower their prices, as the motherboards would be simpler to design. 

Type I - Identical to Type II, except requires extra cables for for connectors like the RJ-11 and RJ-45. However, offers more flexibility to where it can be placed in the computer.  

Type II - Used when size is not important. Type II is able to integrate the RJ-11 and RJ-45 connectors and did away with extra cables.

Type III - SO-DIMM style connector that can be installed with a mere 5 mm overall height above the system board. In addition, cabling to the I/O connectors allow Type III cards to be placed anywhere in the system.

PCI-X

PCI-X is a high performance bus that is designed to meet the increased I/O demands of technologies such as Fibre Channel, Gigabit Ethernet and Ultra3 SCSI. PCI-X capabilities include:

• Up to 133 MHz bus speed
• 64-Bit bandwidth
• 1GB/sec throughput
• More efficient bus operation for easier interface.
• Split Transactions allows an indicator device to make only one data request and relinquish the bus, instead of constantly needing to poll the bus for a response.
• Byte Count that enables indicator to specify, in advance, the specific number of bytes requested, eliminating the inefficiency of speculative prefetches.
• Backwards compatibility

AMR

Released September 8, 1998, AMR is short for Audio/Modem Riser. AMR allows an OEM to create one card that has the functionality of either Modem or Audio or both Audio and Modem on one card. This new specification allows for the motherboard to be manufactured at a lower cost and free up industry standard expansion slots in the system for other additional plug-in peripherals.

CNR

Introduced by Intel February 7, 2000, CNR is short for Communication and Network Riser and is a specification that supports audio, modem USB and Local Area Networking interfaces of core logic chipsets. 

BUS SPECIFICATIONS