Phil Storrs PC Hardware book

The PC Busses

A very important feature of PC Computers is the external bus slots into which various I/O cards are plugged. The video interface is usually provided by a card plugged into a bus slot, and most modern PC's also have a sound interface card in a bus slot.

In the past the serial, parallel, and joystick interfaces were on plug in I/O cards along with hard drive and floppy disk drive interfaces. Over recent years these functions have been included on the system boards but the video and sound functions, and network interfaces are usually still provided by cards plugged into bus slots on the system board.

Bus technology has undergone a process of evolution made necessary by developments in other areas of PC technology.

This is the eight bit external bus used in the PC and XT type computers

It provide 8 Data lines and 20 Address lines, Hardware Interrupt lines and DMA lines
The bus connectors also provide plus and minus 5 volt, and plus and minus 12 volt, power rails

This is the 16 bit external bus, introduced with the PC/AT computer.
Today it is known as the ISA bus (Industry Standard Architecture)

This bus provided 16 Data lines, 24 Address lines and more Interrupt and DMA lines.

This image shows a comparison of the "old" eight bit PC/XT bus and the ISA bus

The EISA Bus was an alternative advanced Bus devised by the other large computer manufacturers. This bus had the advantage of being able to accept ISA and the original eight bit cards used in the PC/XT computers. The MCA bus was incompatible with other bus types. This example of an EISA Bus card is a SCSI Interface card once used in a powerful Network Server computer.

This is the EISA bus, introduced in 1987 as a competitor to the MCA Bus.

This picture shows a comparison of the EISA and ISA bus connectors

The EISA connector has two rows of contacts, one above the other
The EISA bus had the advantage of being able to accept ISA bus and eight bit bus cards.

IBM introduced a new type of bus with its PS/2 range of computers in 1987. This bus is the Micro Channel Architecture, abbreviated to MCA Bus. Here is an example of an MCA Bus Network card out of an IBM PS/2 model 50 computer.

A MCA Bus card

The rest of the PC industry did not make much use of IBM's MCA bus, it was too expensive and would not accept any of the existing bus cards in use (the eight bit PC/XT or ISA bus cards)

Here is an ISA bus card compared to an MCA bus card

Here is the MCA Bus card sitting on top of an ISA bus card for comparison.

Local Bus technology

All the popular busses available up to 1994 transferred data at a maximum of 8 MHz. A local bus on the System Board transferred data between the memory and the processor at up to 33 MHz or even higher in some cases. The logical next step was to extend this bus to a series of bus slots and plug I/O cards that required faster transfer rates, like Video Cards, Hard Drive Interfaces, SCSI interfaces and high speed Network cards, into these slots.

The first attempt at this were proprietary solutions from individual manufacturers. Local Bus technology did not catch on until the Video Electronics Specialists Association (VESA) devised the VESA Local Bus. This example of a VESA Bus card is a Video Interface card.

The VESA Local Bus

The first universal local bus was the VESA bus, here is a comparison with the ISA Bus
This bus was soon superceded by the PCI bus, a more advanced and "platform independent" bus

Today the PCI local bus has taken over form the VESA local bus

Here is a ISA Bus card sitting on top of an VESA bus card for comparison.

This bus used an MCA type connector on the end of the existing ISA Bus slots.

The VESA Local Bus was not used for long, it was soon made obsolete by the PCI Bus. The PCI Bus has the advantage of being used in all modern computer platforms, not only those using Intel family processors.

When compared to ISA, EISA, MCA and VESA cards, the PCI Bus has the components on the other side of the board.

Here is a PCI bus card compared with an ISA bus card.

More images showing the differences between a PCI Bus card and an ISA Bus card.

The PCI connector is near the back of the board but set in from the ISA Bus connectors.

The PC hardware Gallery The opening index The PC Glossary PC Acronyms

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Copyright © Phil. Storr, last updated 26th December 1998