Expansion cards fit into the expansion bus slots on your motherboard and are used to communicate with peripheral devices. The common ones are multi-IO boards, video cards, and sound cards. The motherboard expansion bus must match the expansion card to be attached.
The first IBM compatible expansion bus was called the XT bus. It would support devices using an eight-bit "architecture". On most computers today it is the set of slot sockets nearest the wall of the comupter case.
New developments indicated that newer equipment would require more information forcing more electrical contacts. With the advent of the IBM AT computers came the AT bus. It included the old XT bus with an additional socket about half the size of the original. It sits in line with the other so that the edge of the printed circuit board can be inserted into both sockets at once. As you can imagine, old cards fit in new machines but cards made for the AT won't work in the XT. Nonetheless, this new combination was accepted as the Industry Standard Architecture (ISA) bus.
That worked very well as processor technology advanced from 286 to 386. With the 80486 microprocessor, however, the speed and processing power (8-bits of information per processor clock-pulse for the 8088, 16 for 286 . . . and 32 bit for the 486) became the realization of computer program developer's dreams. The fast 32 bit processor, coupled with a 32 bit bus, could produce startling graphics at remarkable speeds. The VESA (Video Enhanced System Architecture) bus was developed.
The previous edge connectors worked with metal fingers about twice the width of the VESA Local bus contacts. The new architecture allowed more contact points to be made in the same amount of card edge space. So when the Pentium and 586 processors showed up, the technology already existed for high-capacity, high speed expansion cards with small size. The AT and XT bus slots were chucked and in their place sits the PCI bus expansion slot. It handles 64 bit bus applications, making it very fast.
Backward compatability demands, at least for now, that ISA slots be left in modern motherboards. Your old stuff will fit into the new model. But change keeps happening. Consider that right now you can buy a four processor Pentium motherboard.
Updated January 24, 1998, 10:52pm. firstname.lastname@example.org