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I/O Bus Slots
All motherboards have one or more system I/O buses, that are used to expand the computer's capabilities. The slots in the back of the machine are where expansion cards are placed (like your video card, sound card, network card, etc.). These slots allow you to expand the capabilities of your machine in many different ways, and the proliferation of both general purpose and very specific expansion cards is part of the success story of the PC platform.
Most modern PCs have two different types of bus slots. The first is the standard ISA (Industry Standard Architecture) slot; most PCs have 3 or 4 of these. These slots have two connected sections and start about a half-inch from the back of the motherboard, extending to around its middle. This is the oldest (and slowest) bus type and is used for cards that don't require a lot of speed: for example, sound cards and modems. Older systems (generally made well before 1990) may have ISA slots with only a single connector piece on each; these are 8-bit ISA slots and will (of course) only support 8-bit ISA cards.
Pentium systems and newer 486-class motherboards also have PCI (Peripheral Component Interconnect) bus slots, again, usually 3 or 4. They are distinguished from ISA slots in two ways. First, they are shorter, and second, they are offset from the back edge of the motherboard by about an inch. PCI is a high-speed bus used for devices like video cards, hard disk controllers, and high-speed network cards.
Note: Newer PCI motherboards
have the connectors for the hard disks coming directly from the motherboard. These
connectors are part of the PCI bus, even though the hard disks aren't connected to a
physical PCI slot.
The newest PCs add another, new connector to the motherboard: an Accelerated Graphics Port slot. AGP is not really a bus, but is a single-device port used for high-performance graphics. The AGP slot looks similar to a PCI slot, except that it is offset further from the back edge of the motherboard.
Older 486 systems use VESA Local Bus, or VLB slots instead of PCI to connect high-speed devices. This is an older bus which began to be abandoned in favor of PCI at around the time the Pentium was introduced. VLB slots look like ISA slots, only they add third and fourth sections beyond the first two. This makes their connectors very long, and for that reason VLB cards are notoriously difficult to insert into or remove from the motherboard. Care must be exercised to avoid damage.
Some motherboards incorporate a so-called "shared" ISA and PCI slot. This name implies a single slot that can take either type of card, but that isn't possible because the two slot types are physically incompatible. In order to save space while maximizing the number of expansion slots, some designers put an ISA slot on the board right next to a PCI slot; you then have the choice to use either the ISA or the PCI slot, but not both. This design is possible because ISA cards mount on the left-hand side of a slot position, while PCI slots mount on the right-hand side.
Next: Power Connector(s)