expansion slots

The expansion slots allow the computer system to communicate with the outside world. The success of the IBM compatible has been mainly due to its ability to change, and add, new features by doing nothing more than just fitting them into these slots.

The original IBM PC-AT ran its system with a clock speed of 8MHz. Manufacturers, using the PC-AT design for their computers, called "clones", moved in the direction of compatibility with the PC-AT. This meant that any board plugged into the expansion slot of the "clones" could only run at speeds of 8MHz to ensure this compatibility.

As the technology of the clones evolved, with microprocessor speeds of 10,12, and then 16 MHz being reached, the 'plug-in' speed remained at 8MHz because of the need to ensure compatibility with the PC-AT. This resulted in delays as the microprocessor had to wait around for the rest of the system to catch up.The main problem was that the RAM memory of the computer was in the form of plug-in cards.

A way around the problem was for designers to move the RAM (Random Access Memory) off plug-in cards onto the motherboard with the processor. They also gave RAM its own bus to the processor, called the local bus. This now allowed RAM to run at the same speed as the processor. It being many times faster than cards using the original 'Industrial Standard Architecture' (ISA) IBM PC-AT 8MHz system. Effectively Local Bus means that memory is addressed directly by the CPU.

As processor speeds increased the next bottleneck was the presenting of information on the monitor screen. To overcome this, video data was also placed on the local bus away from the 16 bit, 8MHz ISA bus. To allow for the need of users wanting to upgrade to new video standards, a local bus connector was provided allowing the plugging in of common video controller boards, called the 'Video Local Bus' (VLB) cards.

The two most popular designs is the older, VESA(Video Electronics Standard Association), or VLB local bus and the newer PCI(Peripheral Component Interconnect) Local Bus found in newer Pentiums. Both offer 32-bit data transfer rates at up to 33MHz although the PCI has the edge in speed by reducing processor wait state writes to the video memory. PCI also has the advantage of plug-and-play ability as used with Windows 95. All you need do is to install a plug-and-play graphics card and Windows 95 will configure it for best operation with your system. Consequently, most modern systems now use PCI Local Bus which is incompatible with the VESA LB.

VESA LB, or VLB, still have a standard ISA bus slot as an extension of its bus connector. This allows a PC-AT board to be plugged into a standard ISA connector, or the particular connector on the VESA LB slot. A VESA LB board, however, can only be connected into appropriate slot of the combination board connector. The provision allows for the connection of older ISA or VLB boards to be connected in the same expansion slot.

The PCI system has a separate set of PCI and a few standard ISA connectors, boards are not interchangeable between the two connectors.

In the latest systems the disc data is also being moved to the local bus to avoid the slow 8MHz PC-AT bus.

The PCI bus is very fast and is now the standard bus used with new video boards. Running at clock speed of 33 Mhz, the PCI local bus uses 32-bit data bus that supports add-in cards and peripheral components at 132 MB/sec. bandwidth. This compares with the ISA data transfer rate of 5MB/sec. There is a 64-bit data transfer extention to basic PCI which adds a futher 60 pins to the 120 available on the 32-bit card. This doubles the data transfer speed to 264MB per second.

The older 'VESA bus video card' can not be used with the newer PCI bus because the two are incompatible. The 486 processor motherboard used the VESA LB as did some of the older Pentium boards. So when upgrading be aware that you need to check if the motherboard uses VESA or PCI local bus. As all new systems now use PCI Local Bus, you will eventually have purchase a video card that uses PCI LB.

Expansion Slots
Expansion slots openings are located on the back of the computer.They look like the ones shown in the figure above.They provide access to the AGP,PCIe,PCIand ISA expansions slots one the motherboard.To use this expansion slot opening,a person would need an expansion card like the sound blaster live card as shpwn below,which is actually two cards
The top card plugs into a PCI slot, while the bottom card sends and receives its data through the larger card via the connected cable. The smaller card simply needs an empty expansion slot opening on the back of the case to mount to. The expansion slot opening you clicked on would be perfect for the bottom card.