Devices such as keyboards and mice are so common that their controller circuitry and connectors are built right into the motherboard.  Even audio, video and LAN capabilities - at one time considered "specialized" devices - are now built into most motherboards.  Such motherboards are designed for their simplicity, and will usually decrease the cost of the overall PC.  However, it makes the system more expensive to repair, since a faulty component will usually require the entire motherboard to be replaced.

However, integrated motherboards are not always suited for specialized applications.  For example, the data transfer rate for integrated video (133 MB/s) may not be enough for games or high-end graphics applications.  In addition, you may want to connect a specialized device that does not have a suitable port or connector on the motherboard, for example, a Firewire camcorder.  For these devices, the controller circuitry and connectors are built into special circuit boards called expansion cards (or controller cards, or adapter cards), which are physically inserted into special “expansion slots” on the motherboard.

The narrow channel in each expansion slot contains tiny copper contacts, which are connected to expansion buses.  When an expansion card is inserted into the expansion slot, a connection is made between the contacts in the slot and the copper “fingers” on the expansion card.  This allows the expansion card to use the associated expansion bus to communicate with an appropriate support chip, which ultimately allows the  device to communicate with the CPU.

Expansion Bus Evolution
 
● ISA (Industry Standard Architecture)
 

The first (non-proprietary) expansion slots were developed by IBM for the IBM PC in 1981.  The expansion slot (and its supporting bus) was given the name ISA (Industry Standard Architecture), and as the name suggests, became the industry standard for the next 10 years.

The supporting ISA bus had a very low data transfer rate (16 bits @ 8 MHz = 16 MB/s).  However, at the time, expansion slots were only used to connect low-speed peripherals such as mice, modems and sound cards, and so the data transfer rate was sufficient.
 
● PCI (Peripheral Component Interconnect)
 

In the early 1990s, processor speeds were increasing, and applications were switching from text-based to graphics-based.  The ISA bus became a major bottleneck to the overall system performance (increasing the speed of the CPU accomplished little if it was always waiting for a slow bus to transmit data).  The solution was to create a new type of expansion slot that would support a much faster bus.

The result was the Peripheral Component Interconnect (PCI) bus, developed by Intel in 1992.  It had a much higher data rate than the ISA bus (32 bits @ 33.3MHz = 133 MB/s), and could therefore be used for high-speed devices such as video adapter cards and network cards.

Over the years, the PCI standard was enhanced to include a 64 bit bus and faster bus speeds; however, these were found primarily on motherboards intended for server PCs that required high data transfer rates for multiple storage devices (eg, SCSI / RAID).

Modern motherboards usually include one or two standard PCI slots (32 bits @ 33.3MHz), due to the abundance of PCI devices still in existence.  However, as these devices begin to disappear, the PCI slot will soon join the ranks of ISA as obsolete technology.
 
● AGP (Accelerated Graphics Port)
 

By the mid-1990s, most video adapter cards used a PCI slot.  However, with computer graphics becoming more and more sophisticated, the standard PCI bus was no longer fast enough to transmit video data to and from the CPU.  In addition, video adapter cards could not supply enough video RAM for graphics applications involving 3D acceleration and full-motion video.

In response, Intel developed the Accelerated Graphics Port (AGP) in 1997.  The initial version of AGP used a 32-bit bus running at 66.6MHz, giving a data transfer rate of 266 MB/s.  However, the main advantage of AGP was that its bus was attached to the northbridge chip rather than the southbridge.  This allowed data to travel more efficiently between the video adapter card and the CPU, resulting in better performance.  AGP was also able to utilize the PC's RAM for doing its calculations.  As a result, large amounts of expensive video RAM were no longer required on the video adapter card.

AGP was later enhanced to allow signal rates of 2, 4 and 8, giving data transfer rates of 533MB/s, 1 GB/s and 2 GB/s respectively.  However, the emergence of PCI Express has rendered AGP virtually obsolete, as new motherboards rarely include an AGP slot.
 
● PCI Express (Peripheral Component Interconnect Express)
  PCI Express (PCIe) slots are the newest and most advanced expansion slots found on motherboards today.  PCI Express was developed jointly by Intel, IBM, Dell and HP in 2004.

PCI Express is a serial technology, similar to USB.  In other words, a PCIe bus transfers a single bit at a time (unlike a parallel bus, which transfers 32 or 64 bits at a time).  Serial data transfer eliminates many problems associated with parallel transfers, such as skew (bits arriving at different times) and jitter (voltages floating above or below target levels).  This allows serial bus speeds to be increased significantly, and therefore maintain relatively high data transfer rates.

In fact, PCI Express uses a super high-speed serial bus running at 2.5GHz.  Using a signaling rate of 2, and allowing for 20% overhead processing, this gives a data transfer rate of 500 MB/s - which is four times greater than standard PCI, and twice the rate of standard AGP.

In addition, there are five categories of PCI Express, each with an increasing data transfer rate as follows:

Category

Data Transfer Rate
PCIe x1 (read "PCIe by one")
PCIe x2
PCIe x4
PCIe x8
PCIe x16

500 MB/sec
1 GB/sec
2 GB/sec
4 GB/sec
8 GB/sec

The numeric designator in the PCI Express category defines the number of serial "lanes" that are being used to carry data.  Therefore, PCIe x16 has 16 serial lanes, each with a data transfer rate of 500 MB/s, giving an overall data transfer rate of 8 GB/s!

Using multiple serial "lanes" to carry data is similar to a point-to-point network topology.  At the source, data is broken up into smaller units called "packets".  Packets are then numbered, and sent to the destination via the serial "lanes".  When they arrive at the destination, they are re-assembled into the original data (this accounts in part for the 20% overhead processing).  Therefore, the more lanes, the more data can be moved per second - which is why PCIe x16 (at 8 GB/s) is primarily used for high-volume graphics adapter card.

The number of serial lanes also determines the physical size of the expansion slot, with PCIe x1 being the smallest (about 1" long), and PCIe x16 the largest (about 3.5").  In addition, a PCIe card will physically fit (and work correctly) in any slot that is at least as large as it is.  As a result, a PCIe x16 card can only fit in a PCIe x16 slot, whereas a PCIe x1 card can fit in any size PCIe slot.

The increased data transfer rates of PCI Express has led to a gradual "phasing out" of all other types of expansion slots.  For example, most motherboards today have one or two PCIe x1 slots, which are used for moderate-volume devices (modems, sound cards, network adapters etc.) that were previously connected using a standard PCI slot.  Most motherboards also have a single PCIe x16 slot, which has virtually replaced the AGP slot for connecting high-speed video adapter cards.

 


A motherboard with two ISA slots (black), five PCI slots (white) and one AGP slot (brown).
A motherboard with two PCI slots (white), one PCIe x1 slot (small green), and one PCIe x16 slot (large green)