Expansion Slots

The most visible parts of any motherboard are the expansion slots. These look like small plastic slots, usually from 3 to 11 inches long and approximately 1⁄2 inch wide.

As their name suggests, these slots are used to install various devices in the computer to expand its capabilities.

Some expansion devices that might be installed in these slots include video, network, sound, and disk interface cards.

If you look at the motherboard in your computer, you will more than likely see one of the main types of expansion slots used in computers today:

1. ISA
2. PCI
3. AGP
4. PCIe
5. AMR
6. CNR

Each type differs in appearance and function. In this section, we will cover how to visually identify the different expansion slots on the motherboard.

ISA Expansion Slots

If you have a computer made before 1997, chances are the motherboard has a few Industry Standard Architecture (ISA) expansion slots.

They’re easily recognizable because they are usually black and have two parts: one shorter and one longer.

Computers made after 1997 generally include a few ISA slots for backward compatibility with old expansion cards (although most computers are phasing them out in favor of PCI).










picture of ISA slots






PCI Expansion Slots

Most computers made today contain primarily Peripheral Component Interconnect (PCI) slots.

They are easily recognizable because they are short (around 3 inches long) and usually white.

PCI slots can usually be found in any computer that has a Pentium-class processor or higher.






Picture of PCI expansion slots








AGP Expansion Slots

Accelerated Graphics Port (AGP) slots are very popular for video card use. In the past, if
you wanted to use a high-speed, accelerated 3D graphics video card, you had to install the
card into an existing PCI or ISA slot.

AGP slots were designed to be a direct connection between the video circuitry and the PC’s memory.

They are also easily recognizable because they are usually brown, are located right next to the PCI slots on the motherboard, and are shorter than the PCI slots.







AGP slot





PCIe Expansion Slots

The newest expansion slot architecture that is being used by motherboards is PCI Express
(PCIe). It was designed to be a replacement for AGP and PCI.

It has the capability of being faster than AGP while maintaining the flexibility of PCI. And motherboards with PCIe will have regular PCI slots for backward compatibility with PCI.

There are seven different speed levels for PCIe, and they are designated 1X, 2X, 4X, 8X, 12X, 16X, and 32X.

These designations roughly correspond to similarly designated AGP speeds. The slots for PCIe are a bit harder to identify than other expansion slot types because the slot size corresponds to its speed.

For example, the 1X slot is extremely short (less than an inch). The slots get longer in proportion to the speed; the longer the slot, the higher the speed.

The reason for this stems from the PCIe concept of lanes, which are the multiplied units of communication between any two PCIe components and are directly related to physical wiring
on the bus.

Because all PCIe communications are made up of unidirectional coupling between devices, each PCIe card negotiates for the best mutually supported number of lanes with each
communications partner.














PCI Express slots (from top to bottom: x4 , x16, x1 and x16),
compared to a traditional 32-bit PCI slot (bottom),
as seen on DFI's LanParty nF4 SLI-DR.


AMR Expansion Slots

As is always the case, Intel and other manufacturers are constantly looking for ways to improve the production process.

One lengthy process that would often slow down the production of motherboards with integrated analog I/O functions was FCC certification.

The manufacturers developed a way of separating the analog circuitry, for example, modem and
analog audio, onto its own card.

This allowed the analog circuitry to be separately certified (it was its own expansion card), thus reducing time for FCC certification.

This slot and riser card technology was known as the Audio Modem Riser, or AMR. AMR’s 46-pin slots were once fairly common on many Intel motherboards, but technologies including CNR and Advanced Communications Riser (ACR) are edging out AMR.

In addition and despite FCC concerns, integrated components still appear to be enjoying the most success comparatively.











picture of AMR slot





CNR Expansion Slots

The Communications and Networking Riser (CNR) slots that can be found on some Intel
motherboards are a replacement for Intel’s AMR slots.

Essentially, these 60-pin slots allow a motherboard manufacturer to implement a motherboard chipset with certain integrated features.

Then, if the built-in features of that chipset need to be enhanced (by adding Dolby Digital Surround to a standard sound chipset, for example), a CNR riser card could be added to enhance the onboard capabilities.

Additional advantages of CNR over AMR include networking support, Plug and Play compatibility, support for hardware acceleration (as opposed to CPU control only), and no need to lose a competing PCI slot unless the CNR slot is in use.

arrow show the CNR slot