Fig:1.4
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:
ISA
PCI
AGP
PCIe
AMR
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). Figure 1.4 shows an example
of ISA expansion 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. Figure 1.5 shows an example of several 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. Figure 1.6 shows an example of an AGP slot, along with a PCI
slot for comparison. Notice the difference in length between the two.
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.
FIGURE 1 . 5 PCI expansion slots
FIGURE 1 . 6 An AGP slot compared to a PCI slot
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.
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. Figure 1.7 shows an example of an 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.
Figure 1.8 shows an example of a CNR slot.
FIGURE 1 . 7 An AMR slot
FIGURE 1 . 8 A CNR slot
