EXPANSION SLOTS & BUSES

COMPONENTS OF A MOTHERBOARD

2// EXPANSION SLOTS & BUSES:->

The most visible Parts of any motherboard are "Expansion Slots". These look like small plastic slots, usually from 1 to 6 inches long & 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, & 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.

PCI
AGP
PCIe
AMR
CNR

Each type of slots differs in appearance & function. In this session we will learn how to visually identify the different expansion slots on the motherboard.

* PCI (Peripheral Component Interconnect)

Today's most of the motherboards contain 32 bit (PCI) slots. They are easily recognized because they are short (3 inch long) compared to the classic ISA slot & usually white.

PCI slots can usually be found in any motherboard that has a Pentium class processor or higher supportable feature. PCI expansion buses operate at 33 or 66 MHz over a 32 bit (4 - byte) channel, resulting in data rates of 133 & 266 MBps.

PCI slots & adapters are manufactured in 3.3 & 5 V versions. Universal adapters are keyed to fit in slots based on either of the common 5V slots & adapters is oriented towards the front of the mother board, & notch in the 3.3V adapter towards the rear.

* AGP (Accelerated Graphics Port)

AGP slots are known mostly for Video card use & are steadily being supplanted by PCI Express adapters. 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 & PCs memory. They are easily recognizable because they are usually brown in color & located right next to the PCI slots on the motherboard,& are shorter than the PCI slots.

AGP performance is based on the original specification, known as AGP 1X. It uses a 32 bit channel & a 66MHz clock, resulting in a data rate of 266.67 MBps. AGP 2x, 4x, & 8x specification multiply the 66 MHz clock they receive to increase throughput linearly. For instance, AGP 8x uses the 66 MHz clock to produce an effective clock frequency of 533 MHz, resulting in throughput of 2133.33 MBps over the 4-byte channel.

* PCIe(Peripheral Component Interconnect Express)

A newer expansion slot architecture that is being used by motherboards is "PCI express (PCIe)". It was designed to be a replacement for AGP & PCI. It has the capability of being faster than AGP while maintaining the flexibility of PCI. Motherboards with PCIe might have regular PCI slots for backward compatibility with PCI.

True bus architectures are parallel in nature, while PCIe is a serial technology, striping data packets across multiple serial paths to achieve higher data rates. PCIe uses the cncept of "lanes" which are the switched point - to - point signal paths between any two intercommunicating devices comprises a separate pair of wires for both directions of traffic. The signal lane or combined collection of lanes that the switch interconnects between devices is referred to as a link.

There are seven different link widths supported by PCIe, designated x1 (pronounced "by 1"), x2, x4, x8, x12, x16, & x32, with x1, x4, & x16 the most common. Every PCIe slot has a 22-pin portion in common towards the rear of the motherboard.

There are 3 major versions of PCIe currently specified, 1.x, 2.0 & 3.0. For these 3 versions a single lane, & hence a x1 slot operates in cach direction at a data rate of 250 MBps, 500 MBps & 1GBps respectively. A PCIe 1.1 x16 slot is capable of 4 GBps of throughput in each direction, 16 times the 250 MBps x1 rate.

* AMR (Audio Modem Riser)

Intel & other manufacturers are constantly looking for ways to improve the production process. The manufacturers developed a way of separating the analog circuitry, for example, Modem & analog Audio, onto its own card. This allowed the analog circuitry to be separately certified, thus reducing time for FCC certification.

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

* CNR (Communication & Network Riser)

The CNR slots that can be found on some Intel motherboards are a replacement for Intel's AMR slots. One portion of these slots is the same length as one of the portions of the AMR slot, but the other portion of the CNR slot is longer than that of the AMR slot. Essentially, these 60 - pin slots allow a motherboard manufacturer to implement a motherboard chipset with certain integrated features.