Expansion Slots

Before you start

Objectives: learn which slots (buses) are being used to add new features to personal computers.

Prerequisites: you should know what is a Motherboard.

Key terms: pci, slot, pcie, bus, agp, express, card, amr, isa, expansion, pci-x, connector


ISA bus was used on older computers. It was first developed in 1981, and was used in 8-bit systems.

ISA Slots

Image 228.1 – ISA Slots

Although ISA can still be used for specialized purposes, we won’t see it in personal computers any more. ISA was replaced with more advanced bus called PCI.


The first type of expansion slot we need to be familiar with is the PCI orPeripheral Component Interconnect expansion slot. Although it is now being replaced with newer bus, we can still find it in many older computers.

PCI Slots

Image 228.2 – PCI Slots

PCI slots are usually colored white. There is an exception if we’re dealing with a 64 bit PCI slot. They’re usually colored brown. PCI supports a 32 or 64 bit I/O bus providing compatibility with both 486 and Pentium machines. This bus is processor independent which means that the CPU and the PCI bus can process concurrently. PCI is plug-and-play, meaning that newly installed devices can be detected and configured automatically. PCI buses are most commonly used for devices such as sound cards, modems, network cards, storage device controllers, etc.


Small form factor computers, such as laptops or micro-ATX systems, might include a mini-PCI slot. Mini-PCI devices are small cards with either 100 or 124 pins. A typical use for a mini-PCI slot is to add internal cards (such as wireless cards) to laptops.

Orange PCI slot

On occasion we will see a PCI slot, but it will be colored orange. The orange PCI connector is actually just a standard PCI connector, except for the fact that it’s been wired to also function as an AMR or CNR slot. What it does is it takes first sets of pins and also wires them as AMR or CNR pins, so that we can put a little AMR card in the first set of pins, using only the first portion of the connector and have it function as a AMR or CNR connector.


In addition to PCI slots we can also have an AGP slot or Accelerated Graphics Port.

AGP Slot

Image 228.3 – AGP Slot

AGP slots are usually brown in color, like we can see on the picture. They are set farther from the edge of the motherboard. Older AGP slots had a little ledge, like we can see on the picture, but newer ones don’t have that. That ledge is used to keep us from installing a newer AGP board that doesn’t have a notch, into an older slot that it’s not compatible with that card. AGP is similar to PCI, but it is designed specifically for graphics support. Motherboards that provide AGP support have a single AGP slot. AGP is commonly used for video cards in computer systems, but is being replaced by PCIe.


Some motherboards can have a AMR or Audio Model Riser connector.

 AMR Slot

Image 228.4 – AMR Slot

AMR connectors and CNR connectors look alike. They’re very small, much smaller than a PCI or an AGP slot. A riser card is not a bus, but rather a card that attaches to the Motherboard and allows inserting additional cards (called daughter cards). AMR slots typically provide sound or modem functions. CNR is a riser card slot (not a bus) that allows for inserting networking, wireless communication, sound, or modem cards.

PCI Express (PCIe)

PCI is a great bus and it’s been with us a long time, but a new version of PCI has been released called PCI Express. PCIe dramatically increases both the speed and the functionality of PCI. Let’s take a look at what PCIe slots look like.

PCI Express

Image 228.5 – Various PCIe Slots

 PCIe Slots

Image 228.6 – PCIe and PCI on Motherboard

As we can see on the picture we have different PCI Express bus sizes. We have PCI Express X16 slot, X8 slot, X4 and X1 slot. PCI Express (PCIe) is a next generation I/O bus architecture. Rather than a shared bus, each PCIe slot links to a switch which prioritizes and routes data through a point-to-point dedicated connection and provides a serial full-duplex method of transmission. Basic PCIe provides one lane for transmission (x1) at a transfer rate of 2.5 Gbps. It can also provide multiple transmission lanes (x2, x4, x8, x16). In addition to greatly increased speed, PCIe offers higher quality service. PCIe is backwards compatible and allows legacy PCI technology to be run in the same system. That means that we can have both PCIe and PCI buses on the same system, which is often the case at the time of writing this article.

PCI Express Mini Card

PCI Express Mini Card is also known as Mini PCI Express, Mini PCIe, and Mini PCI-E. It is a replacement for the Mini PCI form factor, based on PCI Express.


PCI-X, short for PCI-eXtended, is a computer bus and expansion card standard that enhances the 32-bit PCI bus. It is similar in electrical implementation and it uses the same protocol, but it has higher bandwidth than PCI. PCI-X has been replaced with PCIe, since PCIe is even more advanced. PCI-X is often confused with PCI Express, although the cards are totally incompatible and look different. The reason for this confusion is that PCI-X sounds similar to PCI Express.

Installing Expansion Cards

The first thing we need to do when you’re installing expansion board is to implement proper ESD prevention measures. For example, we can rest the computer case on the antistatic mat and ground it to the case. In addition, we can also ground ourselves to the mat and the case. Also, the expansion board that we are installing should have been inside the static shielding bag. In the old days, when installing the expansion cards, we would also have to configure the IRQ channels, I/O address and maybe even his DMA channels for the card. Today, we are dealing with plug-and-play boards and we don’t have to do that anymore. All we have to do is insert the card into an expansion slot, turn system on, and the BIOS and operating system will take care of IRQ assignments, I/O adresses and DMA channels. More than likely, if we have an operating systems such as Windows installed, we would then also have to load the appropriate driver that will allow the CPU to communicate with with the board we’ve just installed.


We differentiate several different standards when it comes to expansion slots. We have ISA, PCI, AGP, PCI-X, and PCIe. ISA was replaced with PCI, and PCI is now being replaced with PCIe.