What Is an Expansion Slot?

An expansion slot refers to any of the slots on a motherboard that can hold an expansion card to expand the computer's functionality, like a video card, network card, or sound card.

What Are Expansion Slots Used For?

The expansion card is plugged directly into the expansion port so that the motherboard has direct access to the hardware. However, since all computers have a limited number of expansion slots, it's important to open your computer and check what's available before you buy one.

Some older systems require the use of a riser board to add additional expansion cards; however, modern computers not only usually have enough expansion slot options, but they also have features integrated directly into the motherboard, eliminating the need for so many expansion cards.

ASUS 970 Pro Gaming/Aura ATX DDR3 AM3 Motherboard
ASUS 970 Pro Gaming/Aura ATX DDR3 AM3 Motherboard.

Expansion slots are sometimes referred to as bus slots or expansion ports. The openings on the rear of a computer case also sometimes go by this term.

Different Kinds of Expansion Slots

There have been several types of expansion slots over the years, including PCI, AGP, AMR, CNR, ISA, EISA, and VESA, but the most popular one used today is PCIe. While some newer computers still have PCI and AGP slots, PCIe has basically replaced all the older technologies.

ePCIe (External PCI Express) is another kind of expansion method, but it's an external version of PCIe. That is, it requires a specific kind of cable that extends from the motherboard out the back of the computer, where it connects with the ePCIe device.

How Do Expansion Slots Work?

Like mentioned above, these expansion ports are used to add various hardware components to the computer, like a new video card, network card, modem, sound card, etc.

Expansion slots have what's called data lanes, which are signaling pairs that are used for sending and receiving data. Each pair has two wires, which makes a lane have a total of four wires. The lane can transfer packets eight bits at a time in either direction.

Since a PCIe expansion port can have 1, 2, 4, 8, 12, 16, or 32 lanes, they're written with an "x," like "x16" to indicate that the slot has 16 lanes. The number of lanes directly relates to the speed of the expansion slot, which is why video cards are usually built to use a x16 port.

What to Know Before Installing Expansion Cards

An expansion card can be plugged into a slot with a higher number, but not with a lower number. For example, a x1 expansion card will fit with any slot (it will still run at its own speed, though, not the speed of the slot) but a x16 device will not physically fit into a x1, x2, x4, or x8 slot.

When you're installing an expansion card, before removing the computer case, be sure to first power down the computer and unplug the power cord from the back of the power supply. The expansion ports are usually located catty-corner to the RAM slots, but that might not always be the case.

If the expansion slot hasn't been used before, there will be a metal bracket covering the corresponding slot on the back of the computer. This needs to be removed, usually by unscrewing the bracket, so that the expansion card can be accessed. For example, if you're installing a video card, the opening provides a way to connect the monitor to the card with a video cable (like HDMI, VGA, or DVI).

Seating the Expansion Card

When seating the expansion card, make sure you're holding on to the metal plate edge and not the gold connectors. When the gold connectors are properly lined up with the expansion slot, press down firmly into the slot, making sure that the edge where the cable connections are is easily accessible from the back of the computer case.

You can remove an existing expansion card by holding on to the metal plate edge, and pulling firmly away from the motherboard, in a straight, upright position. However, some cards have a small clip that keeps it in place, in which case you have to hold back the clip before pulling it out.

New devices need the proper device drivers installed in order to work properly. See our guide on how to update drivers in Windows if the operating system doesn't provide them automatically.

Do You Have Room for More Expansion Cards?

Whether or not you have any open expansion slots varies with everyone since not all computers have the exact same hardware installed. However, short of opening your computer and checking manually, there are computer programs that can identify which slots are available and which are used.

For example, Speccy is one free system information tool that can do just that. Look under the Motherboard section and you'll find a list of the expansion slots found on the motherboard. Read the Slot Usage line to see if the slot is used or available.

Speccy motherboard details

Method 1: Check With the Motherboard Manufacturer

Another method is to check with the motherboard manufacturer. If you know the model of your specific motherboard, you can find out how many expansion cards can be installed by checking with the manufacturer directly or looking through a user manual (which is usually available as a free document from the manufacturer's website).

If we use the example motherboard from the image at the top of this page, we can access the motherboard's specifications page on the Asus website to see that it has two PCIe 2.0 x16, two PCIe 2.0 x1, and two PCI expansion slots.

970 Pro Gaming/Aura specs summary

Method 2: Check the Back of Your Computer

One more technique is to see which openings are unused on the back of your computer. If there are two brackets still in place, there are most likely two open expansion slots. This method, however, isn't as reliable as checking the motherboard itself, since your computer case might not correspond directly with your motherboard.

Do Laptops Have Expansion Slots?

Laptops don't have expansion slots like desktop computers do. A laptop may instead have a little slot on the side that uses either PC Card (PCMCIA) or, for newer systems, ExpressCard.

These ports can be used similarly to a desktop's expansion slot, like for sound cards, wireless NICs, TV tuner cards, USB slots, additional storage, etc.

  • What is gradually replacing portable-specific expansion slots?

    USB (Universal Serial Bus) is a standard connection for various devices. Many manufacturers now use USB instead of portable-specific expansion slots.

  • What screws do you use on PCI expansion slots?

    Most computer case screws require #6-32 x 1/4-inch screws. They typically have a hexagonal head and can be installed or removed using a #2 size Phillips screwdriver.

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