What Is an Expansion Slot?

Expansion Slot Definition

ASUS 970 Pro Gaming/Aura ATX DDR3 AM3 Motherboard
Expansion Slots (ASUS 970 Pro Gaming/Aura ATX DDR3 AM3 Motherboard). © Amazon.com / Asus

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.

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 but modern computers not only usually have enough expansion slot options but also have features integrated directly into the motherboard, eliminating the need for so many expansion cards.

Note: Expansion slots are sometimes referred to as bus slots or expansion ports. The openings on the rear of a computer case are also sometimes called expansion slots.

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 of the older technologies.

ePCIe, or 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.

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.

Important Facts About 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).

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.

Note: 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 expansion slot is used or available.

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 PDF from the manufacturer's website).

If we use the example motherboard from the image above, 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.

One more method you can use to check the available expansion slots on your motherboard 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, is not as reliable as checking the motherboard itself since your computer case might not correspond directly with your motherboard.

Do Laptops Have Expansion Slots?

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

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

You can buy an ExpressCard from various online retailers like Newegg and Amazon.