EMZETT.Expansion Cards
In short: Add-in cards plugged into a free slot on the motherboard via PCIe, to add extra functions — e.g. graphics cards, network cards or capture cards.
In more detail: Expansion cards give desktop PCs their typical flexibility: instead of permanently built-in functions, almost every component can be individually replaced or added. On laptops, this option is usually unavailable due to limited internal space.
In Depth
The modular construction principle
Expansion cards are the direct result of a modular construction principle: instead of permanently building every possible extra function (graphics, network, additional ports, video capture) directly into the motherboard, the motherboard only offers standardised PCIe slots, into which the appropriate card is plugged as needed. This keeps motherboards cheaper and more flexible — someone who doesn’t need a dedicated graphics card doesn’t pay for one, while someone who needs a special professional graphics card or capture card can choose it independently of the rest of the system and replace it later, without having to replace the entire motherboard.
PCIe lanes and slot sizes
PCIe slots differ in their physical size and the number of electrical contacts (“lanes”, e.g. x1, x4, x8, x16) — more lanes mean more available bandwidth between the card and the rest of the system, since each lane represents its own, parallel-usable data connection. A graphics card typically uses the largest, fastest slot (x16), while simpler expansion cards like network cards are often fine with a smaller x1 slot, since they need considerably less bandwidth.
Sharing the lane budget
How many PCIe lanes a motherboard can offer in total depends on both the CPU and the chipset — a frequently overlooked limit when running several expansion cards at once. If, for example, a graphics card, an NVMe SSD and an additional network card are all run at the same time, the limited total lane budget can result in the graphics card being connected with only 8 lanes instead of the full 16 — for most current graphics cards, a barely measurable performance loss, but theoretically relevant for older or particularly bandwidth-hungry cards.
Typical categories of expansion cards
Besides graphics cards and capture cards, there are numerous other expansion cards: dedicated sound cards (for higher audio quality than integrated motherboard solutions), additional network cards (e.g. for 10-gigabit Ethernet, if the chipset only offers gigabit), RAID controllers (for advanced hard drive management), and additional USB or Thunderbolt controllers, in case the motherboard itself doesn’t come with enough modern ports.
See also: Graphics cards, PCIe, Capture Card, Chipsets