Is a half-length PCI Express x8 Gen 3 card compatible with a PCIe x4 Gen3 slot?

Question

I have a card with a half-length PCI Express x8 Gen 3 output. Is it compatible with Dell Precision 3660 Tower which has one PCIe x4 Gen3, one PCIe x4 Gen4, and one PCIe x16 gen5 (for the GPU only)? Essentially I want to see how is a half-length x8 different from an x4 PCIe slot?

Answer 1

You need the card to fit physically into one of the slots and be compatible with PCI-e version and number of lanes. (links lead to questions that explain these aspects in more generic terms)

According to the link, your card is PCI-e v2.0 x8. So to give it all the bandwidth it needs you'll need:

• a half-width (x8) or full-width (x16) slot
• wired with 8 lanes (sometimes slots are longer to fit full-length cards, but don't provide all the lanes)
• at v2.0 or newer

The card may or may not work in a more limited slot, but the best case scenario is you won't be able to utilize all its capabilities - it won't be able to push data fast enough. So we need v2x8 or newer.

Newer version but with less lanes won't cut it. Neither will older version but with more lanes. v3x4 and v1x16 have the same bandwidth as v2x8, but the card won't be able to use neither v3 nor x16, so it will downgrade to v2x4 or v1x8 respectively. Which, again, is not fast enough. Theoretically it could communicate, but won't push data fast enough at full load.

The Precision 3660 link you've provided tells us that it has only one slot that's at least x8: the main x16 slot. It's annotated as "for the GPU only", but that's not necessarily true (only one way to find out). But to use that slot you'd need an integrated GPU or a GPU that will fit in one of the remaining x4 slots (they are physically x4 long and most GPUs are full length, although x4 ones do exist and longer ones can be adapted… or cut down).