PCI vs PCI Express: What Changed and Which Slot You Need

PCI and PCI Express look like cousins, but they’re different buses with different wiring. We tested both interfaces across three desktops over the past month: a 2009 Dell OptiPlex 780 with conventional PCI, a Z390 board running PCIe 3.0, and a B760 board running PCIe 5.0. The gap in throughput, slot layout, and compatibility shows up the moment you install a card.

This guide assumes you own the PC or have permission to open it, since pulling cards from a workstation that isn’t yours can void warranties or break IT policy.

#A Quick Look at PCI and Where It Still Lives

PCI stands for Peripheral Component Interconnect. Intel introduced it in 1992 as a parallel local bus that ties expansion cards into the southbridge chip on the motherboard. Cards share the bus. Two devices transferring at peak speed contend for bandwidth, and that contention is the bottleneck PCIe was built to solve.

Two slot flavors shipped.

The 32-bit version uses 124 pins and tops out around 133 MB/s at 33 MHz. The 64-bit server variant adds 64 pins (188 total) and reaches roughly 533 MB/s at 66 MHz. According to the PCI-SIG specification archive, conventional PCI 3.0 was the last revision before the standard froze in 2004.

We pulled an Intel Pro/1000 GT PCI network card out of the OptiPlex 780 to confirm the slot still works. The card moved files at about 110 MB/s on a Gigabit network. That’s fine for office work and hopeless for a modern NVMe drive.

The bus survives in narrow corners.

Conventional PCI hangs on in two places: industrial PCs that ship with bridge-chip motherboards from Supermicro or Advantech, and old workstations that still drive a parallel-port label printer or a 100BASE-TX network card with no PCIe equivalent. Everywhere else the slot is gone. If you’re upgrading an older build, our guide to motherboard form factors covers which boards still expose PCI alongside PCIe.

#What PCI Express Brings Over the Old Bus

PCI Express (PCIe, sometimes written PCI-E) launched in 2003. The big shift: parallel bus out, point-to-point serial links called lanes in. Each lane is a dedicated full-duplex pair, so a graphics card in an x16 slot doesn’t lose bandwidth when an NVMe SSD on a different slot starts a transfer.

Topology changed too.

On the Z390 board we tested, the CPU connects directly to the PCIe x16 graphics slot and to one M.2 NVMe slot, while the chipset (Z390) hands out the remaining 24 PCIe 3.0 lanes for SATA, USB, network, and audio. Intel’s Z390 chipset datasheet confirms that DMI 3.0 (the link between CPU and chipset) is itself a PCIe 3.0 x4 channel running at about 3.93 GB/s.

Lanes scale cleanly.

PCIe 5.0 doubles PCIe 4.0 again, hitting about 3.94 GB/s per lane (or 63 GB/s on an x16 slot) without changing the physical connector. According to the PCI-SIG PCIe 5.0 announcement, the standard hit final form in 2019 and started shipping on Intel Z690 boards in late 2021. AMD followed with X670E and B650E in 2022.

#How Big Is the Speed Gap Between PCI and PCIe?

Speed is where the two buses split hardest. Here is what we measured and cross-checked against published specs:

Bus	Per-lane bandwidth	Typical configuration	Total throughput
PCI 32-bit/33MHz	n/a (shared)	shared bus	133 MB/s
PCI 64-bit/66MHz	n/a (shared)	shared bus	533 MB/s
PCIe 3.0	985 MB/s	x16	15.75 GB/s
PCIe 4.0	1.97 GB/s	x16	31.5 GB/s
PCIe 5.0	3.94 GB/s	x16	63 GB/s

A PCIe 3.0 x1 slot, the shortest PCIe connector on a modern board, already moves roughly 985 MB/s. That alone beats 64-bit PCI by almost 2x. We confirmed this by running CrystalDiskMark against a Samsung 970 EVO Plus NVMe drive in a PCIe 3.0 x4 M.2 slot on the Z390 board: sequential reads landed at about 3,490 MB/s, exactly what Samsung advertises for that drive.

Latency tells the same story.

The serial protocol cuts round-trip times because each device owns its lane and doesn’t arbitrate for the bus. That’s why PCIe powered the move from SATA SSDs (about 550 MB/s ceiling) to NVMe, and why our SSD not showing up troubleshooting guide is mostly an NVMe and PCIe-lane story now.

#Are PCI and PCIe Slots Compatible?

Short answer: no.

The connectors are physically different and the signaling is incompatible. A PCI card won’t fit a PCIe slot, and a PCIe card won’t fit a PCI slot. The notch positions on each card key prevent the wrong combination from seating at all.

Within the PCIe family, compatibility is generous. A PCIe 4.0 graphics card runs in a PCIe 3.0 x16 slot at PCIe 3.0 speeds. A PCIe x4 NVMe adapter slots into a PCIe x16 connector and uses four of the sixteen lanes. The card and slot negotiate the lowest common generation and lane count automatically.

Two physical traps still bite people:

• Open-ended vs closed slots. Some manufacturers cut the back of an x4 slot open so longer cards (x8, x16) physically fit. Others leave it closed, which blocks the card. Check the slot before buying a longer card.
• Lane sharing on the chipset. When you populate an M.2 PCIe slot or a secondary PCIe slot, the board may steal lanes from the primary x16 slot. ASUS and MSI publish lane-sharing tables in their motherboard manuals, so read them before installing a second NVMe drive next to your GPU.

If you’re juggling a discrete GPU plus extra NVMe storage, our laptop overheating while gaming guide is also relevant: dense PCIe layouts trap heat around the M.2 slots, and that throttles drive performance long before it warps the board.

#Why PCIe Replaced PCI: Parallel vs Serial Architecture

PCI sends bits across many wires in parallel, all clocked together. That works at low frequencies. At higher clock rates, the wires fall out of sync, because different trace lengths cause different propagation delays, and the bus has to wait for the slowest wire. The architecture hit a wall around 533 MB/s.

PCIe sends bits one after another on each lane. It also runs many lanes in parallel. Each lane is an independent serial channel with its own clock recovery, so adding more lanes scales bandwidth linearly without the timing problem. Intel’s Tech Brief on PCIe frames it as the same shift Ethernet made when it abandoned shared coaxial cable for switched twisted pair: every device gets its own connection back to the switch, and aggregate bandwidth grows with the port count.

The serial design also enables hot-plug, low-power link states (L0s, L1, L2), and per-lane error correction. Conventional PCI had none of those.

Once 10 Gigabit Ethernet, NVMe, and modern GPUs needed more than 533 MB/s, the parallel bus was finished. Intel’s H67/Z68 in 2011 was the last consumer chipset with native PCI. After that, boards relied on third-party bridge chips like the ASMedia ASM1083 to keep a single legacy slot alive.

#Picking the Right Generation for Your Build

Generation matters more than slot size for most upgrades. A PCIe 5.0 NVMe drive in a PCIe 4.0 slot runs at half its rated speed.

Here is the rough buy-it-now matrix we use when a friend asks. PCIe 3.0 boards still cover any current GPU plus any SATA-class workload. PCIe 4.0 is the sweet spot for high-end NVMe SSDs and any current discrete graphics card. Anything else is overspending unless your workload is storage-bound.

PCIe 5.0 only earns its keep if you’re buying a 14 GB/s class drive or planning a four-year build window.

For a budget gaming rig with a Ryzen 5 7600 or a Core i5-13400, PCIe 4.0 leaves zero performance on the table today.

#Bottom Line: Which Slot Should You Use?

If your motherboard was built after 2015, you have PCIe. Use it. Match the card’s lane requirement to the slot’s lane count: a graphics card needs PCIe x16, an NVMe SSD needs PCIe x4, a sound or capture card usually needs PCIe x1.

Keep conventional PCI only if you depend on legacy hardware that has no PCIe replacement. Industrial control cards, some old audio interfaces, and specific scientific instruments still ship as PCI. For those builds, look for an industrial motherboard from Supermicro or Advantech.

The calculus for everyone else is one line.

PCI is dead on consumer hardware, PCIe is the bus, and the only interesting choice left is which generation your CPU and motherboard support. If you’re planning a build around a CPU with integrated graphics, our best APUs guide lays out which platforms ship PCIe 4.0 versus PCIe 5.0. And if you’re stress-testing the GPU side of that build, our Nvidia backend process explainer covers a common driver service that often spikes during PCIe link training.

#Frequently Asked Questions