The Death Of The PCI Express Expansion Card

The Death of the PCIe Expansion Card

With the AM5 platform from AMD on the horizon, five major motherboard manufacturers have annonced their flagship motherboards with the X670E chipset. Some of them are having fun with this generation’s multi-faceted step into “five”: AM5, PCIe Gen 5.0, DDR5, 5nm process, boost clocks over 5GHz, you catch the drift. But do you know what every single announced motherboard has fewer than five of? PCI Express (PCIe) slots.

PCIe, In Brief

As a quick high-level overview, a CPU has a given number of PCIe lanes of a given generation (speed) that can be assigned as the motherboard chooses between any number of devices -- usually PCIe slots, M.2 slots, and the chipset. Zen 4 (along with Zen 3 and Zen 2 before it) CPUs have 24 PCIe lanes, and most motherboards divvy them up as such: 16 lanes go to the x16 PCIe slot intended for use by the graphics card; 4 lanes go to the M.2 slot intended for use by the boot drive; and 4 lanes go to the chipset. The chipset then acts as a PCIe switch, enabling additional PCIe and M.2 slots to exist and operate with a bit more latency, as they sometimes have to wait for their turn to send/receive data.

Slots vs. Lanes

If you need a lot more PCIe lanes because switching would be too slow or too high-latency, high-end workstation CPUs like Threadripper or server CPUs like EPYC have more lanes for more connections to the CPU without going through the chipset. But most casual, enthusiast, low-end workstation, home theater, and gaming PCs don’t need that many directly-connected lanes. Common PCIe expansions in those kinds of systems rarely need to be sending and receiving data at the exact same millisecond as each other, and even rarer that the latency of waiting their turn in the chipset’s switching queue would matter enough to spend five times the money on a system built around CPUs with a higher PCIe lane count.

The Wonderful World of Expansion Cards

The kinds of useful expansion and upgrades that can be had via a PC’s PCIE slots, even through the chipset, include:

An array of many extra USB ports for extra HCI devices
~~USB 3.2 Gen 1~~ ~~USB 3.2 Gen 2~~ ~~USB 3.2 Gen 2x2~~ ~~USB 4~~ USB 4 Version 2.0 ports so you can upgrade to the latest and greatest external peripherals
Thunderbolt ports so you can attach monitors, USB devices, and even externally-mounted PCIe cards with a single daisy-chainable cable
WiFi cards so you can get higher network speeds and lower latency without a cable
Network cards so you can have additional and/or faster Ethernet ports
TV tuners so you can receive, watch, and time-shift local over-the-air content
Video capture cards so you can stream or record video feeds from game consoles or non-USB cameras
SATA or SAS adapters so you can attach additional hard drives and SATA SSDs
M.2 adapters so you can attach additional NVME SSDs
Sound cards so you can run a home theater surround sound system from your PC without needing a receiver (RIP Windows Media Center)
Legacy adapters for older devices that use serial, parallel, or PCI (non-Express) to connect to the computer
And finally, the belle of the PCIe ball, graphics cards.

Falling Slot Counts

In their infinite wisdom and knowing how useful these PCIe expansions can be for extending the life and usefulness of your computer, case manufacturers have continued to add covers for PCIe slots aplenty. By way of example, Fractal Design’s Define series of PC cases range from the gargantuan Define 7 XL with its 12 slot covers to the tiniest option, Define 7 Nano, which still manages to find room for 3 slot covers. In between you’ll find the Define 7 Mini (4), Define 7 Compact (7), and Define 7 (9). However, motherboard manufacturers have begun to let that potential go to waste. Even though all of the motherboards announced so far for the AM5 platform by five distinct manufacturers are the ultra-high-end X670E variety, not one includes more than four PCIe slots. Why?

Actually, I have a few ideas why.

Built-In Functionality

First and foremost, many of the things in the above list of uses for PCIe slots now come built into motherboards. Some announced motherboards already include 10GbE LAN, WiFi 6E, USB 4 and/or Thunderbolt 4, and a multitude of lower-speed (but still high-speed) USB-A and/or USB-C ports. These things are considered value-adds by motherboard manufacturers: ways for their premium products to stand out from the competition, including their own budget offerings. And each inclusion deals a double-whammy to the inclusion of more PCIe slots. First, it reduces the number of people who need to include an expansion card for that functionality. And second, it uses up some of the switching bandwidth that the chipset would otherwise be reserving for PCIe slots.

A Mismatched Pace of Innovation and Upgrades

Second, technology has continued to move at such a fast-yet-slow pace that the dream of moving all of your add-on cards from one PC to the next is mostly dead. PCs are powerful enough now (and recently, PCs were stagnant enough) that most people don’t need to build a new PC every couple of years. And by the time you’re building a new PC the USB 3.0 ports, Wireless N card, Bluetooth 4.0 adapter, and 1080p capture card are woefully out-of-date and begging to be replaced by USB 4, WiFi 6E, Bluetooth 5.3, and a capture card that can grab 4K 120Hz feeds from today’s consoles and mirrorless cameras. Thus the potential cost savings from paying less for each motherboard but more to buy into PCIe add-on cards that move between motherboards evaporates for most people. Even though you pay more for each motherboard, you save money long-term.

M.2 Is More

It’s also true that some of the space and bandwidth that could’ve gone to PCIe slots is instead being dedicated to M.2 slots. I was bewildered at this because I couldn’t imagine anybody who would find 5 NVME SSDs and two PCIe cards more useful than the inverse -- but then I realized that because the M.2 slot is still PCIe electrically, it’s not just “for SSDs” any more than PCIe slots are “for graphics cards.” In fact, you can buy SATA adapters, WiFi cards, and more in the M.2 form factor. Heck, you can even adapt an M.2 slot to a PCIe x4 slot, assuming you can find anywhere to mount the PCIe card. (By the way, if you know somewhere less sketchy than a very-medium-looking Ali Express listing to buy those adapters, please let me know.)

USB And Thunderbolt

And finally, the decisive victory of laptops over desktop PCs in the marketplace combined with the incredible capability of USB4 and Thunderbolt 3 (and 4) means that many peripherals that would have been a PCIe expansion card in the past are instead USB/Thunderbolt devices these days. There’s enough electricity and bandwidth in those slots to take care of almost any non-GPU, non-NVME use case from the above list of expansions: TV tuners, capture cards, storage, sound cards, and if you want to go really meta: PCIe slots. And by choosing an external form factor and port, manufacturers widen their market to include “everybody with a new-ish laptop” without actually excluding desktop PC users.

Moving On

My current desktop PC, the one I wrote this article on, has been my technological Ship of Theseus. When I build a Zen 4 PC next quarter, I’ll retire most of my PCIe expansion cards, and my PC will truly be new for the first time in a long time. But I won’t be giving up any functionality or expandability, even if it’ll take a different form (and clutter my desk more) from now on.

Kerrick Long’s

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