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Comment by hinkley | Hacker News Reader

Maybe I'm crazy, but I feel like the big inflection point with these single board computers will really come when they have PCI express slots on them.

There's a whole bunch you can do with peripherals and networking over PCIe that would make those chips useful for a new set of situations.

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I think it all depends on the use case.

For example an IOT device probably doesn't need PCIE as it would be more focused on sensors, power consumption, ultra low BOM, etc.

But in the mid - high end embedded space PCIE is already taking over. WiFi chips are switching from SDIO interfaces -> PCIE (finally!), storage is shifting from eMMC -> NVMe (uses PCIE), NICs have been PCIE for a long time, multi board designs are using PCIE b/w boards, etc.

For the higher end embedded systems that need the latency and throughput PCIE is king.

Other than graphics, is there anything available over PCIe that isn't available over USB?

Any sensor activity that is sensitive to inconsistent latency, like a GPS time source. I don't think you'll find any trusted stratum 1 ntp servers using USB for clock sources.

So use GPIO?

For this specific device? Sure, but that wasn't the question that I was answering.

Right, any low-latency low-bandwith application can use GPIO (or in the case of audio, isochronous USB); high-bandwidth non-latency-sensitive applications use USB.

This leaves just latency-sensitive high-bandwith applications, of which the most common is video.

Just because you can do it over USB doesn't mean you should. PCIe gets you more bandwidth and vastly better latency, and the peripherals that use PCIe tend to be higher quality and have better drivers than their USB competition. Just compare any USB NIC against Intel Ethernet or Atheros WiFi, or NVMe and AHCI devices against the mess that is UASP. You can't build a decent router or NAS around USB devices.

(And DisplayLink offers graphics over USB, but it's so bad that in almost any other context it should be regarded as not counting.)

DisplayLink has a large number of customers, so obviously it works well enough for a lot of use-cases. I've used it for status displays, and the additional monitors that most engineers like to have. I can't even remember which one of the two monitors in my home office is DisplayLink. I'd imagine that I'd notice if I tried to play a game...

On quite a few of these devices, some of the 'on board' connectors are running over USB as well, so they'll never perform even though they're soldered.

TCP/IP over PCIe is a hell of a lot faster than USB, for one. Peripheral management is better as well.

The cost of the PCIe connector starts to be significant then, and even the X1 kind are comparatively huge.

Almost every one of these devices has 3 connectors I'd never use. One PCIe connector would be smaller, especially at X1, and X4 is probably a wash.

Maybe not the same 3 you wouldn't use, but that's part of the 'be all things to all people' problem.

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