How to connect 127 USB devices to your PC. Kinda...

USB galore
(Image credit: Linus Tech Tips)

Who hasn't wondered or dared to dream exactly how many USB devices you can connect to a single PC? Or how many unpowered USB hubs you can daisy chain? Or what about daisy chaining powered USB hubs or the limits to slotting in USB controller cards?

The possibilities are almost endless, with the emphasis on almost. Because YouTube channel Linus Tech Tips has the answers to all these questions, and more.

For reasons that we won't bore you with, the theoretical limit for USB devices connected to a single PC is 128, but the practical limit is one fewer. At least, that's the, er, theoretically practical limit.

In actual practice, it typically turns out to be far fewer. That's usually down to the USB hubs being used to expand the number of ports and how they're connected.

In reality, pretty much every physical USB hub actually contains multiple internal hubs. And however many hubs there are, you can usually double that ostensible number because each hub in any reasonably recent device can typically run in both USB 2.1 and USB 3.1 modes.

The rough upshot is that if you attempt to daisy chain three of any given hub, Windows will get cranky on you and inform you, "Error. Too many USB hubs. A hub will not function when it's connected more than 5 hubs away from the root port."

In simple terms, each hub acts like a double hub, so three of them daisy chained is actually six hubs away from the system root USB port. You'll also very quickly run into power delivery problems using unpowered hubs. So, there's zero chance of hitting 127 devices using unpowered hubs.

Even using larger 16-port powered hubs connected separately to the PC can create problems, because such physical hubs, again, contain numerous internal USB hubs.

So, to really test the limits of USB device support, what you really need is a crypto mining motherboard with a zillion PCIe slots, each of which can house a USB controller card and thus avoid the need to daisy chain external hubs.

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Your next problem is finding enough USB devices to plug in. And no, you can't just use a load of cheap USB memory sticks because Windows will run out of letters of the alphabet to with to assign to the drives.

Anyway, armed with 20-odd USB sticks, a rack full of keyboards and a basket of mice, plus that ex-crypto platform stuffed with USB cards, Linus Tech Tips managed to get the USB device count up to 110 before the system suffered some kind of crisis, possibly driver related.

The channel also noted that, at that point, the CPU was running at 30 to 40 percent load just sitting at idle with over 100 USB device connected.

Anyway, despite the eventual limitations and not quite achieving the 127-device maximum, it's impressive to see just how tolerant a PC is to loading it up with far more USB devices than you're ever likely to need and in countless configurations.

Jeremy Laird
Hardware writer

Jeremy has been writing about technology and PCs since the 90nm Netburst era (Google it!) and enjoys nothing more than a serious dissertation on the finer points of monitor input lag and overshoot followed by a forensic examination of advanced lithography. Or maybe he just likes machines that go “ping!” He also has a thing for tennis and cars.