I've been looking at USB extension cables recently (type A male to type A female), however I've been surprised by the number of cables describing themselves as USB 2.0 or even USB 2.0 only. However, as far as I can tell the type A connector hasn't changed between USB 2.0 and USB 3.0.

While I realise that a USB 2.0 type A cable should work just fine with USB 3.0 capable devices at USB 2.0 speeds, what I'd like to know is whether a type A to type A cable can in fact be limited to USB 2.0 only speeds, even when plugged into USB 3.0 port and connecting a USB 3.0 device, or if this is simply a mislabelling?

I'm generally wary of this sort of thing, as manufacturers have touted misleading cable compatibility in the past, for example expensive "SATA III" cables, even though any cable suitable for SATA II works just fine, provided it has decent shielding etc.

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A USB 2.0 cable plugged into a USB 3.0 port will only achieve USB 2.0 speeds.

USB3.0 Type A provides 4 pins for USB 2.0/1.x backwards compatibility and 5 pins for the USB 3.0 standard.

USB 3.0 Type A Receptacle Drawing


A USB2 Type A to Type A cable only has 4 pins (and 4 wires) and will be limited to USB2 speeds.

USB3 uses a completely separate bus to maintain backward compatibility with USB2, so there are separate wires.

Even though the Type A connectors appear physically identical at first glance, upon further inspection you'll see that the USB3 Type A connector has 9 pins, compared to USB2's 4 pins.

So for USB3 speeds, you'll generally need a USB3 cable with all 9 pins (and 9 wires).

Hypothetically someone could make a USB3-only cable with just the 5 USB3 pins and wires, but you'd lose backward compatibility with USB2 when using that cable.

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