UK How a USB hub works
A USB hub is a device that connects to one USB port, called the upstream port, and provides several downstream ports for other devices. It is part of the USB specification itself, so the computer recognises the hub and every device behind it without extra drivers.
Every computer already contains one: the root hub inside the USB controller, which provides the ports on the case. An external hub simply extends that tree. The USB standard allows up to 127 devices on one host controller and up to five hubs connected in series between the computer and a device. In practice, power and bandwidth run out long before those limits.
All the devices on a hub share the single link back to the computer. A hub does not add bandwidth. It divides the bandwidth of the port it plugs into.
Powered vs unpowered USB hubs
The biggest practical difference between hubs is where their power comes from. A standard USB 2.0 port supplies up to 500 mA at 5 V, and a USB 3.x port up to 900 mA.
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Unpowered (bus-powered) hubs. Take all their power from the computer’s port and share it between the downstream ports. They suit keyboards, mice and memory sticks, but a portable drive or camera can draw more than the hub has to give, and devices then disconnect or fail to start.
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Powered (self-powered) hubs. Have their own power supply, so each downstream port can deliver its full rated current, whatever the computer provides. They are the right choice for hard drives, cameras, several devices at once or long cables.
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USB-C hubs with Power Delivery. Pass a charger’s power through to a laptop while also running the downstream ports, using the USB Power Delivery standard.
USB hub speeds and bandwidth
A hub’s speed is set by its USB version, and the total is shared between the devices behind it.
| Hub type | Upstream link | Shared between | Suits |
|---|---|---|---|
| USB 2.0 hub | 480 Mb/s | All ports | Keyboards, mice, scanners, licence dongles |
| USB 3.2 Gen 1 hub | 5 Gb/s, plus a separate USB 2.0 path | All ports | Flash drives, single cameras, card readers |
| USB 3.2 Gen 2 hub | 10 Gb/s, plus a separate USB 2.0 path | All ports | External SSDs, several fast devices |
| USB4 or Thunderbolt dock | 40 Gb/s or more | Ports, displays and Ethernet | Desk setups with screens and networking |
A USB 3 hub is really two hubs in one box: a SuperSpeed hub for USB 3 devices and a separate USB 2.0 hub for older ones, so slow devices do not take bandwidth from fast ones. Inside a USB 2.0 hub, a transaction translator lets slower keyboards and mice share the 480 Mb/s link. Multi-TT hubs have one translator per port and handle several slow devices better.
USB hub vs dock
A hub only adds USB ports. A dock adds other interfaces as well, usually display outputs, wired Ethernet, an SD card reader and laptop charging, all through one USB-C or Thunderbolt cable. Docks rely on features of the USB-C port such as DisplayPort Alt Mode, which is why they need a port that supports them. Our explainer on what USB4 adds over USB 3 covers the bandwidth that makes single-cable docks possible.
USB hubs in industrial systems
On a desk, a hub is a convenience. In a machine, vehicle or cabinet, it is an extra box, power supply and pair of cables that can work loose, and a bus-powered hub can brown out a camera at the worst moment. For permanent installations it is usually better to remove the hub altogether.
Cable length is a second limit. USB 2.0 cables are specified to around 5 m, and USB 3 cables are typically reliable to around 2 to 3 m, so a hub is sometimes used as a repeater to reach further. Active extension cables, or USB over Cat6 or fibre extenders, do that job more reliably. Our white paper on industrial USB applications covers these options and more uses.
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Choose a computer with enough ports. Many of our embedded box PCs provide four or more USB ports, split between USB 2.0 and USB 3.2, so each device gets its own port without a hub in between.
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Add a PCIe USB card for cameras. Machine vision systems that stream from several USB 3 cameras need full bandwidth on every port. USB frame grabber cards give each port its own controller, which a hub cannot.
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Use industrial hubs where a hub is unavoidable. DIN-rail hubs with screw-terminal power, isolation and wide temperature ratings suit control cabinets better than desktop hubs.
Quick answers on USB hubs
USB hubs: common questions
What is a powered USB hub?
A hub with its own power supply. It plugs into the mains or a DC supply as well as the computer, so every port can deliver full USB current to drives, cameras and chargers.
Does a USB hub add latency?
Only a very small amount. A hub adds a few microseconds, which keyboards, mice and most devices never notice. Shared bandwidth matters more than latency when several busy devices use the same hub.
Does a USB hub slow down devices?
Only when they compete for bandwidth. One device on a hub runs at full speed. Several fast devices share the upstream link, so two drives copying at once each get roughly half.
How many devices can I connect to a USB hub?
As many as the hub has ports. USB allows up to 127 devices per host controller, but power and bandwidth set the practical limit well below that.
Can a USB hub charge a phone?
Yes, best from a powered hub. A bus-powered hub shares limited current, so charging is slow. A powered hub, or one with USB Power Delivery, charges at full rate.
We fit the USB ports, PCIe USB cards and connectors each system needs as standard, and test every connected device before the system ships.