USB Hubs
A passive hub (or bus-powered hub) is a hub that draws all its power from the host computer's USB interface. Often, USB connected devices require more power and will not work in this type of hub.
An active hub (or self-powered hub) has it’s own power supply so can provide full power to each of its ports.
Powered versus unpowered hubs
It's important to use the correct type of Hub as the upstream port may not be able to supply enough power to the downstream outputs of a connected Hub. This could damage the upstream port by drawing too much current. In addition, some devices connected to the hub may not work or present issues.
Watch out for the shonky hubs: (The ‘active hub’ claim being just a selling point for the Hub)
- Some passive hubs wrongly report themselves upstream as powered.
- Some active hubs do not have a big enough power supply power every port. They save money on cheap power supplies and circuitry by assuming most devices plugged in are low power.
A USB 3.0 hub is the logical combination of a USB 2 hub and a USB 3 hub. The USB 2.0 hub component is connected to the High-Speed data lines of a USB cable and the USB 3 hub component is connected to the Super-Speed data lines of the USB cable. As such, the USB 3.0 hub presents itself upstream as two devices, a USB 3 hub on the Super-Speed bus and a USB 2 hub on the High-Speed bus.
A maximum of 7 tiers (or levels) or a maximum of 5 hubs in a chain from the host (or computer) to a device.
A single hub might have two levels of hubs in the one device. For example, a common implementation of a 7 port hub is to utilise two 4 port hubs. The 4th port of the first hub connects to the input of the second 4 port hub. As such, the first 3 ports are at one layer, and the following 4 ports are an extra layer down in the USB hierarchy. While that 7-port hub might on the surface, appear to be just one layer because it’s just one box, it’s effectively two layers in the USB chain.
Hubs might also be in unexpected locations. For example, the bulge in the middle of a 10m Active USB cable is a two-port hub. This is necessary because a USB cable can only be 5m long before requiring a hub. The 10m cable is electrically two 5m cables with a Hub in the middle. This also adds one extra level of a hub which must be included in the end to end count of a maximum of 5 hubs.
A very useful and free application to figure out what USB port is connected to what and what it is called is an application called UsbTreeView. The application can be used to document all the USB connections to allow a complete understanding of any USB configuration.
UsbTreeView can be found at https://www.uwe-sieber.de/usbtreeview_e.html
Microsoft Windows will associate a physical USB port to a software COM port. As such, it’s advisable to always plug your gear in the same way (ie. use the same plug spots on the computer and if USB hubs or active cables are used, reconnect to all the same ports on the hubs etc.).