This article explains USB communication in a simplified way, consisting of 3 basic elements that form the chain of communication.
- USB Host Controllers
- USB Hubs
- USB Devices (Functions)
USB operates in 7 Tiers (layers) of communication.
Tiers describe the order in which ports, hubs and devices (also called "Functions") are connected.
The USB host controller mostly occupies Tier 1 and sometimes Tier 2.
Both hubs and USB extenders occupy Tiers 2 through 6. Hubs, depending on their size, would normally occupy 2 Tiers.
Devices (Functions) are always on Tier 7 since it is the end of the daisy-chain.
Issues can occur when a USB tree goes deeper than Tier 7 with hubs. In such cases, devices are simply not discovered. Take this example:
Laptop [USB Host Controller] (Tier 1 - 2)
Solvo (Tier 3)
USB Extender (Tier 4)
7 port USB Hub (Tier 5 - 6)
Interactive Whiteboard with built-in USB Hub (Tier 7)
In the example shown above, devices plugged into the whiteboard will not be discovered and will note work. In order to solve it, change the chain of USB connections, so that the devices will be connected on a lower tier level.