Up to 127 devices can theoretically be connected.
The Universal Serial Bus (USB) specification permits the connection of up to 127 devices to a single USB bus. This number includes the USB host controller itself, which counts as one device. Therefore, a computer's USB port, acting as the host, can interface with a maximum of 126 external peripheral devices. It is important to note, as highlighted in the reference from 12-Jan-2023, that this figure is a theoretical limit.
Understanding the Theoretical Limit of 127 Devices
The limit of 127 devices stems from the addressing scheme used by the USB protocol. Each device on a USB bus is assigned a unique 7-bit address, allowing for 128 possible addresses (0-127). Address 0 is reserved for the host controller, leaving addresses 1 through 127 available for connected peripheral devices.
Practical Limitations and Real-World Connectivity
While the specification allows for 127 devices, reaching this number in practice is uncommon and often not feasible due to several factors:
- USB Hubs: To connect more than one or two devices to a single physical port, you need a USB hub. Each hub also counts as a device on the bus. So, connecting numerous devices typically involves a tree-like structure of hubs, with each hub consuming one of the 127 available addresses.
- Bandwidth: All devices connected to the same USB bus share the available bandwidth. High-bandwidth devices like external hard drives, webcams, or video capture devices can quickly consume the bus's capacity, leaving insufficient bandwidth for numerous other devices to function correctly simultaneously.
- Power: Devices draw power from the USB port. While powered hubs provide their own power, bus-powered hubs and devices rely on the host port's power, which is limited. Connecting many bus-powered devices can exceed the available power.
- System Resources: The operating system and system hardware need to manage each connected device, which can consume system resources (CPU, memory).
Practical Connectivity Summary
Here is a simplified look at the limits:
| Aspect | Limit | Notes |
|---|---|---|
| Theoretical Limit | 127 devices (including the host) | Defined by the USB specification's addressing scheme. |
| Practical Limit | Significantly fewer than 127 | Limited by bandwidth, power, and the number of hubs used. |
| Host Port Limit | 1 device directly (without a hub) | Typical physical connection. |
| Limit via Hubs | Can connect many more, up to the bus limit | Each hub uses an address; performance degrades with many. |
Tips for Connecting Multiple USB Devices
If you need to connect numerous peripherals, consider these tips:
- Use powered USB hubs for devices that require significant power or when connecting many devices.
- Distribute high-bandwidth devices across different USB host controllers on your computer if possible (often corresponding to different physical port groups).
- Choose USB devices and hubs that support the highest USB standard your computer's port supports (e.g., USB 3.0/3.1/3.2 or USB4) for better performance and bandwidth.
- Be mindful that performance may degrade when many devices are active simultaneously on the same bus.
In summary, while the technical specification allows for a large number of connections, the actual number of devices you can practically use effectively on a single USB bus is considerably lower than the theoretical maximum of 127.
