USB fundamentals for PC technicians How many peripheral devices can be addressed on a single USB host controller or hub tree (USB logical bus), excluding the host itself?

Introduction / Context:Universal Serial Bus (USB) is ubiquitous for connecting keyboards, mice, storage, and many other peripherals. Technicians frequently need to know the addressing limits of a single USB host controller when planning hub deployments and troubleshooting enumeration issues.

Given Data / Assumptions:

• The question concerns the logical addressing capacity of one USB host controller tree (root hub plus downstream hubs and devices).
• USB specification reserves address 0 during enumeration.
• Counting refers to total device addresses, including hubs and functions, not the number of physical ports on any one hub.

Concept / Approach:

Per the classic USB design (USB 1.x/2.0), the bus provides 7-bit addresses for devices. One address is reserved for the default state during setup, leaving a maximum of 127 usable addresses for attached functions and hubs on a single logical bus.

Step-by-Step Solution:

Verification / Alternative check:

Hub planning guidelines and certification tests reference the 127-device limit per host controller tree, confirming this capacity independent of physical port counts.

Why Other Options Are Wrong:

125 and 100 are arbitrary and below the specification limit. 225 exceeds the address space. “None of the above” is incorrect because 127 is a standard, well-known value.

Common Pitfalls:

Confusing the number of physical hub ports with the total logical device limit; forgetting that hubs themselves consume addresses; assuming multiple root ports on a motherboard share the same logical bus when each host controller has its own 127-device capacity.

Final Answer:

127.