How Devices Connect (USB, PCIe, GPUs & Peripherals)
You've plugged in a thousand USB sticks, slotted a graphics card into a motherboard, or stared at a device that should work and quietly didn't. Most of us treat the ports on a machine as magic holes: push the connector in, hope a light comes on. When it works, great. When it doesn't - "device not recognized," a GPU the system swears isn't there, a webcam that's invisible - you've got nothing to reason with.
This guide gives you something to reason with. There are really only two ways the outside world reaches the brain of a computer: a slow, friendly, universal door (USB) for the things you plug and unplug all day, and a fast, internal highway (PCIe) for the heavy hardware that lives inside the case. Once you can see those two paths - and the little translator (the driver) that sits at the end of each - every port, slot, and "why won't this work" stops being mysterious.
How to read this
- Here to fix one thing? If a device isn't being detected, start with Phase 1: USB & the Host/Device Model - detection and drivers live there. If you're choosing a slot for a card, jump to Phase 2: PCIe.
- Want it to finally make sense? Read in order. Phase 1 builds the mental model (host, device, enumeration, driver) that the later phases reuse.
The phases
- USB & the Host/Device Model - the universal port: one host (the computer) in charge of many devices, how plugging something in leads to it being detected and a driver loaded, hubs and daisy-chaining, and the genuinely confusing USB naming and USB-C situation.
- PCIe - the High-Speed Internal Highway - the expansion bus inside the case: lanes as parallel data paths, the slots on the motherboard, and what plugs in (GPUs, NVMe SSDs, network cards). Why lane count and generation matter.
- GPUs & Peripherals - why a GPU exists (massively parallel work: graphics, and now ML), how it connects and gets fed data, and a short tour of how everyday peripherals - keyboard, mouse, display, webcam - present themselves to the system.
This is the physical/electrical side of how parts talk to each other. The story of how data actually travels between the CPU, RAM, and these buses lives in How Data Moves Inside a Machine; the software layer that turns "a device exists" into "an app can use it" is covered in What an Operating System Is.