USB Reference Guide

USB is genuinely simple technology wrapped in some of the most confusing naming in consumer electronics. A connector that looks identical can run at 480 Mbps or 80 Gbps. A cable that charges a laptop at 240W might move files at 2001 speeds. The same standard has been renamed three times without changing.

This reference decodes all of it - versions, speeds, connectors, Power Delivery, and Thunderbolt - and shows how to check what your own hardware is actually doing, on Mac and on Windows.

Start here: what are you trying to find out?

USB versions and speeds

Every USB version, in one table

The renaming mess, decoded

The USB Implementers Forum has renamed the 5 Gbps standard three times without changing it. This is the single biggest source of USB confusion, and it’s worth internalizing:

The rule: ignore the version number entirely and read the Gbps figure. A product advertising “USB 3.2” with no Gen suffix is usually the slowest of the three, because if it were faster the marketing would say so.

The USB-IF now recommends plain speed labels - USB 5Gbps, USB 40Gbps - which is a real improvement, but a decade of old naming remains on packaging and in operating systems.

Why you see a different speed than advertised

The negotiated speed is set by the slowest link in the chain. Four things can cap it:

• The cable. The most common culprit by far. A USB 2.0 cable caps a 10 Gbps drive at 480 Mbps, and USB-C cables give no visual clue about their speed tier.
• The port. Laptops frequently mix port types - one Thunderbolt port and one plain USB-C port that looks identical.
• The device. The drive or peripheral may simply not support the faster tier.
• Overhead. Real throughput always lands below the theoretical maximum; 5 Gbps yields roughly 400 MB/s in practice, not 625 MB/s.

The fix is always the same: read the negotiated speed - what the two ends actually agreed on - then change one variable at a time. USB Connection Information shows that number live on Mac and Windows, so you can swap cables while watching it update.

Connectors

USB-A, USB-B, and USB-C

USB-A - the familiar rectangle. Runs USB 1.1 through USB 3.2 Gen 2 (10 Gbps). One-way insertion. Still the most common port in the world.

USB-B - square with beveled top corners, used on printers and audio interfaces. Largely legacy, though USB-B still appears on studio gear.

USB-C - small, oval, reversible. Carries data, video, audio, and up to 240W of power in one connector. Everything from USB 2.0 to USB4 v2.0 and Thunderbolt 5 uses this shape.

Micro-USB / Mini-USB - older small connectors. Micro-USB persists on budget devices and is nearly always capped at USB 2.0 speeds.

Why USB-C is so confusing

Because USB-C is a connector shape, not a capability. Everything else about it is optional. Two identical-looking USB-C ports can differ in:

• Data speed - 480 Mbps to 80 Gbps
• Power - 15W to 240W, in or out
• Video output - full DisplayPort, or none at all
• Thunderbolt / PCIe - present or absent

Note row three: high wattage does not imply high speed. Plenty of 240W cables carry USB 2.0 data only - we’ve read the chip data proving it in the eMarker guide.

Can USB-C carry video?

Yes, through Alternate Modes, where the connector’s high-speed pairs are reassigned to another protocol:

• DisplayPort Alt Mode - the common one, up to 8K
• Thunderbolt - carries DisplayPort natively, plus PCIe
• HDMI Alt Mode - defined but effectively abandoned; HDMI output is nearly always via an adapter

Not every USB-C port supports video. A port that charges your laptop perfectly may output nothing to a monitor - the manufacturer chose not to wire it.

USB Power Delivery

How Power Delivery works

Older USB supplied a fixed 5V and hoped for the best. USB Power Delivery (USB-PD) replaces that with a negotiation:

1. You plug in. Before meaningful power flows, the charger advertises a menu of profiles - voltage and current pairs it can supply.
2. Your device examines the menu and requests the best profile it supports.
3. The charger switches to that voltage, and both sides agree on a contract.

This is why a 100W charger might deliver 60W to your laptop: the two sides found no better mutually supported profile, or the cable couldn’t carry the current.

Anything above 60W requires an e-marked cable rated for 5A. Above 100W you’re into Extended Power Range (EPR), which needs both an EPR charger and an EPR cable.

There are also two adjustable variants: PPS (Programmable Power Supply) and AVS (Adjustable Voltage Supply), which let a device request a fine-grained voltage rather than a fixed rung - used for efficient phone fast-charging and high-tier laptop charging respectively.

Why your device charges slowly

In order of likelihood:

1. The cable is 3A-rated, capping you at 60W no matter what the charger can do.
2. No matching profile - your laptop wants 20V but the charger’s ladder tops out at 15V.
3. The port is shared. Multi-port chargers split a total budget; using a second port can drop the first from 65W to 45W.
4. The charger is smaller than you think. Advertised wattage is often the sum of all ports, not what one port delivers.

The only way to settle it is to read the profiles the charger actually advertises and the contract actually negotiated. That’s what USB Connection Information shows on macOS, and what every charger in our tested library has been measured for.

USB vs Thunderbolt

Thunderbolt is a certified superset of USB4. Every Thunderbolt 4 port is a valid USB4 port, but not every USB4 port meets Thunderbolt’s floor. The practical difference is the guaranteed minimum: a USB4 port may legally deliver half the bandwidth and support one display instead of two.

For distinguishing them on your own machine, see Thunderbolt 4 vs USB4.

How to check USB on your computer

macOS

• USB Connection Information - live negotiated speed, PD profiles, and device identity from the menu bar.
• System Information - Apple menu → About This Mac → More Info → System Report → USB. Shows a Speed field per device, but doesn’t update live.
• Terminal - system_profiler SPUSBDataType prints the same data, verbosely.

Most Macs from 2012 support USB 3.0 or better; every Apple Silicon Mac supports USB4 or Thunderbolt on its USB-C ports.

Windows

• USB Connection Information - live negotiated speed per device from the system tray.
• Device Manager - right-click Start → Device Manager → View → Devices by Connection. Shows the controller hierarchy, but not negotiated speed.
• USBView - Microsoft’s free utility, more detailed per-port information, not live.

Windows labels speeds with the old marketing names. Translate them:

One caveat worth knowing: Windows does not expose USB-C cable e-marker data or charger PD profiles to applications the way macOS does. Speed readings are available on both platforms; detailed charging analysis is a macOS capability today.

A short glossary

• Negotiated speed - what the two ends actually agreed on, as opposed to what either is capable of. The only number that matters.
• E-marker - a chip in a USB-C plug declaring the cable’s speed and current ratings. Full guide.
• PD profile / PDO - one voltage-and-current pair on a charger’s advertised menu.
• Contract - the profile the device and charger settled on.
• EPR - Extended Power Range, PD 3.1’s above-100W tier, up to 240W at 48V.
• PPS / AVS - adjustable-voltage PD modes for efficient fast charging.
• Alt Mode - reusing USB-C’s high-speed pairs for another protocol, usually DisplayPort.
• SOP′ - the addressing channel used to talk to a cable’s e-marker chip rather than the device on the far end.

Related guides

• How to check USB-C cable speed
• USB-C eMarker chips explained
• Why is my device connecting at USB 2.0 speed?
• Thunderbolt 4 vs USB4
• Monitoring USB-C Power Delivery on a Mac
• Charging speed vs data speed
• All guides · Chargers we’ve measured