USB, Thunderbolt, and cables that charge but do not carry data

Why this matters

USB-C is the first connector in computing history where the plug tells you almost nothing about what the port does. The same shape may carry 480 Mbps or 40 Gbps, may deliver 2.5 W or 240 W, may output video or not, and may be a Thunderbolt port or not — in any combination.

That is a genuine problem in the field and the exam treats it as one. The questions are about identifying capability, choosing a cable, and explaining why a working-looking setup does nothing.

Lesson 4 covered the mobile side of this. Here the concern is the standards themselves, the power negotiation, and the diagnostic method — because a technician who can test a suspect cable with what is already on the bench solves this class of problem in a minute.

The lesson

USB generations and their renamings, and how to read what a port actually is

The speeds, in order, with the names they have been given:

• USB 1.1 — 1.5 Mbps low speed and 12 Mbps full speed.
• USB 2.0 — 480 Mbps, "High Speed". Still everywhere, and still what most keyboards, mice and printers use.
• USB 3.0 / 3.1 Gen 1 / 3.2 Gen 1 — 5 Gbps. Three names, one speed.
• USB 3.1 Gen 2 / 3.2 Gen 2 — 10 Gbps.
• USB 3.2 Gen 2x2 — 20 Gbps, using two lanes, USB-C only.
• USB4 — 20 or 40 Gbps, built on the Thunderbolt 3 protocol, USB-C only.
• USB4 Version 2.0 — up to 80 Gbps.

The renaming is why the marketing names are unreliable and the number of gigabits is the thing to hold. Recent branding has moved toward naming ports by speed for exactly that reason.

How to read a port in practice:

• Colour is a hint, not a standard. Blue often means 5 Gbps, teal 10 Gbps, yellow or red often means always-on charging. Manufacturers vary.
• The symbol beside the port is more reliable: the SuperSpeed logo, a lightning bolt for Thunderbolt, a battery or "SS" with a number.
• The system's own device information is definitive, and on a laptop the manufacturer's specification page is the fastest route.

Two constants worth carrying: everything is backward compatible, so a device always works at the lower of the two speeds; and a USB 2.0 cable in a USB 3 port limits the link to 480 Mbps, which is the most common cause of "the external drive is slow".

Connector shape against capability: why the same plug does four different jobs

USB-C carries four largely independent capabilities, and any port may implement any subset:

1. USB data, at any of the speeds above.
2. Power Delivery, at any of several wattages.
3. DisplayPort Alternate Mode, for video.
4. Thunderbolt or USB4, which includes PCI Express tunnelling.

A port that does all four looks exactly like a port that does only the first. That is why the field advice is to check the specification rather than the shape — and why lesson 4's dock symptoms are diagnostic: each symptom names which capability is missing.

Cables share the problem. A USB-C cable may be:

• Charge-only, with no data conductors at all. Extremely common in the box with phones and power banks.
• USB 2.0 data plus power, which is most cheap cables.
• Full-speed data, which requires more conductors and better shielding.
• Thunderbolt or USB4 certified, which is the most capable and the most expensive, and which is required for 40 Gbps.

Passive USB-C cables are limited in length for high speeds; long runs need active cables with electronics in the connector.

Power delivery: negotiation, wattage, and the cable that limits charging

USB Power Delivery is a negotiation. The sink states what it can accept, the source states what it can provide, and they agree on a voltage and current. Without that negotiation a port delivers the legacy baseline — 5 V at a low current, enough for a mouse and not for a laptop.

The examinable points:

• Standard USB-A ports deliver small amounts: 2.5 W on USB 2.0, 4.5 W on USB 3.
• Power Delivery negotiates higher voltages — 9, 15, 20 V and above — to reach 60 W, 100 W, and in the latest revision 240 W.
• The cable is part of the negotiation. Cables above 60 W must contain an identifier chip declaring their rating. A cable without it is capped at 60 W no matter what the source and sink can do.
• Power direction is negotiated too. Two devices that both want to be the source will not charge each other.

So "my laptop charges slowly on this charger and fast on that one" has three candidate causes: the charger's maximum, the cable's rating, and what the laptop requested. All three are readable — chargers and laptops are labelled, and most operating systems report the negotiated wattage.

Thunderbolt over the same connector, and how to tell whether a port supports it

Thunderbolt 3 and 4 use the USB-C connector and carry PCI Express and DisplayPort over it, at 40 Gbps. That PCI Express tunnelling is what makes external graphics enclosures and very fast external storage possible, and it is what distinguishes Thunderbolt from plain USB at the same speed.

Identification:

• The lightning bolt symbol beside the port. This is the reliable marker.
• Thunderbolt 4 guarantees a minimum feature set: 40 Gbps, at least 32 Gbps available to PCI Express, two 4K displays or one 8K, and charging support on at least one port.
• USB4 adopted the Thunderbolt 3 protocol, so the two are closely related and largely interoperable, but USB4 makes several features optional where Thunderbolt 4 requires them.

Compatibility runs one way: a Thunderbolt port accepts USB-C devices; a plain USB-C port does not run Thunderbolt devices. A user who bought a Thunderbolt dock for a machine without a Thunderbolt port has bought a partially working dock, and the missing parts are precisely the ones that need PCI Express.

The security note the exam expects: PCI Express tunnelling means a Thunderbolt device has a path toward direct memory access. Modern systems mitigate this and require authorisation for new devices, but the principle — do not connect unknown Thunderbolt devices — is the same reasoning as not connecting unknown storage.

Testing a suspect cable with equipment you already have

This is the practical skill, and it needs nothing bought.

Is it charge-only? Connect a device that appears as storage — a phone in file transfer mode, a flash drive with an adapter. If power flows and no device enumerates, the data conductors are absent or broken. Confirm by substituting a cable known to carry data.

Is it a slow cable in a fast port? Check the negotiated speed in the system's device information, or copy a large file and observe the rate. A 5 Gbps link should comfortably exceed 200 MB/s to a fast drive; a link sitting at 30 to 40 MB/s is running at USB 2.0 speed.

Is it the cable or the port? Substitute one at a time. Same cable, different port; same port, different cable. Two tests isolate it definitively.

Is it intermittent? Move the cable at each end while a transfer runs. A transfer that stalls when the connector is flexed has found a broken conductor at the strain point, which is where they always break.

Is it a power problem? Check whether the device works on a powered hub or a different port. Bus-powered drives and hubs are the classic case: a device that works on one port and not another is usually drawing more than the weaker port supplies.

Recording the result matters as much as getting it: "works at 480 Mbps on any port, so the cable is USB 2.0" is a finding a colleague can act on, and "something is wrong with the USB" is not.