How it works: US B

One simple socket that’s evolved throughout the years

The Universal Serial Bus (USB) arrived on 6 May 1998, at Flint Center, Cupertino. Specified by an Intel-led industry partnership, it had already appeared in a few PCs, but it was only when Apple unveiled the first-gen iMac, with the traditional interfaces abolished, that we knew USB was the future.

USB 1.1, capable of speeds up to 12Mbps, was “the new generation of I/O,” announced Steve Jobs. Simple, he said. Elegant, he said. And he was basically right. Although, if he’d envisaged that 20 years later USB would cover two data protocols running over six physical connections in five speed classes, he might have used some different words.

The iMac’s ports were USB Type-A, the now familiar rectangular receptacle with a plastic tab across the middle. Its mouse and keyboard had matching steel-sheathed plugs attached, while peripherals like disk drives and printers would come with square USB Type-B ports, connecting with an A-to-B cable that established a two-way data flow and a one-way power supply, from computer (host) to accessory (device).

Socket to me

USB 2.0, which arrived in 2001, boosted transfer rates to 480Mbps, competing with FireWire 400, which Apple had added to the iMac DV for video transfer. Smaller accessories, like digital cameras, didn’t have room for the full-size Type-B socket, and some manufacturers had invented their own. Now there were two official solutions: the sharp-cornered trapezoid Mini-B, and Micro-B.

What made USB so successful was that the spec covered more than just the physical connection and communication protocols. It defined device classes — printer, mass storage, communications

(networking); and human interface (keyboards and mice) — so that ‘class compliant’ products could identify themselves to the computer and start working, with little need to install drivers. This also meant Macs and Windows PCs could use the same USB devices.

Generation gain

Nearly a decade later, USB 3.0 — branded SuperSpeed and indicated by an SS logo and a blue insert in the Type-A port — multiplied transfer rates tenfold to 5Gbps. But with more pins required, Micro-B had to be upgraded to what looked like two plugs stuck together. USB On-The-Go (OTG) devices got the even more esoteric Micro-AB socket. OTG allowed role-swaps between computer and device, so a phone could supply data to and draw power from a Mac as an accessory, but then connect to a camera or keyboard as a host. USB 3.0 killed FireWire, but its new rival was Thunderbolt. After the USB Implementers Forum (USB-IF) declined to back it, Thunderbolt was shoehorned into Mini DisplayPort, creating a 10Gbps connector. USB 3.1, arriving in 2014, matched Thunderbolt’s speed, but only in Gen 2 ports, branded SuperSpeed+. Thunderbolt retained the advantage of daisy-chaining, allowing up to six devices per port, versus USB’s one.

Mobile hone

Smaller devices remained resistant to standardization, and in 2003 the iPod switched from FireWire not to USB, but to Apple’s own 30-pin Dock connector. Even so, it was USB-compatible, with a Type-A plug on the other end of the cable.

The same arrangement was used for the iPhone in 2007, but when it began to feel out of scale with modern devices, Apple replaced it (in 2012) with Lightning: as compact as Micro-B, yet reversible and with a simpler plug, like the tab in USB ports, and a hollow socket.

USB Type-C emerged from the USB-IF two years later, similarly compact and reversible — finally scotching the joke that there were four ways to insert a USB plug, three of them wrong — but retaining the form of a tab inside a receptacle, with a hollow plug fitting concentrically. Most Type-C ports are USB 3.1 Gen 1 or 2, but just to complicate matters, Thunderbolt 3 jumped formats and became a superset of USB Type-C.

Every Thunderbolt 3 port will work with any USB Type-C cable and device, but not every USB Type-C port supports Thunderbolt 3 devices. All Thunderbolt 3 cables work with USB Type-C devices, and good-quality USB Type-C cables will work for Thunderbolt 3 devices, but you’ll only get Thunderbolt 3’s maximum 40Gbps with a 0.5m or shorter cable, unless you buy an ‘active’ Thunderbolt 3 cable with electronics that boost the signal.

40Gbps should satisfy most of us for a while, but with Apple now sitting on the USB-IF board, what comes next will be interesting. Adam Banks