Making Sense of the Nonuniversal Technology of USB

Any engineer old enough to see — or design — consumer electronics in the 1990’s knows the trauma of a world without standard data and power connections. This “pre-” universal serial bus (USB) age is why you still have a drawer filled with cables of various shapes and sizes.

A USB-C receptacle connector. (Image: DigiKey, Amphenol ICC.)

The universal in USB may feel like a misnomer — given all the available options. But before its inception, every company and model had proprietary power and data linkups. It’s taken time, but USB has significantly whittled the field. Now, the engineering looks to simplify further as USB-C may become the truly-universal USB.

What about all of those different USB options?

When designing USB compatible electronics, engineers still need to make various decisions. Here is a basic rundown of what you need to know. First, USB is broken into four different categories:

1. Plug (a.k.a. connector): any physical part of a USB cable/device that inserts into another device
2. Port: the part of a device where a plug is inserted
3. Type: defines the physical shape of a USB plug and/or port
4. Version: the particular USB protocol that a device is compatible with

Plugs used to come in only Type-A and Type-B.

A USB-A receptacle connector. (Image: DigiKey, Amphenol ICC.)

Type A appeared to stand the test of time and newer versions were backwards compatible. However, each version would change the port’s color and add pins (to improve data and charging speeds). Now this iconic USB plug is being replaced by Type-C.

USB-B receptacle connectors for USB versions 1.0 to 2.0 (left) and versions 3.0 to 3.2 (right). (Image: DigiKey, On Shore Technology Inc. [left], Amphenol ICC [right].)

Type B is uncommon and reserved for large electronics like printers. For different USB versions, the shape of the larger Type-B ports and plugs will change.

Larger USB plugs eventually gained the title of Standard size with the emergence of USB mini-A, micro-A, mini-B and micro-B. Unless the host device was small, most electronics opted for Standard Type-A plugs and ports — making mini-A and micro-A rare. Most peripherals have opted for mini-B and micro-B based on whichever was available at the time. In fact, USB mini-B and micro-B are the shapes people think of when colloquially saying “mini-USB” and “micro-USB.”

A USB mini-B receptacle connector (left) and USB micro-B receptacle connector (right). (Image: DigiKey, Hirose Electric Co Ltd [Left], Amphenol ICC [right].)

Similar to its predecessor, micro-B will change shape when working with USB version 3.2, but these are also rare.

A USB version 3.2 micro-B receptacle connector (Image: DigiKey, Amphenol ICC.)

Older USB cords almost always went from a Type-A on one side to a Type-B — be it Standard, mini or micro — on the other. This is because USB data transmission has always been bidirectional, but power supplied used to only move from the host device (with Type-A ports) to a peripheral (with Type-B ports). With the latest versions of USB4 and USB4 2.0, power is bidirectional, requiring the use of USB-C at both host and peripheral.

Which USB options should engineers choose when developing electronics?

The answer really depends on the application.

Standard USB-A is still widely used. So, if backwards compatibility is of concern, its best to have this option available.

For high charging and data rates, USB-C is the best option as it’s able to charge in both directions while maintaining 40 Gbps and 240 W at 48 V and 5 A. USB-C can also be described as the true universal USB option. It can be used with a host device and peripheral devices. Its cords are bidirectional. And its ports are fully reversible — it is never upside down. Since it’s the only port compatible with current USB versions, it also represents the future of the technology. In fact, its popularity has grown to where USB chargers and host devices are now opting out of Type-A.

But engineering isn’t always about future proofing and backwards compatibility. Cost, part availability and legacy systems will dictate decisions. As a result, the following chart should be helpful when making final decisions.

In conclusion, the “U” in USB doesn’t mean that the connector is universal — yet. As a result, considerations must accompany any design decision. Data transfer rates, power transfer specifications and connector sizes will all play a role in the USB specification suitable for your design.