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For a quarter of a century, the USB port has been a loyal old friend. Connecting our everyday gadgets and peripherals and powering them, all we need to do is plug them in and watch them work magically.

Over time, the sockets have changed, but no matter what you plug in, the host always seems to know what the device is. But how exactly does this happen? How does it know when a mouse is connected instead of a printer? What is the difference between USB 2.0 and USB 3.2 SuperSpeed?

Welcome to our USB inner workings explainer to see how it has survived for so long as others come and go.

A DE-15 VGA socket, two DE-9 serial ports, and a DB-25 parallel port, along with two PS/2 connectors.

Mice and keyboards almost always use the serial PS/2 ports, each with a dedicated 6-pin port. Printers and scanners connect to the parallel port through a 25-pin connector, and everything else connects through the classic serial port.

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What if you accidentally plug the mouse into the keyboard slot? It just doesn't work because the PC doesn't know an incorrect device has been inserted. In fact, these interfaces cannot recognize what the device is: essentially, you would tell the computer what it is and then manually install the correct driver for it.

If all goes well, after the driver is installed, a quick reboot, and a bit of luck, this is all it takes to get everything running smoothly. However, usually, PC users need to delve into the Windows Control Panel or motherboard BIOS to get it to work smoothly.

For 90s PC users trying to connect peripherals, this is a familiar sight.

Naturally, consumers wanted something better: it could be said, "one port to rule them all." A socket where you can plug and unplug devices without restarting the machine, and it can immediately identify and configure the device for you.

System providers also wanted something more universal to replace the need for many different sockets and lower production costs. Over the years, it also needed to have scope for development and improvement while maintaining backward compatibility.So, the requirements were not high at that time.

Universal Serial Bus: A Rare Unified Moment

In the world of computing, planets occasionally align and initiate a period of harmonious productivity, benefiting everyone. Such an event occurred in 1994 when Intel, Microsoft, IBM, Compaq, DEC, and Nortel formed an alliance, unanimously agreeing that it was time to create a new connection system to meet everyone's desires and needs.

Intel took the lead in technology development, with Ajay Bhatt becoming the project's chief architect—someone who would go on to do the same for AGP (Accelerated Graphics Port) and PCI Express. Within two years, a complete specification was released, along with the chips to control it all.

The Universal Serial Bus (USB) was born as an alternative to serial, parallel, and PS/2 ports. It prided itself on a clean, simple design and offered a lot of performance. The new system started slowly, not really taking off until the release of version 1.1 in 1998.

The changes in the revision were quite small, mainly concerning power management and device compatibility, but these were not the reasons for the USB application's launch. Instead, Microsoft added support for USB 1.1 to Windows 95 in the fall of 1997 through an update.

Microsoft also heavily promoted the phrase "plug and play"—a PC design philosophy and system requirement aimed at eliminating the complexity of setting up computers and peripherals. Although not the most powerful system, USB was the perfect example of it.

However, the biggest advertisement for USB came from Apple's decision to fully commit and release a product that would shake up the entire PC industry.

Farewell to Beige and Old Ports - Apple's First iMac

The original iMac was launched in August 1998, bright and bold, and was one of the first so-called "no legacy" PCs. The term was used to indicate that the machine avoided all old ports and devices: everything in it would be the latest hardware. Although it was not initially well-received by critics, it continued to sell in large quantities—its popularity brought USB truly to the map, even though Windows computers would still need to sell for many years without compromising on the past interfaces.The USB specification has undergone several revisions, with the main revisions being the 2.0 version in 2001, the 3.0 version in 2008, and the latest 4.0 specification released in 2022. However, we will come back to discuss this later.

Now, let's look at how USB actually works.

It's simple on the surface: the working principle of USB

Let's first take a look at the overall layout connected in a typical PC.

The USB ports in the lower left part are directly connected to what Intel calls the PCH: Platform Controller Hub. At the time when USB first appeared, this chip was usually called the South Bridge, which manages the flow of instructions and data to components such as hard disk drives, network adapters, audio chips, etc.

The PCH still plays the same role, although it now has more things to handle. By the way, AMD Ryzen CPUs can actually handle these tasks directly: they do not need a PCH/South Bridge, although most Zen motherboards come with an additional controller to provide more ports and sockets.

Deep in the silicon internals of the X299 chip is a part called the USB host, which contains two key elements: the USB controller and the root hub. The former is a small processor responsible for issuing all instructions, managing power transfer, etc. Like all such integrated circuits, it requires drivers to operate, but these drivers are almost always built into the operating system.

The root hub is the main stage for connecting USB devices to the computer, but not every system is set up this way. Sometimes devices are connected to other hubs, which are then daisy-chained back to the USB host (green box at the top of the image).

The latest specification allows for up to 5 hub chains, although this may not sound like much, but the same standard also stipulates that a single USB controller must support up to 127 devices. Need more? Then just add another controller—this is actually the default requirement in the USB 3.0 standard.

Hubs and devices communicate with each other through a set of logical pipes, with each connected peripheral device having up to 32 communication channels (16 upstream, 16 downstream). However, most only use a few and enable them as needed.Pipes can be simply categorized based on what they are doing: sending/receiving commands or transferring data. In the case of the latter, the logic system used only sends in one direction, while commands are always bidirectional.

For example, a USB scanner will only send data to the hub, while a printer will only receive data. Hard disk drives, network cameras, and other multifunctional devices do both, so there will be more active pipes working.

So how is all this information transmitted?

In the case of USB 1.0 to 2.0, it can be done with only 2 wires, which is significantly fewer than the wires used in older parallel ports.

USB 2.0 Pins - Ground, Data Pair, Power

This specification's connector contains 4 pins: one for 5-volt power, two for data, and one for grounding. The 5 V pin provides all the current needed to operate the electronic devices in the connector and the device itself, up to the following limits:

USB 2.0 = 2.5 watts

USB 3.0/3.1 = 4.5 watts

USB 3.2/4 = 7.5 watts

By charging or powering via battery, USB 2.0 or higher can bypass these limits. When used in this way, no data can be transferred, but more power can be provided—something old ports could never do.Data lines operate as differential pairs — the voltage patterns between them provide a bit stream to the host controller. When a device is plugged into a USB port, the controller detects a voltage change on one of the data pins, which initiates a process known as device enumeration. The peripheral device is first reset to prevent it from being in an incorrect state, and then the controller reads all relevant information (such as device type and maximum data speed).

USB devices fall into one of many categories, each with a fixed code — for example, a Bluetooth adapter falls into the wireless adapter category, while a steering wheel with force feedback is a physical interface device.

A very important group is the mass storage class. Initially set up for devices like external hard drives and CD burners, it has expanded over the years to include flash drives, digital cameras, and smartphones — the latter having seen a tremendous increase in storage capacity and often using USB connections to transfer files to a computer.

Only one device can be managed at a time (hence it is a serial bus), but the controller can switch between them very quickly, giving the impression that they are all being handled simultaneously. Although the bus is not as fast as the SATA interface, for example, a computer using a USB drive can boot from them and also run portable applications on the device without installing them.

Speaking of speed, let's delve into this aspect of the communication system.

In the early drafts of the USB 1.0 specification, the data lines in the interface were designed to operate at only one speed: 5 MHz. Since the lines work in pairs, the bus is 1 bit wide, providing a maximum bandwidth of 5 Mbits per second (or 640 kB/s).

This was a huge improvement over the ancient serial ports, but it was below what a parallel port configured in ECP mode (20 Mbits/s) could achieve. However, at the time, this speed would have excluded many very simple devices, such as mice and keyboards, so the specification was extended to work at two clock rates, providing data rates of 1.5 Mbits/s or 2 Mbits/s. With a liberal dose of artistic license, the designers labeled these as low speed and full speed.

When USB 2.0 was finalized in 2001, the bus offered a higher clock rate, providing a peak bandwidth of 480 Mbits per second — what could be faster than "full speed"? Of course, high speed.

When version 3.0 appeared seven years later, this naming confusion reached its peak.

4 pins for 1.1/2.0 and 5 data pins (on the back) for 3.0Two data lines have already reached their maximum capacity. To continue increasing bandwidth, the only option is to add more pins. The original USB design considered such changes, which is why the sockets are relatively spacious and neat.

These additional pins allow data to flow simultaneously in both directions (i.e., duplex mode) and provide a theoretical peak bandwidth of 5 Gbits per second—more than 400 times higher than the original specifications. Since these channels are located in the space above the old channels, USB 3.0 retains full backward compatibility.

Then things started to get quite silly...

Version 3.1 was introduced in 2013, featuring faster data channels (10 Gbits/s), but for some reason, this revision was labeled as USB 3.1 Gen 2. Why Gen 2? Because 3.0 was renamed to 3.1 Gen 1.

When the USB 3.2 specification appeared five years later, the organization that helped develop the USB standard and reach a consensus decided that the more powerful features of 3.2 (up to 20 Gbits/s) required another renaming:

USB 3.1 Gen 1 --> USB Gen 3.2 1x1

USB 3.1 Gen 2 --> USB Gen 3.2 2x1

The new system has two versions on top of this: Gen 3.2 1x2 and 2x2, where two sets of data lines are used in parallel. With so many different specifications and speeds available, you would think there would be a fixed standard to help identify things. But you would be wrong—just look at this back panel on a Gigabyte motherboard:

Manufacturers can use official logos to indicate which version it is, but since there is no enforcement of their use in any way, few people use them. Another renaming activity has recently taken place, suggesting that manufacturers use SuperSpeed USB 5 Gbps, SuperSpeed USB 10 Gbps, and so on, highlighting how chaotic USB has become.

When USB4 (this is not a typographical error, it is not USB 4.0) was introduced in 2019, people hoped things would become clearer. Unfortunately, the speed grades and labels still lack clarity. If anything, it has actually become more confusing because it was soon announced that Thunderbolt 3 would be integrated into USB4—effectively becoming the same thing (unless some additional adjustments are made to the latter)...The port labels are the same, but they are black and do not have the "Certified" section.

A further revision of USB appeared in August 2022 in the form of USB4 2.0, offering faster data transfer rates and improved backward compatibility. Soon after, there was another attempt to organize the naming conventions, adding a large number of new logos for USB cables and ports.

These changes are welcome and long overdue, but because they ultimately cannot be implemented in any meaningful way, manufacturers and retailers can mix and match the names, colors, and logos with their products.

For example, although AMD uses the new system in its chipset, motherboard suppliers continue to release new products with old names. It will take many years for each company to do this correctly.

USB Type: As simple as A, B, and C?

When designing USB, engineers wanted to make the system as simple and easy to use as possible to avoid wasting time trying to configure everything. This concept was carried out to the format of the socket - one shape for the USB host, and another shape for the device to be connected. They were eventually called Type A and Type B connectors.

The idea behind this is that users can clearly see which end of the cable is connected to where. Unfortunately, the designers also wanted the implementation cost of the system to be as low as possible, and the design of Type A sometimes makes it difficult to insert.

Another problem with the first generation of USB is that the Type B plug is too bulky for small devices such as media players and mobile phones. Therefore, when version 1.1 was released in 1998, a reduced version was introduced, called Mini-A and Mini-B. They were quickly adopted by mobile phones and tablets, although they are also known for being quite fragile.

But even these were too large, once smartphone manufacturers began to seek thinner devices. USB 2.0 solved this problem, not only providing faster speeds, but also providing us with Micro-A and B connectors.Next to the Big Daddy Type A is the Baby micro-B USB 2.0 also provided a Micro-AB socket (accepting micro-A and micro-B plugs), then although USB 3.0's Type A is backward compatible with USB 2.0, the Type B is not—it is physically unsuitable for insertion into a 2.0 Type B socket—although old cables can be inserted into a USB 3.0 Type B connector.

In addition, the same specification also has a somewhat bulky Micro-B SuperSpeed connector, which contradicts its "miniature" purpose.

Dysfunctional old series of USB connectors

All these changes are in pursuit of higher performance (you can clearly see the additional data pins in USB 3.0) and to appease the growing members of the guiding group, which is called the USB Implementers Forum (USB-IF).

There is obviously a need for something better...

Manufacturers and consumers both hope that the connector is small in size, the same on both ends, and provides an improved range of performance. Therefore, with the emergence of USB 3.1 (developed separately), the USB-C plug was born.

It not only replaces the requirement for different A/B sockets but can also be inserted in any direction and is used for connection systems other than USB (such as DisplayPort, HDMI, and Thunderbolt).

The data lines of the USB-C connector are much more than those of USB 3.0 Type A (sorry, USB 3.2 SuperSpeed) - two are completely dedicated to USB 2.0 support, and another four sets of differential pairs provide bidirectional communication. These changes provide a bandwidth of up to 80 Gbits/s in the latest specifications.

With USB4, the connection with the old socket has been completely abandoned—it is either USB-C or nothing—but we still need many years to say goodbye to the Type A sockets on PCs and other devices.Hello USB, My Old Friend

USB has been widely adopted in computers and other gadgets for over 25 years. Although the latest versions bear little resemblance to the original design, the basic premise still applies: plug it in, and the device will work properly.

Each revision of the specification has provided higher performance (USB4 2.0 is nearly 7,000 times faster than 1.1) and the ability to supply more power to devices (currently up to 100 watts when used in power delivery mode).

USB4 2.0 is nearly 7,000 times faster than USB 1.1.

But why or how has USB lasted so long? Is there something better that can provide more bandwidth or power? The simple answer is no, or at least not anymore.

Twelve years ago, Intel released Thunderbolt. At the time, it seemed more attractive than USB 3.0, with greater bandwidth and greater flexibility. As mentioned earlier, the latest version, called Thunderbolt 3, now serves as a superset of USB-C, abandoning its original connector (Mini DisplayPort), and has the same maximum bandwidth as USB4. It offers more features, such as the ability to supply more power to running devices, but it has not replaced USB; instead, it has essentially been integrated into USB4.

Yes, it's a USB-C connector, but it's actually a Thunderbolt cable.

There's also FireWire, which at some point offered better performance than USB 2.0 and supported full-duplex data transfer, but when USB 3.0 arrived and improved in many aspects, including performance, FireWire no longer offered any obvious advantages and was not widely adopted.

Part of the appeal of USB to system vendors and manufacturers is its relatively open specification. Unlike Thunderbolt or FireWire, you can make a "USB 3.2" cable and sell it as is, without fully complying with all the details in the specification. For example, it may not support full bandwidth or provide the maximum available power.

While this makes such products cheap to manufacture and purchase, it does mean that it is a potential minefield when it comes to getting the cable you actually need. The fact that USB offers multiple transfer speeds and power modes further complicates the issue—although there have been significant improvements in logos and certifications, it will continue to be so for the foreseeable future.The old logo reads USB 3.2 Gen 2x1, but it is marketed as 3.2 Gen 2x2 - there is no simple way to distinguish what it is.

However, despite the flaws of loose standards, confusing naming schemes, and various socket types, USB still remains as ubiquitous as ever. Almost every computer peripheral uses it to connect to the host—even if it is wireless, it is almost certainly going to use a USB dongle.

One day, USB may eventually follow the fate of its predecessors, but for now, its simple appeal and ongoing development will keep it moving forward. A loyal old friend, indeed.