It was 1997 when discussions started about a single connector that would be able to connect different types of devices to computers using only one type of cable and one type of port.
Thus, as time goes by, ports such as COMs (often used for modems and other types of devices), LPTs (for printers) and PS2s (typically for mouse and keyboard) are practically disappeared and have given way to a type of port – and its connector – of which no one could do without now: the Universal Serial Bus, which we all know as USB.
Although we all understand what a port and a USB cable are, it is not always clear the difference between the standards that have occurred over time: leaving aside the standard 1.1, which now fallen into disuse, in this article we will see what is the difference between USB 2.0 and USB 3.0.
Our guide will analyze what are the practical differences between the two standards, leaving out voluntarily what is related to the circuitry and electronic design. Finally, we will analyze the latest generation of USB, or USB 3.1 with its USB Connector Type-C, which aesthetically is different from the previous one but which retains some backward compatibility.
Point 1: aesthetic differences
Usually we read that a device has a number of USB 2.0 and USB 3.0 ports: Although these may seem identical, you will discover later that the USB devices of the two categories are capable of transmitting data at different speeds as well as absorbing a different amount of electricity (and therefore feed different types of devices).
Distinguishing visually one USB 2.0 to USB 3.0 is simple: the classical laminated plate of the firsts is grey or black white, while the seconds have one light blue or dark blue.
As for the leads, instead, if the plastic plate is blue, this is certainly a USB 3.0 – while the white plate could also be a USB 2.0 cable.
Step 2: The energy required for power supply
The great success of USB connectivity has certainly been the ability of many compatible devices to “feed themselves” simply by using that type of connection, effectively eliminating the need for an alternative source of power (and hence another, annoying cable).
The combination power source + link is born in USB 2.0 and also remains in USB 3.0, but with different numbers: the USB 2.0 devices can absorb a maximum current of 100 mA, while the USB 3.0 can absorb a maximum current of 150 mA.
These values change dramatically during the enumeration phase – one in which the host (the operating system) identifies the type of device connected to the port: USB 2.0 devices at that stage can absorb current up to 500 mA, USB 3.0 devices can be up to 900 mA.
So, concluding, USB 3.0 differs from its predecessor even for the remarkable power gains that devices need – USB 2.0 can power devices up to 2.5W, USB 3.0 comes up to 4.5W (using a 5V input ) – although specific energy-optimization mechanisms are taking place.
Step 3: Data transfer performance
USB 2.0 standard (which we remember has been around for 14 years) the maximum theoretical transfer speed is equal to 480 Mbit/s, while that of USB 3.0 (which is “only” 4 years old) is over ten times higher, or to be more precise 5 Gbit/s.
Although for some types of devices this difference is remarkably noticeable right away, it is virtually impossible for a USB device to effectively communicate with the host using the maximum theoretical speed due to the structural limitations of the device itself.
For example, talking about USB storage devices (disks, USB sticks or flash memories), the performance of the memory type will affect the performance of USB, impeding the attainment of the maximum transfer rate.
This does not mean, however, that the difference between USB 2.0 and USB 3.0 is not perceived in the vast majority of situations, making it also a slight difference in price – USB 2.0 devices are usually cheaper than USB 3.0 ones.
Step 4: Backward Compatibility
It seems obvious, but in fact it is not: unlike what’s normal you can use USB 2.0 devices – with obvious limitations – both in USB 3.0 or USB 2.0 ports, with no compatibility issue. Both standards are completely compatible with each other with regard to transfer data.
Clearly, this will affect performance: a USB 2.0 device plugged into a USB 3.0 port may not use the newer standard transfer speed, as well as the transfer rate of an USB 3.0 device connected to a USB 2.0 will be “adequate” to the previous standard.
This becomes slightly different regarding the current: no problem for USB 2.0 devices connected to USB 3.0 ports, they will get surely the amount of current required; instead the opposite scenario is different: a USB 3.0 device may fail to get the necessary current to feed itself if connected to a USB 2.0 port, although that scenario is rarely seen.
It should be noted, however, that with USB 3.0 the compatibility with USB cables and microUSB 2.0 type A is interrupted.
USB Type C (USB 3.1)
The USB 3.1 specification was introduced last December 2013, and with it all its novelties and improvements in terms of energy and data transfer rates. First of all, USB 3.1 allows to power devices up to 100W (in some Chromebook and the latest Macbook, the USB 3.1 connector feeds and recharges batteries and devices alone); secondly, the theoretical speed reaches ten times as high as its predecessor, touching the 10 Gbit / s.
The most interesting innovation, however, is the connection cable – far different from its predecessors – which is now on more and more devices: the USB Type-C connector has a thickness similar to that of a current microUSB, is slightly longer and – very much appreciated – it is double-sided, meaning that it can be inserted into the port in both directions, eliminating the danger of damaging the cable by inserting it incorrectly, which – strange but true – has happened and still happen very often with USB 2-3.0 cables.
The USB Connector Type-C is backward compatible with USB 2.0 ports up, but not the other way round. Today are extremely popular adapters and hybrid cables (USB 3.0 Type-B – USB 3.0 USB Type-C) to make it less “traumatic” the gradual transition from a standard (and its dedicated cable) to another.