Universal Serial Bus (USB) jacks and ports are tools that allow you to connect computer peripherals to your computers, keyboards, external hard drives, or storage keys.
The numbers following the USB symbol simply correspond to the version of the USB standard concerned, the 3.0 and 3.1 series being the most recent at the time these lines are written.
In addition to the color of their connectors (version 3.0 is usually blue), it is the speed of data transfer that is the main difference between these two standards.
Thus, the USB 2.0 standard, introduced in 2000, made it possible to guarantee a transfer speed much higher than that of the previous standard: from 1.5 MB per second to theoretical 60 MB! USB 3.0, appeared in 2008, has multiplied this transfer rate by 10, reaching the theoretical rate of 625 MB per second!
Be careful, if backward compatibility is supported in the case of USB 3.0 jacks (that is, your device with a USB 3.0 port will work on your computer with USB 2.0 sockets), the speed of transfer will remain blocked to 2.0 standards…
USB standards. As early as 1995, the USB standard was developed for connecting a wide variety of devices.
The USB 1.0 standard offers two modes of communication:
12 Mb / s in high-speed mode.
1.5 Mb / s at low speed.
The USB 1.1 standard provides some clarifications to USB device manufacturers but does not change the bit rate.
The USB 2.0 standard provides speeds up to 480 Mbit / s.
The USB 3.0 standard provides speeds up to 4.8 Gbps.
In the absence of a logo the best way to determine whether these are USB devices at low or high speed is to consult the product documentation as long as the connectors are the same.
Compatibility between USB 1.0, 1.1 and 2.0 devices is assured. However, using a USB 2.0 device on a low-speed USB port (i.e. 1.0 or 1.1) will limit the bit rate to 12 Mbps. In addition, the operating system may display a message explaining that the flow will be restricted.
There are two types of USB connectors:
The so-called type A connectors, whose shape is rectangular and generally used for devices low in bandwidth (keyboard, mouse, webcam, etc.);
The so-called type B connectors, whose shape is square and used mainly for high-speed devices (external hard drives, etc.).
Description: USB Type A and Type B Connectors
1. Power + 5V (VBUS) 100mA maximum
2. Data (D-)
3. Data (D +)
4. Mass (GND)
USB bus operation
The USB architecture has the characteristic of providing power to the peripherals it connects, up to a maximum of 15 W per device. It uses a cable consisting of four wires (the GND ground, the VBUS power supply and two data wires called D- and D +).
Description: the USB cable
The USB standard allows the chaining of devices, using a bus or star topology. The devices can then either be connected one after the other, or branched.
The branching is done using boxes called “hubs”, with a single input and several outputs. Some are active (providing electrical power), others passive (powered by the computer).
Description: Bus topology of USB ports
Description: Star topology of USB ports
The communication between the host (the computer) and the peripherals is done according to a protocol (communication language) based on the principle of the token ring. This means that the bandwidth is shared temporally between all connected devices. The host (the computer) sends a sequence start signal every millisecond (ms), a time interval during which it will simultaneously give the “speech” to each one of them. When the host wants to communicate with a device, it sends a token (a data packet, containing the device address, encoded on 7 bits) designating a device, so it is the host who decides the “dialogue” with peripherals. If the device recognizes its address in the token, it sends a packet of data (8 to 255 bytes) in response, otherwise, it forwards the packet to other connected devices. The data thus exchanged are coded according to the NRZI coding.
Since the address is 7-bit coded, 128 devices (2 ^ 7) can be simultaneously connected to a port of this type.