How USB communication works

Communication technology

Release date:2022/5/25         

 ・In Japanese
<Premise knowledge>
 ・Serial communication
 ・Differential transmission communication
 ・Half-duplex communication、Full dual communication
 ・AC-DC converter

■USB communication connector type-A, type-B, type-C

USB is an abbreviation for Universal Serial Bus, which is one of the standards for serial communication. USB types are divided according to the connector shape, and are mainly divided into Type-A, Type-B, and Type-C. From Type-C, it is a vertically and horizontally symmetrical connector and can be inserted in either direction.

In addition, the USB version is divided according to the communication method, and there are mainly USB 1.x to USB 3.x . The feature of USB3 is that the number of connector terminals has increased to realize high-speed communication.

■How USB communication works

To understand how USB communication works, it's best to understand Type-A first. The functions of each terminal are as follows.

There are two terminals for data transfer, which is called a differential transmission line. By reversing the polarity of the terminals, it becomes more resistant to noise and high-speed communication is possible. The figure below is a circuit diagram when a PC and a USB memory are connected. The side that supplies power (control side) is called the host, and the side that supplied power (controlled side) is called the device. This is a diagram when sending from a PC to a USB memory, but there is also a driver in the internal circuit on the USB memory side and a receiver in the internal circuit on the PC side for sending from USB to the PC. On the other hand, the data communication terminal is shared for transmission and reception. This method is called half-duplex communication.

Type-B, like a mobile phone, has an additional terminal to identify it when it becomes a host or a device depending on the application.

The following is the case when using a mobile phone as a device. It becomes a device by recognizing that terminal 4 is an open terminal.

The following is the case when using a mobile phone as a host. In that case, a dedicated cable called OTG (On-The-Go) cable is required. Terminal 4 and terminal 5 are connected in the cable, and the mobile phone recognizes it and switches to the host. The method of determining whether or not a terminal is connected is to detect the voltage level of the terminal using a pull-up resistor or pull-down resistor.

The following is the case when connecting the charger. By connecting terminals 2 and 3 in the charger with a resistor, the mobile phone recognizes it and charges it. The charger only contains an AC-DC adapter. When charging a lot of mobile phones at once, there are people who prepare a large capacity AC adapter separately and then connect the resistor and the USB terminal by themselves.

A major feature of Type-C is that the terminal arrangement is point-symmetrical so that it can be inserted even if the connector is turned upside down (instead, some terminals are superfluous). Also, in order to increase the communication speed, the terminal for transmission and the terminal for reception are separated as the Super Speed terminal. Such a method is called full-duplex communication. There are also conventional terminals (D +, D-) that share transmission and reception (half-duplex communication) so that it can also support Type B to Type C conversion adapters. Therefore, the communication speed does not increase when converting from Type B to Type C.

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