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2023

The principle of data line transmission

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With the continuous improvement of social level, USB cable is used more and more frequently in our life, but we can know the principle of USB data cable transmission, the following is a small series to tell you:

The host controller is responsible for the transmission of data streams between the host and the USB device. These transmission data are treated as a continuous bit stream. Each device provides one or more interfaces that can communicate with the client program, and each interface consists of 0 or more pipes, which independently transmit data between the client program and the specific terminal of the device. USBD establishes interfaces and pipelines for the real-world needs of the host software. When a configuration request is made, the main controller provides services according to the parameters provided by the host software.

USB supports four basic data transfer modes: control transfer, isotime transfer, interrupt transfer and data block transfer. Each transmission mode applied to a terminal having the same name has different properties.

The control transmission type supports the transmission of control, status, configuration and other information between the peripheral and the host, providing a control channel between the peripheral and the host. Each peripheral supports a control transfer type so that configuration and command/status information can be transferred between the host and the peripheral.

The isochronous (lsochronous) transmission type (or synchronous transmission) supports data transmission between the peripheral and the host with periodicity, limited delay and bandwidth, and the same data transmission rate. This type is error-free, so it cannot guarantee correct data transmission. It supports data transmission such as computer-telephone integration system (CTI) and audio system with the host computer.

Interrupt transmission types support input devices such as gamepads, mice, and keyboards. These devices have small and non-periodic data transmissions between the host, but are sensitive to response time and require immediate response.

The data block (Bulk) transmission type supports peripherals such as printers, scanners, digital cameras, etc. The amount of data transmitted between these peripherals and the host is large. USB performs this type of data transmission only when the bandwidth is met.

USB uses a block bandwidth allocation scheme, and if the peripheral exceeds the current bandwidth allocation or potential requirements, it cannot enter the device. Synchronous and interrupt transmission type terminals reserve bandwidth and ensure that data is transmitted at a certain rate. The centralized and controlled terminal transmits the transmission data in the best available bandwidth.

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