The USB control transmission in the veterinary B-ultrasound system has the characteristics of small amount of data transmission and no requirements for data transmission time and transmission rate. The USB bus in the veterinary B-ultrasound system always reserves a certain amount of bus bandwidth for control transmission, and in order to enable data to be transmitted as quickly as possible, the veterinary B-ultrasound host USB software can also dynamically adjust the frame time per frame/d required for control transmission. In addition, in order to ensure the correctness of the transmitted data, the USB control transmission in the veterinary B-ultrasound system enables the two mechanisms of retry and error control during the transmission process. Since the control transmission has the above-mentioned characteristics, it is mainly used to send and receive control data related to the configuration information of the USB device, such as device address, device configuration descriptor, etc.

Control transmission can be used for all USB devices in the veterinary B-ultrasound system, and all of these devices must support this transmission mode.

Unlike other data transmission types, the transaction processing of control transmission is bidirectional. That is to say, in the process of transmitting control data once, the USB in the veterinary B-ultrasound system must complete both the processing of IN transactions and the processing of OUT transactions. The transaction processing of control transmission generally includes three stages: establishment, data and status, and each transaction processing stage is composed of other specific transaction processing. Among them, the establishment phase is responsible for completing the host sending a control request signal to the USB device. The specific transactions in this phase are implemented by SETUP transactions: the data phase is an optional phase, mainly used to send control data whose format is defined by the USB standard in the veterinary B-ultrasound system or customized by the device supplier. The transactions in this phase are implemented by one or more IN/OUT transactions; the status phase is a phase used by the USB device to report the data transmission results of the previous two phases to the host. The transactions in this phase are implemented by an IN transaction or an OUT transaction.