Devices must detect communications directed to the device’s address on the bus. The device stores received data in a buffer and returns a status code or sends requested data from a buffer or a status code. In almost all chips, these functions are built into the hardware and require no support in code beyond preparing the buffers to send or receive data. The firmware doesn’t have to take other action or make decisions until the chip has detected a communication intended for the device’s address.

SuperSpeed devices have less of a burden in detecting communications because the host routes SuperSpeed communications only to the target device.On power up or when a device attaches to a powered system, a device must respond to standard requests sent by the host computer during enumeration. The host may also send requests any time after enumeration completes. All devices must respond to these requests, which query the capabilities and status of the device or request the device to take other action.

On receiving a request, the device places data or status information in a buffer to send to the host. For some requests, such as selecting a configuration, the device takes other action in addition to responding to the host computer.The USB specification defines requests, and a class or vendor may define additional requests. On receiving a request the device doesn’t support, the device responds with a status code.