When I used the ModBus Master course, I found that the serial port was opened correctly after I called the bool QModbusDevice :: connectDevice () method, but when I call the void QModbusDevice :: disconnectDevice () method, the serial port is always on QModbusDevice :: ClosingState. I browse the source code and found that although the serial port is in QModbusDevice :: ClosingState, d-> m_queue queue is not empty, so the process has been empty d-> m_queue, which will lead to the serial port is in a closed state, but it shows the serial port is still in closing.