The reasons why the 485 connection port is plugged in for a flash and does not display the port are the wrong port selection, the serial port is occupied, the serial port is damaged and the driving problem.

1. Port selection error: Please ensure that the selected serial number is correct. If you are not sure, please check the serial number list in Device Manager to confirm the correct serial number.

2. The serial port is occupied: If other programs occupy the serial port, you will not be able to see the serial port in the serial port management tool. You can try to use another serial port or stop other programs that occupy the serial port.

3. The serial port is damaged: If the serial port is damaged, it may flash after being plugged in, but the port will not be displayed. You can try to replace the serial port or repair the equipment.

4. Driver problem: If the driver is installed incorrectly or incompatible with the system, the serial port may not work normally. You can try to update the driver or use the "Rollback Driver" function in Device Manager.