1. Check whether the correct serial port number, baud rate, and other parameters are selected.

• Check **Device Manager → Ports** to see if "USB-SERIAL CH340 (COMx)" appears, confirming that the driver is properly installed.  
• Try to use the rear USB ports of a desktop computer. The front USB ports of some laptops or desktops may not provide sufficient power.  
• If the driver has never been installed, visit the official website or use the installation CD to download and install the driver following the instructions.  
• If the serial port number still does not appear after driver installation, try using a different USB port.  
• Note the correct COMx port number, use that port in the software to connect, and select the correct baud rate and other parameters.

2. Is the connection cable original? A non‑original cable refers to one that has been cut and extended in the middle, or a user‑made cable.  

• The connection cable must be a 4‑core cable, crimped with a 6P4C RJ11 connector, with the two ends reversed (reverse order).  
• One end: yellow‑green‑red‑black; the other end: black‑red‑green‑yellow.  
• It is recommended to keep the cable length within 20 meters. Beyond that, communication performance cannot be guaranteed.  
• The C18 series software includes send/receive verification, so extension may be attempted.  
• Other series software have no send/receive verification, so extension is not recommended.

3. If the serial port number is visible, the connection parameters are correct, and the cable is original, but the connection still fails.

• Use a terminal emulator (e.g., HyperTerminal) or a serial debug assistant to open that serial port.  
• Use a conductive tool (e.g., tweezers) to short yellow‑green or black‑red, attempt to send data, and check whether the sent data is received.
• If data is received, the USB‑to‑serial adapter + connection cable are good.  
• However, it is still recommended to try using a rear USB port of a desktop computer once more.  
• If the connection still fails after that, the switch's serial port is generally considered faulty.  
• If no data is received, the USB‑to‑serial adapter may be damaged, or there may be a quality issue with the non‑original cable.