The first step is to check whether the serial cable is normal, and use the serial port assistant software to short-circuit the 2 and 3 lines to check whether the serial port can work. The second step is to connect the MAX232 chip circuit and short-circuit the corresponding data output pin on the chip, or use the serial port assistant software to judge whether the MAX232 chip is good or bad. (If the previous step is normal) Find the data of the upper computer and whether it is connected to MAX232. Step 4: If the circuit is normal, edit the software to test whether the circuit connected to the upper computer and MAX232 can send connection instructions. Step 5: Use the voltmeter to test whether the level information of the hardware circuit meets the requirements of RS232 communication