Mini computer serial port self-loop test steps under Windows system (RS232, RS422, RS485)
author: VENOEN
2025-07-09
Procedure for testing serial ports (RS232, RS422, RS485) in the Windows operating system of minicomputers
The Loopback Test for serial ports is a method used to verify the functionality and performance of serial communication ports such as RS-232, RS422, RS485. In self-loop testing, the output signal from the sender (TX, transmission) is directly connected to the receiver (RX, receiver), forming a closed loop. This test method can ensure that the hardware and drivers of the serial port themselves work properly, and do not depend on external devices.
Preparatory work
Mini computer with serial port (most models support only RS232 working mode, a few models support switching RS232/RS422/RS485 working mode)
Dupont wire 4
Serial debugging tool (UartAssist V5.0.10)
Step 1: download and install UartAssist V5.0.10, download address: //nitwo.com/en/download/UartAssist.html
Mini computer with serial port (most models support only RS232 working mode, a few models support switching RS232/RS422/RS485 working mode)
Dupont wire 4
Serial debugging tool (UartAssist V5.0.10)
Step 1: download and install UartAssist V5.0.10, download address: //nitwo.com/en/download/UartAssist.html
- Step 2: RS232 operating mode test
Generally, the serial port works in RS232 mode by default. You do not need to change this mode in the BIOS.
Use the DuPont wire to connect the pin2(SIN) and pin3(SOUT) pins of the COM port, as shown below:
- Run UartAssist V5.0.10, select the serial port number (such as COM1) on the software page, click open to enable the serial port, click Send, and check whether SEND Data and RECV Data exist in the Data Log. If both are displayed in the Data Log, check whether SEND data and RECV data exist. If the serial port can communicate properly, the port is normal.
Note: The interconnection port must correspond to the port number in the software; otherwise, the test data will be abnormal.
- Step 3: RS422 operating mode test
Before testing this working mode, you need to confirm whether the serial port of your computer supports this mode. After confirming that the computer supports this working mode, you need to enter the BIOS to modify the serial port working mode. To modify the serial port working mode, refer to the following steps:
For example, press the Sw button and press Del key to enter the BIOS. Choose SIO MISC Configuration> Select the port number to be changed and set it to RS422. After the configuration is complete, press F10 to save the configuration and exit.
- Using the DuPont wire, connect pin1(TX-) and pin4(RX-) and pin2(TX+) and pin3(RX+), as shown below:
- Run UartAssist V5.0.10, select the serial port number (such as COM1) on the software page, click open to enable the serial port, click Send, and check whether SEND Data and RECV Data exist in the Data Log. If both are displayed in the Data Log, check whether SEND data and RECV data exist. If the serial port can communicate properly, the port is normal.
- Step 4: RS485 working mode
Because the RS485 works only in half-duplex mode, the self-loop test cannot be performed. Therefore, you must connect the RS485 serial port to another RS485 serial port device for testing. Here we use two identical computers for the communication test.
Test preparation:
Both computers need to be powered on;
Run UartAssist V5.0.10 on both computers at the same time, and open the corresponding COM port in the software.
Use a DuPont cable to connect the pin1(RS485-) and pin2(RS485+) ports of PC1 to the pin1(RS485-) and pin2(RS485+) ports of PC2.
The operating mode of the two computers must be changed to RS485 in the BIOS.
- Click Send on PC1 to Send data. Click Send on PC2 to send data from PC1. Click Send on PC2 to send data from PC2. If PC1 and PC2 can both send and receive data from each other, confirm that the RS485 can communicate properly.
- Tips: If you encounter abnormal communication or unstable communication with serial port peripheral products in the process of using serial port, you can try to check whether the serial port of the computer is normal according to the above steps. If the serial port of the computer is all normal after the test, but the connection is interrupted or the connection is unstable when connecting the peripheral product, please check one by one according to the following steps:
The serial port device may be disconnected due to insufficient or unstable power supply. Ensure that the serial port device is supplied with sufficient and stable power. If possible, use a stable power adapter. Because the serial port of the mini computer is usually passive serial port, there may be type problems during long-term operation, you can try to replace the active serial port adapter to connect the serial port peripheral product test;
Check and replace cables:
Check the serial cable and connector to ensure that they are not damaged or in poor contact. If there is a problem, try replacing the cable.
Configure serial port parameters:
Ensure that serial port configuration parameters (such as baud rate, data bits, stop bits, parity) are consistent between the minicomputer and peripherals. Try lowering the baud rate and see if it improves.
Monitor serial buffer:
Check the usage of the serial buffer to ensure that there is no overflow. If the data transfer speed is too fast, consider increasing the buffer size or adjusting the data transfer strategy.
Remove distractions:
Ensure that there are no strong electromagnetic interference sources around the minicomputer and serial device. If possible, move the device to a less intrusive environment.
Operating system Updates and Fixes:
Make sure the operating system is up to date and install all relevant patches and updates. If the problem persists, consider reinstalling the operating system.
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