Before diving into the PLC programming content for everyone to share, today I’d like to present a useful maintenance tip that might help you in your daily work. Let’s get started!
First, when the CPU is malfunctioning, it's important to check whether all components connected to the internal bus are properly seated. A common troubleshooting method is to systematically replace suspected faulty modules to isolate the issue and resolve it accordingly.
Second, if the memory is acting up, reprogramming the system can help identify if the problem lies in the program itself. If the error reappears after reprogramming, it may be due to electrical noise or a failing memory chip, which would require replacement.
Third, issues with input/output units or expansion modules often start with checking the physical connections—both the insertion of the modules and the integrity of the cables. Once the faulty unit is identified, it should be replaced to restore normal operation.
Fourth, if the program isn’t being executed, follow the standard process: check the input, then the program execution, and finally the output.
(1) For input checks, use the LED indicators on the module or an input monitor. If the LED doesn't light up, there may be a problem with the external input system. A multimeter test can confirm if the voltage is within the expected range. If the LED is on but the internal monitor doesn’t show the signal, the issue could be with the input unit, CPU, or expansion module.
(2) When checking program execution, use the writer’s monitor to verify the status of the ladder logic. If the actual contact state doesn’t match the expected result, it could indicate a programming error or a hardware issue in the execution part.
(3) For output checks, observe the LED indicator. If the output LED shows an error despite correct operation, the CPU or I/O interface may be at fault. If the LED is on but no output is detected, the output unit or the external load could be the problem.
Keep in mind that different PLC models may have varying LED configurations. Some LEDs are connected directly to the I/O unit, while others are linked through the CPU, so the diagnostic results may differ accordingly.
Fifth, if certain parts of the program aren’t executing, the same steps apply as before. However, if the input time for elements like counters or step controllers is too short, it may cause a non-response failure. To avoid this, ensure the input time is sufficient—ideally, less than the maximum response time of the input unit plus twice the scan time.
Sixth, if the power is briefly cut and the program disappears, check the battery first. Additionally, try repeatedly powering the PLC on and off to see if the microprocessor starts correctly. If the program still fails to save, it could be due to a faulty circuit or increased leakage current in the memory or external circuits. Also, check for noise interference from the machine system, as sudden power interruptions often coincide with motor or winding noise.
Seventh, if the PROM won’t run, check if it’s properly inserted. If not, consider replacing the chip.
Eighth, if the system stops after power-on or reset, it might be due to noise interference or poor internal contact. Gently tapping the PLC body can help detect loose connections. Also, check the cable and connector insertions to ensure they’re secure.
Ninth, interference from inverters can affect the analog signals in a PLC. This is a common issue in automation systems where both PLCs and inverters are used together. For example, in a Siemens PLC setup, a 4-20mA signal sent to an inverter may fail to start it. After testing, it was found that the inverter was causing interference on the analog channel. By adding a signal isolation module (such as TA3012), the issue was resolved.
Based on my experience, many engineers face similar problems during system debugging. To prevent such issues, consider the following best practices:
1. Use separate power supplies for the PLC and the inverter, and choose an isolation transformer for the PLC.
2. Keep power lines and signal lines separated, and ensure signal lines are properly shielded.
3. Always use signal isolation modules for analog inputs and outputs.
4. Implement software filtering in the PLC program to reduce noise impact.
5. Separate the signal ground from the power ground to minimize interference.
By following these steps, you can significantly reduce the risk of inverter-induced interference on your PLC’s analog signals. I hope this guide helps you troubleshoot more effectively in your projects!
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