The Siemens S5 series PLCs commonly found in the industrial market in China include three main models: U-type (general-purpose), W-type (word processing), and R-type (switching). Different models of PLCs exhibit different fault manifestations and diagnostic methods. Among these, software faults can be resolved using Siemens-specific programming tools, as Siemens PLCs are equipped with communication PC interfaces that can address almost all software issues through dedicated servo programmers.Software PC programs can determine whether a fault is software-related. If it is a hardware fault, specialized chip-level circuit board repair engineers are required for repairs. PLCs are generally modular in structure, and a straightforward approach to handling faults is to replace the faulty board.
01Diagnosis and Handling of Software Faults
The S5 PLC has self-diagnostic capabilities and can often issue alarms and respond according to pre-programmed instructions when a module function error occurs. Faults can be diagnosed through fault indicator lights. When the power supply is normal and all indicator lights are functioning correctly, especially when the input signals are normal but the system functions are abnormal (no output or erratic output), the first step is to check whether there is an issue with the user program, following the principle of addressing easier issues first, and software issues before hardware issues. The user program for the S5 is stored in the PLC’s RAM, which is volatile and can be lost or corrupted if the backup battery fails or if there is a power interruption. Strong electromagnetic interference can also cause program errors.
For PLCs with EPROM storage cards and slots, recovering the program is relatively simple; copying the program from the EPROM card back to the PLC usually resolves the issue. Users without EPROM sub-cards must use the PG’s online function to send the correct program to the PLC. It is important to note that sometimes a simple program overwrite does not resolve the issue, and it is necessary to clear the user program in RAM before re-copying the program.
By toggling the “RUN” and “ST” switches in the sequence RUN—ST—RUN—ST—RUN or executing the “Object—Blocks—Delete—inPLC—allblocks—overall—Reset” function on the PG, the RAM program can be completely cleared. Additionally, the program stored in the EPROM is not infallible; over-reliance on the program in the EPROM can sometimes complicate repairs. Therefore, regular checks and verifications of the program in the EPROM, especially the backup program in the PG, are particularly important.
02Hardware Faults in PLCs
Hardware faults in PLCs can be more readily identified, and the basic method of repair is to replace the module. Judging the faulty module based on the fault indicator lights and symptoms is key to effective repair; blind replacement can lead to unnecessary losses.1) Power Module FaultsA normally functioning power module should have its working indicator lights, such as “AC”, “24VDC”, “5VDC”, and “BATT”, lit in green. If any of these lights change color, blink, or go out, it indicates a problem with that part of the power supply.
When the “AC” light is off, it usually means there is no working power supply, and the entire PLC stops. At this point, the power supply fuse should be checked for a break. When replacing a blown fuse, it is essential to use a fuse of the same specifications and model. If the fuse blows repeatedly, it indicates a short circuit or damage on the circuit board, necessitating the replacement of the entire power supply.
When the “5VDC” and “24VDC” lights are off, it indicates no corresponding DC power output. If the power deviation exceeds the normal range by 5%, the indicator lights will blink. At this point, although the PLC may still operate, it should be taken seriously, and maintenance should be performed if necessary. The “BATT” indicator light shows the status of the backup power supply: green is normal, yellow indicates low power, and red indicates a fault. If the yellow light is on, the backup battery should be replaced. The manual recommends replacing the lithium battery every two to three years. When the red light is on, it indicates a fault in the backup power supply system, and the entire module needs to be replaced.
2) I/O Module FaultsInput modules are generally composed of optocoupler circuits; output modules may vary in type, including relay outputs, transistor outputs, and optoelectronic outputs. Each input and output point has a corresponding LED indicator. If there is an input signal but the LED does not light up, or if there is confirmed output but the output light does not illuminate, the I/O module should be suspected of having a fault.
Input and output modules typically have 6 to 24 points. If only one point is damaged, replacing the entire module is not economically viable. The usual practice is to find a spare point to replace it and then change the corresponding address in the program. However, it should be noted that if the program is large, it may be difficult to locate the specific address. It is particularly emphasized that whether replacing an input module or an output module, it must be done with the PLC powered off; hot-swapping modules in the S5 is ***not allowed.3) CPU Module FaultsThe CPU module of the general-purpose S5 PLC often includes communication interfaces, EPROM slots, and run switches, making its faults less obvious. Because the cost of replacing a CPU module is high, careful analysis and judgment of its faults are particularly important.
Repair example: A PLC could not switch to the RUN state when powered on, with the error indicator light flashing and then remaining lit. After a power reset, the fault persisted. After replacing the CPU module, it operated normally. During chip-level repairs, the CPU was replaced, but the fault light continued to flash until the communication interface board was replaced, at which point functionality was restored.
03Peripheral Circuit Faults
According to relevant literature, the fault rates in PLC control systems are as follows: CPU and memory account for 5%, I/O modules account for 15%, sensors and switches account for 45%, actuators account for 30%, and wiring and other aspects account for 5%. It can be seen that over 80% of faults occur in peripheral circuits. Peripheral circuits consist of field input signals (such as push buttons, selector switches, proximity switches, and some sensor output signals, relay output contacts, or analog signals converted by analog-to-digital converters) and field output signals (such as solenoids, relays, contactors, motors, etc.), as well as wires and terminals.
Loose connections, component damage, mechanical faults, and interference can all cause peripheral circuit faults. Careful inspection is necessary, and replacement components should be selected for reliable performance and high safety factors. Some powerful control systems use fault code tables to indicate faults, greatly facilitating fault analysis and elimination, which should be utilized effectively.
04Conclusion
The fault diagnosis and handling methods for other series of Siemens PLCs follow the same principles and can be inferred accordingly. Shanghai Caiya Circuit Board Repair Co., Ltd. reminds that for any chip-level precision equipment encountering faults, it is essential to seek professional technical service companies for handling, as unauthorized modifications to programs and repairs of tightly integrated equipment circuit boards may lead to increased repair difficulties or even severe consequences such as scrapping the equipment.