Siemens S5 series PLCs are commonly found in the industrial market, primarily comprising three models: the U-type (Universal), W-type (Word Processing), and R-type (Relay). Different models of PLCs exhibit distinct failure symptoms and require varying diagnostic approaches. Among these, software faults can typically be resolved using Siemens' dedicated programming devices. Siemens PLCs are equipped with communication PC interfaces, enabling the resolution of nearly all software issues through specialized servo programmers.
The determination of whether a fault is software-related can be made through software PC programs. In cases of hardware failures, however, specialized chip-level circuit board repair engineers are required for restoration work. PLCs are generally modular in structure, and a relatively straightforward approach to handling hardware failures is to replace the faulty board or card.
The S5 PLC possesses self-diagnostic capabilities, often alerting and responding according to pre-programmed procedures in the event of modular functional errors, which can be identified through fault indicator lights. When the power supply is normal, all indicator lights are functioning correctly, particularly the input signals, yet the system functionality is abnormal (no output or erratic output), troubleshooting should follow the principle of addressing the easier issues first, then the more complex, and addressing software issues before hardware. The first step is to check for issues with the user program. The S5 user program is stored in the PLC's RAM, which is volatile and prone to loss or corruption in the event of power surges or battery failures, as well as intense electromagnetic interference.
For PLCs equipped with EPROM memory cards and slots, restoring the program is straightforward, typically involving copying the program from the EPROM card back to the PLC, which often resolves the issue. Users without EPROM subcards must utilize the online functionality of the Programmable Gateway (PG) to transmit the correct program to the PLC. It's worth noting that sometimes a simple program overwrite is insufficient, and it becomes necessary to clear the user program in RAM before re-copying the program.
Clearing the user program in RAM can be achieved by cycling the "RUN" and "ST" switches on the PLC in the sequence RUN---ST---RUN---ST---RUN, or by executing the "Object—Blocks—Delete---inPLC—allblocks---overall—Reset" function on the PG. Additionally, relying solely on the program stored in EPROM is not foolproof, and overconfidence in its integrity can sometimes lead to troubleshooting confusion. Regularly verifying the program in EPROM, especially the backup program in the PG, is therefore crucial.
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PLC hardware failures are often straightforward to identify, and the basic approach to repair is to replace faulty modules. Accurately identifying the faulty module based on fault indicator lights and symptoms is crucial to avoid unnecessary replacements.
- Power Module Failures
A functioning power module should have its operational indicator lights, such as "AC," "24VDC," "5VDC," and "BATT," illuminated in green continuously. Any change in color, flickering, or extinguishing of these lights indicates an issue with the corresponding power section.
The "AC" light represents the PLC's AC main power supply. If the "AC" light is off, it typically indicates a lack of operating power, causing the entire PLC to stop. In this case, check if the power fuse is blown and replace it with the same rating and type. If no identical imported fuse is available, use a fast-acting fuse with the same current rating as a substitute. Repeatedly blown fuses suggest a short circuit or damage to the circuit board, necessitating the replacement of the entire power supply.
The "5VDC" and "24VDC" lights extinguishing indicate a lack of corresponding DC power output. When the power deviation exceeds 5% of the normal value, the indicator lights will flicker. Although the PLC may still function, attention should be given, and a shutdown for inspection is advised if necessary. The "BATT" light, a backup power indicator, is green when normal, yellow when low on battery, and red when faulty. Replace the backup battery when the yellow light is on, as per the manual's recommendation to change the lithium battery every two to three years. A red light indicates a failure in the backup power system, requiring the replacement of the entire module.
- I/O Module Failures
Input modules typically consist of photocoupler circuits, while output modules vary by model, including relay, transistor, and photoelectric outputs. Each input/output point has a corresponding LED indicator. Suspect an I/O module failure if an input signal is present but the corresponding LED does not light up, or if an output is confirmed but the output LED remains off.
I/O modules have 6 to 24 points, and replacing the entire module due to a single faulty point is economically unfeasible. Instead, find a spare point to substitute and modify the corresponding address in the program. However, note that finding specific addresses in large programs can be challenging. Importantly, both input and output module replacements must be performed with the PLC powered off, as hot-swapping modules in S5 is strictly prohibited.
- CPU Module Failures
The CPU module of a universal S5 PLC often includes communication interfaces, EPROM slots, and run switches, making faults more concealed. Given the high cost of replacing CPU modules, their failure analysis and diagnosis require particular care.
Repair Example: A PLC failed to switch to the RUN state upon powering on, with the error indicator light first flashing and then remaining lit. The fault persisted after power cycling and reset. The PLC operated normally after replacing the CPU module. However, during chip-level repairs, replacing the CPU alone did not resolve the flickering fault light, which ceased only after replacing the communication interface board.
Peripheral Circuit Failures
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According to relevant literature, the failure rates in PLC control systems are distributed as follows: CPU and memory account for 5%, I/O modules for 15%, sensors and switches for 45%, actuators for 30%, and wiring and other aspects for 5%. This indicates that over 80% of failures occur in peripheral circuits. Peripheral circuits comprise field input signals (such as pushbuttons, selector switches, proximity switches, and switching outputs from sensors, relay output contacts, or analog signals converted by analog-to-digital converters), field output signals (solenoid valves, relays, contactors, motors, etc.), as well as wires and terminal blocks.
Loose connections, damaged components, mechanical failures, and interference can all lead to peripheral circuit malfunctions. Careful troubleshooting is essential, and when replacing components, it is crucial to select devices with reliable performance and high safety factors. Some advanced control systems utilize fault code tables to indicate failures, significantly facilitating fault analysis and troubleshooting, which should be fully utilized.
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The fault diagnosis and handling methods for other series of Siemens PLCs follow the same principles and can be deduced accordingly. Shanghai Caiya Circuit Board Repair Co., Ltd. reminds that for any chip-level precision equipment encountering faults, without sufficient confidence, it is imperative to seek professional technical service enterprises for handling. Unauthorized modification of programs and repair of intricate equipment circuit boards may lead to increased difficulty in equipment repair or even the severe consequence of equipment scrapping.