Siemens PLC common faults - solutions - Huaqiang Electronic Network

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Question: Which settings are retained after a full CPU reset?

Solution: When a CPU is reset, not all data is lost. The main memory is cleared completely, but the data stored in the Flash-EPROM memory card (MC) or micro memory card (MMC) remains intact. Additionally, the load memory, timers (except for the CPU 312 IFM), and diagnostic buffers are preserved. For CPUs with an MPI interface or a combined MPI/DP interface, the current address and baud rate used by the interface are retained before the reset. However, any other PROFIBUS address is erased and cannot be accessed anymore. The protection level set prior to the reset is also kept.

Question: Are MPI and PROFIBUS interface settings retained after updating the operating system of the CPU 41x?

Answer: After updating the operating system, a full reset occurs, so the program must be reloaded. CPUs with an MPI or combined MPI/DP interface retain their current address and baud rate before the update. However, other PROFIBUS addresses are deleted and become inaccessible. It's important to note that after resetting the PG/PC, communication with the CPU can only occur via the MPI or MPI/DP interface. Also, the protection level set before the OS update is the same as the MPI address. Detailed instructions on how to update the operating system are available in the provided documentation for various CPUs.

Question: How do you change the response monitoring time in a PROFIBUS DP network?

Answer: If the bus profile is set to "user-defined," the response monitoring time can be adjusted manually. Otherwise, the option will be disabled. To change it, select a bus component, double-click it, go to the "PROFIBUS" tab under "General," then click "Parameter" and "Properties." In the "Network Settings," you can adjust the bus profile. Clicking "Bus Parameter" will display the response monitoring time. This time can be recalculated automatically by clicking "Recalculate" or entering a value between 15.000 and 975.000.000 t_bit. The setting applies to the entire PROFIBUS DP network.

Question: What information is stored in the diagnostic buffer of the SIMATIC S7-CPU?

Answer: The diagnostic buffer stores detailed error information from the system. Each module with diagnostic capabilities includes a buffer that logs events like error numbers, locations, times, and descriptions. These errors are recorded automatically without user intervention. System diagnostics include both OS-related errors and certain user program events. Users can also add custom diagnostic messages. The buffer acts as a circular log, where the most recent event appears first. If full, older entries are overwritten. The buffer size may vary depending on the CPU model and configuration. Entries include fault events, mode transitions, and user-defined diagnostics. However, temporary statistics or service quality data are not included.

Question: Why doesn’t the date change correctly from February 29th to March 1st after shutting down CPU 945?

Answer: If a non-zero correction factor is set for the hardware clock and the CPU is powered off during the date change, the date may not update properly. For example, if the date is set to 29.02 at 23:59:00 and the CPU is turned off, it may still show 29.02 when restarted. This happens because the correction factor causes incorrect time calculations. To fix this, use a correction factor of zero, which prevents time drift issues.

Question: Which drives support the new “Clock Synchronization” function of SIMATIC?

Answer: Starting with firmware version V3.1, the S7-400 CPU supports the TIA system function “Clock Synchronization.” This feature allows direct synchronization between I/O modules and the user program within the DP cycle. It is supported by the complete product lines “SIMODRIVE” and “MASTERDRIVE MC.” Configuration requires Drive ES Basic V5.2 or later, STEP 7 V5.2, and firmware V3.1 for the S7-400 CPU. Note that only the internal DP interface of the CPU is available for communication.

Question: What is entered in the image when there are differences on redundant digital input modules?

Answer: In the process image (PII), the last average of the redundant digital input modules is valid until an error occurs. If a discrepancy is detected, the faulty module is put into a passive state, and the CPU no longer reads its inputs. At this point, the value from the non-passive module is used. Once the error is resolved, the faulty module can be identified using tools like the IO type (interconnect) and FLF (fault localization tool).

Question: Why do I need to use a SIFI-C filter in some peripheral modules? How are these modules connected?

Answer: SIFI-C filters are required for certain peripheral modules to meet CE certification standards related to high-frequency absorption and emissions. Information about which filter to use can be found in the latest catalog or system manual. For digital output modules, the filter should be connected to the load voltage source. For digital input modules, it should be connected to the module/sensor supply. For analog modules, the filter connects to the module power supply. The same filter can be used for both input and output modules.