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. While the main memory is cleared, data stored in Flash-EPROM memory cards (MC) or micro memory cards (MMC) remains intact. In addition to this, data loaded into memory, timers (except for CPU 312 IFM), and diagnostic buffers are also preserved. A CPU with an MPI interface or a combined MPI/DP interface retains its current address and baud rate before a full reset. However, any other PROFIBUS address will be erased and no longer accessible. The protection level set prior to the reset is also maintained.

Q: 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. A CPU with an MPI or combined MPI/DP interface will keep its current address and baud rate before the OS update. However, other PROFIBUS addresses will be deleted and cannot be accessed. Important: After resetting the PG/PC, communication with the CPU can only be done via the MPI or MPI/DP interface. Note: The protection level is the same as the MPI address. Detailed instructions on how to update the OS are provided in the downloadable 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 manually adjusted. Otherwise, the option is disabled. To change it, select the bus component, double-click, go to the "General" tab, click "PROFIBUS," then "Parameter." Click "Properties" to access the "Network Settings." From there, you can adjust the bus profile. Clicking "Bus Parameter" will show 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 CP or module with diagnostic capabilities has a buffer that records errors, including their cause, location, and timestamp. The buffer logs events automatically without user input. It includes system error events, some user program sequences, and user-defined diagnostics. These entries help identify faults quickly, improving system availability. The buffer is circular, meaning older entries are overwritten when full. Entries include fault events, mode transitions, and user-defined messages. In STOP mode, fewer entries are stored to help locate the cause of the stop. The buffer does not include temporary error statistics or service quality data.

Question: Why doesn’t the date change correctly from 29.02 to 01.03 after shutting down CPU 945 on February 29th?

Answer: If the hardware clock has a non-zero correction factor 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, the next day might still show 29.02 at 23:52:50. This happens because the correction factor causes incorrect time calculations. The fix is to set the correction factor to zero, which prevents the issue.

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

Answer: Starting with firmware V3.1, the S7-400 series supports the TIA system function “Clock Synchronization.” This feature links I/O modules directly with the user program in 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. Only the internal DP interface of the CPU is used for communication.

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

Answer: In the input process image (PII), the last averaged value of the redundant digital input modules is valid until an error occurs. If a discrepancy arises, the faulty module is put into a passive state, and the CPU stops reading from it. The non-passive module’s value remains active. Once the error is resolved, the faulty module is identified through the IO type or FLF tool.

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

Solution: Some peripheral modules require a SIFI-C filter to meet CE certification standards for HF absorption and emissions. The correct filter for each module is listed in the catalog or system manual. For digital output modules, the filter should be connected to the load voltage. For digital input modules, it should be connected to the module/sensor supply. Analog modules require the filter to be connected to the module power supply. A single filter can be used for multiple input and output modules.