When designing a safe plc, there are many factors to consider and many special designs are required. For example: a safety PLC emphasizes internal diagnosis, combined with hardware and software, allows the device to detect the discomfort of its working state at any time; a software with a safety PLC must use a series of special technologies to ensure the reliability of the software; A safety PLC has redundancy function, which can maintain system operation even if it is partially disabled; a safety PLC also has an additional safety mechanism, which does not allow random reading and writing of internal data through the digital communication interface.
The difference between safety PLC and conventional PLC is that safety PLC needs to be certified by third-party professional organizations to meet the strict international standards of safety and reliability. A systematic approach must be thoroughly adopted to design and test a safety PLC. German TUV experts and US FM experts will provide third-party independent validation and verification of the safety PLC design and testing process.
Special electronic circuitry, detailed diagnostic software analysis, and integrity design for all possible failures ensure that the safety PLC has the ability to measure more than 99% of the potential hazards of internal components. A Failure Mode, Impact, and Diagnostic Analysis (FMEDA) approach has been a guide to designing how each component causes system failure and tells you how the system should detect the failure. TUV engineers personally perform failure testing as part of their certification process.
Strict international standard software is applied to safety PLCs. These standards require special techniques to avoid complexity. Further analysis and testing, carefully examine the task interaction of the operating system. Such tests include real-time interactions such as multitasking (when used) and interrupts. A special diagnosis is also required, called "program flow control" and "data validation." Program flow checking ensures that basic functions are performed in the correct order, data validation allows all critical data to be stored redundantly in memory, and validity testing is performed prior to use. In the software development process, a safety PLC requires additional software testing techniques. In order to verify the data integrity check, a series of "software failure injection" tests must be performed, that is, the program is intentionally destroyed to check whether the PLC response is operating in the expected safe manner. The software is designed and tested with detailed documentation so that third-party inspectors can understand the operating principle of the PLC, and most software development does not use this standard operating procedure, which explains why many garbage software will appear. So many bugs can't be found.
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