What is PLC redundancy - Solutions - Huaqiang Electronic Network

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Redundancy in PLC systems can be categorized into software redundancy and hardware redundancy. Hardware redundancy typically involves specific hardware configurations and connection methods, while the software requirements are less strict. In a typical setup, one PLC acts as the master, and the other functions as the slave. The slave continuously copies all data from the master to ensure a backup is always available.

When the system detects that the master PLC has failed or is malfunctioning, it automatically switches control to the slave without any interruption. This ensures continuous operation, which is critical in industrial environments where downtime could lead to serious consequences.

Software redundancy, on the other hand, usually requires a smaller investment. It allows the software to read and store data, and automatically switch to a backup system when an issue is detected. Some systems also distinguish between cold, warm, and hot redundancy based on how quickly the system can take over after a failure.

Looking at Siemens' hardware solutions, two models support redundancy: CPU414H and CPU417H. While both provide reliable performance, the CPU417H is faster and offers more memory capacity. However, its cost is significantly higher than the CPU414H.

These Siemens PLCs support redundancy between two units on the same backplane or on different backplanes, using fiber-optic daughter cards for communication. After configuration, testing, and initial setup, I manually stopped the master PLC during operation, and the system seamlessly switched to the slave without any noticeable disruption. The result was very impressive.

In industrial applications, redundant systems are commonly used in control sections that are critical to human safety or equipment integrity. Failures in such areas can lead to severe damage or hazardous situations. Based on my experience with the Siemens S7-400 system, the cost of a redundant controller setup is higher than using two independent controllers. However, the benefits include stable performance and smooth, seamless switching during failures.