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Fault tolerance - Hot Standby with SIMATIC S7- 400H
The SIMATIC S7-400H is a controller with two H CPUs of the same type; in the event of a fault, changeover takes place from the master system to the standby station. It is suitable for high-availability processes with hot standby requirements (processes with changeover times shorter than 100 ms).

Operating System Updates for CPU412-3H / CPU414-4H / CPU414-4H PG / CPU417-4H


The method of event-driven synchronization supports fast and bumpless changeover to the redundant CPU in the event of a fault. It resumes processing at the point of interruption without any loss of information or interrupts. The operating system ensures that all commands, which if executed independently would produce different states in the two systems, operate in synchronism. No programming or parameterization has to be performed by the user for this purpose.


The central devices can be configured in two different ways:

  • When the subunits have to be completely separated from each other for availability reasons, it is appropriate to use two standard racks (UR1 and UR2). Each rack accommodates one CPU and one power supply (PS). If extremely high availability is required, two redundant power supply modules can be used. The distance between the two racks can be up to 10 km.
  • Two CPUs, each with either a single or a redundant power supply, are plugged into the UR2-H rack with a segmented backplane bus. This supports an extremely compact configuration.


Depending on the type of connection, the following I/O components can be used:

  • All PROFIBUS slaves for single-sided connection
  • ET 200M for switched and redundant connection

How do you connect an operator panel (WinCC flexible) to an H system (S7-400H)?

Your RK Controls representative will be happy to work with you side by side as you select the 400H components for your particular application.


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