• High-availability automation system with redundant design.
• For applications with high fail-safety requirements.
  Processes with high restart costs, expensive downtimes, little supervision, and few maintenance options.
• Redundant central functions.
• Increases availability of I/O: switched I/O configuration.
• Also possible to use I/Os with standard availability: single-sided configuration.
• Hot standby: automatic reaction-free switching to the standby unit in the event of a fault.
• Configuration with two separate or one divided central rack.
• Connection of switched I/O via redundant PROFIBUS DP and/or PROFINET.
  

Catalog ST 70:

You can also find information about SIMATIC S7-400 in Catalog ST 70:

http://www.automation.siemens.com/salesmaterial-as/catalog/en/simatic-st70-chap06-english-2015.pdf