S7 Software Redundancy-
Warm-Standby for SIMATIC Controllers

SIMATIC S7 Software Redundancy is a program package containing function blocks for SIMATIC S7 which, in the event of a fault, change over from the master system to the standby system.
It is suitable for high-availability processes with warm standby requirements (processes that are not time-critical with changeover times of the order of seconds). During changeover, the outputs retain their status.

Changeover time
To determine suitability for specific applications, the changeover time must be used as the selection criterion. It lies in the range of a few seconds and depends on several factors:

• Communications performance of the CPU
• used Communications medium, connection type used and transmission rate
• Transmitted quantity of data
• Cause of fault Data
• transfer rate of the PROFIBUS DP system and
• Number of DP slaves

With S7-300, two separate racks are assembled for the two CPUs. With S7-400, the CPUs can either be plugged into one or into two racks. The redundant link between the systems is implemented over the standard bus systems of PROFIBUS, PROFINET, or MPI.

In this example, a larger controller is used as a master controller for processing the redundant and normal areas. A smaller controller is adequate as a backup controller because it is only responsible for the redundant area in the event of a fault.

Available CPUs
The standard CPUs of the S7-300 and S7-400 as well as WinAC can be used for the master and slave stations. Different CPUs can be used for the master and backup stations.

Features of S7 Software Redundancy
S7-Software Redundancy is characterized by the following features:

• Distributed I/O
  The ET 200M distributed I/O station is connected over two IM 153-2 redundant DP slave interfaces over a single channel to both PROFIBUS DP lines. The complete I/O range of ET 200M is available. Either the complete process or only an especially critical part of it can be redundantly configured. Redundant I/O, if required, must be implemented in the user program.
• Engineering
  The requirements for program development are STEP 7 and S7 Software Redundancy. The redundancy blocks are called at the beginning and end of the program and parameterized. The redundant data areas are specified. The redundancy data are then automatically transferred by the FBs. The application program must be loaded on both CPUs.
• Diagnostics and repair
  All standard diagnostic functions are available, e.g.:
  • Module status in the overview display
  • Status and modification of inputs and output
  • Program status of function blocks
  • Variable status at the end of a cycle

  If a CPU has to be repaired, the CPU must be replaced and the relevant program must be loaded onto the new CPU..

• Communication
  Communication with other devices is supported as follows:
  • Redundancy scripts are available for linking to WinCC (not WinCC flexible).
  • For linking to OP, TP, MP and TD, devices that can be switched over must be used (OP 7, OP 17, WinCE-based).
  • Data communication with the PC and PLC must be programmed.