Choosing a suitable redundancy for Siemens S300/400 PLCs.

I agree with “Control&Automation Systems” and “Mr. Masood Syed”, it is possible to use S300 with software redundancy. But you need to take into account many aspects, from choosing compatible hardware, to the changes you need to apply to the program. Even size of program has effect on the choosen hardware. The size of data to be kept redundant is also a role player in this respect. In its simple case, you may use CPU S300 with HMIs also, even on MPI.

So I propose to first follow all the rules to convert a normal program based on a single CPU to its redundant counter part. I am sure in many cases you may succeed to do this replacement. But in cases you need fast responses, and big control programs and data, as it is proposed by “Mr. Stephen Burke” and “Mr. Rodrigo Pinto Cardenas”, it is better to deploy S400 redundancy. In this case also you have two choices, software and hardware redundancy. The software redundancy aspects in S400 is very similiar to that of S300, but you may use the benefits of more speed and better memory structure of S400 series.

Also keep into account if you need to have redundant I/O. But in your case, as I understood you don’t need it.

So follow the following steps. Have in mind that using S400 let you have more options and senario to choose, but the price may be considerably high.

  • Check for harware needs and compatibility. Sometimes even the Stand version of backplane can also be a concern. Once I have faced with this problem that even Siemens experts was not aware of it.
  • After changing the PLC program, check for memory usage. The aspects to change are clearly written in Siemens manual for software redundancy. You should then choose a proper CPU, having enaugh memory, plus some more space in work memory, as when the program starts, the redundancy software needs more than what you see in S7 manager, to build its own working memory data structures.
  • If you have an HMI also, please choose a proper communication bus. In many normal cases, like what I worked in Water and Waste Water Treatment, a MPI bus is the best option. Its cheap and simple to implement.
  • Check exactly for hardware, software and licenses. You may build a comparision table, and then decide, based on price, availability, future expansion, and flexibility.

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