Hi SME_1

reliability (and/or availability) depends on the chosen architecture and in particular on what level of I/O redundancy is implemented (e.g. single sided or switched I/O's or redundant I/O's) as well as used fieldbus (e.g .PROFIBUS OR PROFINET).

It's all about the the so called Common Cause Failure (CCF) factor in combination with MTBF and example calculation can be found in the S7-400H manual (e.g. Appendix A of the S7-400H manual assuming yours is Firmware 6 one)