Probabilistic Modeling of the PLC

Each input circuit contains the electronics required to read one sensor input. The input module subsystem includes all the electronics common to all input channels on a module. The main processor encompasses all components common to any PLC function. The output module subsystem contains all components common to the output channels on one module. The output circuit consists of the components needed to interface to one final elements device. 

Problem A safety instrumented function uses three analog transmitters as process sensors, a safety PLC and two remote actuated valves interfaced to the PLC with 24 VDC three way solenoids. What portion of the safety PLC is used in this safety instrumented function  

Solution Three analog input channels are needed as well as all common circuitry in an analog input module. All common circuitry in the PLC is required (Main Processor). The common circuitry for one digital output module is required and two digital output circuits are required. 

There are significant differences in performance that can only be shown by subtle changes in the model (Ref. 8). Simplistic models that do not account for all variables would not be accurate and thus provide the wrong calculation for sophisticated technology. This is ttie reason why some manufacturers, like Triconex, provide TUV approved Markov modeling perforrriance calculation sheets for any specific I/O configuration and some TUV approved safety verification tools, like the exida SlLver tool, use these models supplied by the manufacturer. 

PROBLEM Given the failure rate data from Table 10-2, what is the total failure rate of the PLC portion of the safety instrumented function from Example 10-1  

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