Purpose: To present three probabilistic modeling for PLC systems, considering both the hardware and the software components of which the first one is an input based analysis, the second an action based analysis and the third an action-traverse analysis. Summary: A programmable logic controller (PLC) system has a microprocessor, a memory, input devices and output devices for receiving and sending signals. The PLC ladder program is executed through periodic scanning. PLCs operations are critical to the processes in which they are used. The reliability analysis is performed to estimate PLCs period of flawless functioning and the effect of input errors and processing errors. Combinatorial methods such as Fault Trees (FT) and Reliability Block diagrams are used for such analysis. Ft identifies a top event for failure and then leads to other events which caused the top event. An RBD on the other hand, uses a graphical description of the system components and connectors to estimate the overall system reliability based the reliability of its components. Bayesian Networks are also used by some analysts as a reference engine which is found to have superior properties compared to FT.
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