  Much digital instrumentation and control systems embedded in the critical
medical healthcare equipment aerospace devices and nuclear industry have
obvious consequence of different failure modes. These failures can affect the
behavior of the overall safety critical digital system and its ability to
deliver its dependability attributes if any defected area that could be a
hardware component or software code embedded inside the digital system is not
detected and repaired appropriately. The safety and reliability analysis of
safety critical systems can be accomplished with Markov modeling techniques
which could express the dynamic and regenerative behavior of the digital
control system. Certain states in the system represent system failure while
others represent fault free behavior or correct operation in the presence of
faults. This paper presents the development of a safety and reliability
modeling of a digital feedwater control system using Markov based chain models.
All the Markov states and the transitions between these states were assumed and
calculated from the control logic for the digital control system. Finally based
on the simulation results of modeling the digital feedwater control system the
system does meet its reliability requirement with the probability of being in
fully operational states is 0.99 over a 6 months time.
