Safety PLCs are qualified to be used in safety critical systems. Safety critical systems are those systems, failure of which to perform its intended function may lead to loss of life. For example a malfunction of a nuclear reactor shutdown system may be catastrophic.
Safety PLCs are guaranteed to perform its intended function when needed. So in order make their PLC extremely reliable, safety PLC manufacturers use something called a triple modular redundancy (TMR). A TMR PLC will have three sets of input-output boards and three redundant processors for the same function.
The outputs from the output boards are voted as per 2 out of 3 logic. Also such safety PLCs have a large number of diagnostic functions to alarm the plant about a component failure.
one must read the Triconex Safety PLC architecture in details to get a good insight into safety PLCs. Triconex Safety PLCs have been qualified for use in nuclear reactors.