The logic solver evaluates these input signals, determines if a potentially hazardous condition exists and energizes or de-energizes the solid-state output.
In a de-energized to trip safety system, the output de-energizes to move the process to a safe state. If any of the components in the single path fail in such a way that the output cannot be de-energized, the PLC will not provide its desired safety protection function.
Other limitations include limited fail safe characteristics, high risk of covert failures through lack of diagnostics, problems of reliability of softwares (also stability of versions), flexibility without security, unprotected communications, and limited redundancy.