What are the requirements for safety PLCs used in SIS applications?

Safety PLC requirements: IEC 61508 certification to required SIL level (SIL 2 or SIL 3 typical), hardware fault tolerance (redundant processors with cross-checking, voting on outputs), diagnostic coverage (watchdog timers, memory checks, I/O verification), deterministic scan time (predictable response time), separate power supplies and communications, failure mode to safe state, and documented safety manual with failure rate data. Programming: application software developed per lifecycle requirements, certified development tools, restricted instruction set for safety applications, and verification testing. Safety PLC must be separate from BPCS or use certified integrated systems with proven separation.