What is a Safety Instrumented System (SIS) and how does it differ from DCS?

SIS is a dedicated system for process safety functions - detecting dangerous conditions and taking safe action (shutdown, isolation). Key differences from DCS: SIS is designed for high availability and reliability (redundant, fault-tolerant), certified to IEC 61511/61508 functional safety standards, uses fail-safe design philosophy, independent from basic control to prevent common cause failures, and strictly managed with higher change control discipline. SIS handles trips and interlocks; DCS handles regulation and non-safety logic. SIL (Safety Integrity Level) ratings define required reliability.