AbstractProcess industries rely on various safeguards to minimize the potential and consequence of hazardous incidents. Active safeguards, such as safety instrumented systems (SIS), fire and gas systems, and emergency vents, use sensors to detect process upsets, then take action to bring it to a safe state.Many users implement active safeguards using programmable logic controllers (PLCs). The PLC uses a combination of hardware and software to respond to inputs (process variables) by generating outputs (safety actions). The execution of the safe state logic is dependent on the application program integrity, which may be developed by the PES vendor, engineering contractor, system integrator, or the user. Successful implementation and thorough verification of the application program is essential for correct SIS operation.This paper will first discuss the various types of SIS software, including full variability, limited variability, and fixed programming languages. It will then look at proposed application program requirements, and guidelines in the international process industry sector standard, draft IEC 61511.