[en] I and C systems important to safety may be implemented using conventional hard-wired equipment, computer-based (CB) equipment or by using a combination of both types of equipment. This International Standard provides requirements and recommendations for the total I and C system architecture which may contain either or both technologies. In the following, the term requirements is used as a comprehensive term for both requirements and recommendations. The distinction appears at the level of the specific provisions. where requirements are expressed by 'shall' and recommendations by 'should'. This standard highlights the need for complete and precise requirements, derived from the plant safety goals, as a pre-requisite for generating the comprehensive requirements for the total I and C system architecture, and hence for the individual I and C systems important to safety. This standard introduces the concept of a safety life cycle for the total I and C system architecture, and a safety life cycle for the individual systems. The life cycles illustrated in, and followed by, this standard are not the only ones possible; other life cycles may be followed, provided that the objectives stated in this standard are satisfied.