This International Standard is applicable to safety related electronic systems using for digital communication purposes a transmission system which was not necessarily designed for safety related applications and which is • under the control of the designer and fixed during the lifetime, or • partly unknown or not fixed, however unauthorised access can be excluded, or • not under the control of the designer, and also unauthorised access has to be considered. Both safety related equipment and non-safety related equipment can be connected to the transmission system. This Internaitonal Standard gives the basic requirements needed to achieve safety related communication between safety related equipment connected to the transmission system. This International Standard is applicable to the safety requirement specification of the safety related equipment connected to the transmission system, in order to obtain the allocated safety integrity requirements. Safety requirements are generally implemented in the safety related equipment, designed according to IEC 62425. In certain cases these requirements may be implemented in other equipment of the transmission system, as long as there is control by safety measures to meet the allocated safety integrity requirements. The safety requirement specification is a precondition of the safety case of a safety related electronic system for which the required evidence is defined in IEC 62425. Evidence of safety management and quality management has to be taken from IEC 62425. The communication related requirements for evidence of functional and technical safety are the subject of this standard. This International Standard is not applicable to existing systems, which had already been accepted prior to the release of this standard. This International Standard does not specify • the transmission system, • equipment connected to the transmission system, • solutions (e.g. for interoperability), • which kind of data are safety related and which are not. A safety related equipment connected through an open transmission system can be subjected to many different IT security threats, against which an overall program has to be defined, encompassing management, technical and operational aspects. In this International Standard however, as far as IT security is concerned, only intentional attacks by means of messages to safety related applications are considered. This International Standard does not cover general IT security issues and in particular it does not cover IT security issues concerning • ensuring confidentiality of safety related information, • preventing overloading of the transmission system.