This European Standard is applicable to safety-related electronic systems using an open transmission system for communication purposes. It gives the basic requirements needed, in order to achieve safety-related communication between safety-related equipment connected to the open transmission system. This standard is applicable to the safety requirement specification of the safety-related equipment, connected to the open transmission system, in order to obtain the allocated safety integrity level. The properties and behaviour of the open transmission system are only used for the definition of the performance, but not for safety. Therefore, from the safety point of view the open transmission system can potentially have any property, as various transmission ways, storage of messages, unauthorised access, etc.. The safety process shall only rely on properties, which are demonstrated in the safety case. The safety requirement specification is a precondition of the safety case of a safety-related electronic system for which the required evidences are defined in ENV 50129. The communication related requirements for evidence of functional and technical safety are the subject of this standard. This standard is not applicable to existing systems, which had already been accepted prior to the release of this standard. This standard does not specify: - the open transmission system, - equipment connected to the open transmission system, - solutions (e.g. for interoperability), - which kinds of data are safety-related and which are not.