This European Standard is applicable to safety-related electronic systems using a closed 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 transmission system. This standard is applicable to the safety requirement specification and design of the communication system in order to obtain the assigned safety integrity level (SIL). The safety requirement specification is a precondition of the safety case of a safety-related electronic system for which the required evidence is defined i EN 50129. Evidence of safety management and quality management has to be taken form EN 50129. Evidence of functional and technical safety is the subject of this standard. This standard is not applicable to existing systems which has already been accepted prior to the release of this standard. However, as far as is reasonably practicable, this standard shall be applied to modifications and extensions to existing systems, subsystems and equipment. This standard applies to a closed transmission system with the following preconditions, for which evidence shall be provided: 1 Only approved access is permitted. 2 There is a known maximum number of connectable participants. 3 The transmission media is known and fixed. Closed transmission systems are not necessarily data buses. They can also include for instance balise links or simple serial links between two safety-related computers.