This Technical Report addresses the application of closed serial digital communications systems (often termed fieldbuses) used for transmission of safety-related data in the realisation of safety functions at machinery. It offers guidance on the issues that need to be considered during the specification, system design, installation, commissioning, modification and maintenance of such applications. NOTE A closed serial digital communications system is considered to have a fixed number or fixed maximum number of participants linked by a transmission system with well-known and fixed properties, and where the risk of unauthorized access is considered negligible. This Technical Report assumes that the SRECS safety requirements specification (SRS) has been developed and the design of the SRECS (Safety-Related Electrical Control Systems) is intended to include a safety-related communication system. This Technical Report is intended to be used in conjunction with IEC 62061. This Technical Report does not address the design of the safety-related communication system itself.