[en] This international Standard defines the functional requirements for multiplexed data transmission systems arising from IAEA Safety Guide 50-SG-D8, section 4.12 (reproduced in annex A), and its extension to category A systems (According to IEC 1226 classification). Its also provides manufacturers with quidance on nuclear power plant requirements. Multiplexing equipment can be used for special purposes, for example for electrical isolation or for reduction of fire loads due to cabling. In a distributed computer-based system, multiplexing equipment is an essential part of the system. Applications exist in the systems of nuclear power plants important to safety (categories A, B and C). Requirements of the IAEA and IEC for multiplexing and for digital computer hardware and software are therefore applicable. Specific aspects of importance considered in this standard are:-the functional requirements for multiplexing equipment for category A systems, which were not considered by IAEA 50-SG-D8 and IAEA 50-SG-D3;-the conditions for transmission of multiplexed information between category A system and a category B or C system or to a system not important to safety;-the internal redundancy and failure detection features necessary within a multiplexed data transmission subsystem.