IEC 61500:2009 establishes requirements for data communication which is used in systems performing category A functions in nuclear power plants. It covers also interface requirements for data communication of equipment performing category A functions with other systems including those performing category B and C functions and functions not important to safety. This second edition is intended to accomplish the following: - to change the focus from multiplexed data transmission to data communication; - to restrict the scope to communication in systems performing category A functions.