This International Standard 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. The scope of this standard is restricted to the consideration of data communication within the plant I&C systems. It does not cover communication by telephone, radio, voice, fax, email, public address etc. The internal operation and the detailed technical specification of data communication equipment are not in the scope of this standard. This standard is not applicable to the internal connections and data communication of a processor unit, its memory and control logic. It does not concern the internal processing of instrumentation and control computer systems. This standard gives requirements for functions and properties of on-line plant data communications by reference to IEC 60880 and IEC 60987, produced within the framework of IEC 61513. It requires classification of the communication functions in accordance with IEC 61226, which in turn requires environmental and seismic qualification (i.e., the environment where the safety function is required to operate) according to IEC 60780 and IEC 60980.