This document 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 document is restricted to the consideration of data communication within the plant I&C safety 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 document. This document is not applicable to the internal connections and data communication of a processor unit, its memory and control logic. It does not address the internal processing of instrumentation and control computer based systems. This document gives requirements for functions and properties of on-line plant data communication by reference to IEC60880 and IEC60987, produced within the framework of IEC61513. It requires categorisation of the communication functions in accordance with IEC61226, which in turn requires environmental and seismic qualification (i.e., the environment where the safety function is required to operate) according to IEC/IEEE 60780-323 and IEC60980.