IEC 62026-3:2014 specifies an interface system between single or multiple controllers, and control circuit devices or switching elements. The interface system uses two conductor pairs within one cable - one of these pairs provides a differential communication medium and the other pair provides power to the devices. This part establishes requirements for the interoperability of components with such interfaces. This third edition of IEC 62026-3 cancels and replaces the second edition published in 2008. This third edition constitutes a technical revision. The main changes with respect to the previous edition are the followings: specification of group 4 messages (5.1.2); clarifications on messaging protocol (5.2); addition of I/O multicast poll messages (5.5.2 and 5.5.8); clarifications on slave behaviour (5.5.4 and 5.5.9); clarifications on physical layer (transceiver) in 5.7; miscellaneous corrections/clarifications on cable (8.2); clarifications on EMC testing (9.2.10) and logical testing (9.3). The contents of the corrigendum of March 2015 and corrigendum of October 2019 have been included in this copy.