IEC 62026-3:2008 specifies an interface system between single or multiple controllers, and control circuit devices or switching elements. The interface system uses two twisted shielded 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 interchangeability of components with such interfaces. This second edition of IEC 62026-3 cancels and replaces the first edition published in 2000. This second edition constitutes a technical revision. The main changes with respect to the previous edition are listed below: - addition of new services to the base messaging protocol in 5.2; - replacement of detailed application layer requirements in former annexes by references in 5.3 to corresponding specifications in the new edition of IEC 61158 (2007); - addition of requirements for a corresponding safety protocol in 5.6 (by reference to IEC 61784-3); - modified presentation of cable and connector specifications in 8.2 and 8.4, and addition of a new type; - addition of references for safety tests for both EMC and logical tests in 8.10 and 9.3.