This part of IEC 61139 specifies the extensions to SDCI in IEC 61131-9 for functional safety.

This comprises:
- a standardized OSSDe interface for redundant switching signals based on IEC 61131-2,
- minor modifications/extensions to state machines of SDCI to support the safety operations,
- a lean functional safety communication protocol on top of the standard SDCI communication which is a black channel according to IEC 61784-3:2021,
- protocol management functions for configuration, parameterization, and commissioning,
- IODD extensions for functional safety,
- a Device tool interface to support Dedicated Tools according to functional safety standards.

This document does not cover:
- communication interfaces or systems including multi-point or multi-drop linkages,
- communication interfaces or systems including multi-channel or encrypted linkages,
- wireless communication interfaces or systems,
- integration of SDCI-FS into upper-level systems such as fieldbuses/FSCPs.