IEC 61158-5-21:2019 specifies the structure and services of the type 21 IEC fieldbus Application Layer, in conformance with the OSI Basic Reference Model (ISO/IEC 7498) and the OSI Application Layer Structure (ISO/IEC 9545). This second edition cancels and replaces the first edition published in 2010. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous edition:
？added Write and Read service;
？miscellaneous editorial corrections.