IEC 61158-3-21:20 19以抽象的方式根据服务的原始动作和事件定义了由类型21现场总线数据链路层提供的外部可见服务；与每个原语动作和事件相关联的参数，以及它们采取的形式，以及这些动作和事件之间的相互关系，以及它们的有效序列。它取消并取代了2010年出版的第一版，构成了技术修订。主要变化如下: ？增加了网络控制消息类型； ？杂项编辑更正。

IEC 61158-3-21:2019 defines in an abstract way the externally visible service provided by the Type 21 fieldbus data-link layer in terms of the primitive actions and events of the service; the parameters associated with each primitive action and event, and the form which they take and the interrelationship between these actions and events, and their valid sequences. It cancels and replaces the first edition published in 2010 and constitutes a technical revision. The main changes are listed below:
？Added Network Control Message Type;
？Miscellaneous editorial corrections.