数据链路层在自动化环境中的设备之间提供基本的时间关键型消息通信。 该协议为所有参与的数据链路实体提供通信机会，a）以同步启动循环方式，b）以循环或非循环异步方式，根据这些数据链路实体中的每个周期的请求。 因此，该协议可以被描述为一个异步提供循环和非循环访问的协议，但每个循环都有一个同步重启。交叉引用:IEC 61158-3-12:2019 IEEE标准802.1QISO/IEC 7498-1IEC 61588ISO/IEC 7498-3ISO/IEC/IEEE 8802-3IETF RFC 768IETF RFC 791ISO/IEC 10731ISO/IEC 9899IEC 61784-1IETF RFC 1643ISO/IEC TR 8802-1ITU-T V.41IEC 61158-6-12IETF RFC 1213IEEE标准802.1DIEC 61784-2IEC 61131-2IEC 61131-3IEC 61158-1ISO/IEC 1582-1IEC 1582-1ANSI X3。66（R1990）BS EN IEC 61158-5-12:2019 IEC 61158- 5-12:2019IEC 61158-2:2014取代了BS EN 61158-4-12:2014，该标准仍然有效。购买本文件时，所有当前可用的修订版均包含在购买本文件中。

The data-link layer provides basic time-critical messaging communications between devices in an automation environment. This protocol provides communication opportunities to all participating data-link entitiesa) in a synchronously-starting cyclic manner, andb) in a cyclic or acyclic asynchronous manner, as requested each cycle by each of those data-link entities. Thus this protocol can be characterized as one which provides cyclic and acyclic access asynchronously but with a synchronous restart of each cycle.Cross References:IEC 61158-3-12:2019IEEE Std 802.1QISO/IEC 7498-1IEC 61588ISO/IEC 7498-3ISO/IEC/IEEE 8802-3IETF RFC 768IETF RFC 791ISO/IEC 10731ISO/IEC 9899IEC 61784-1IETF RFC 1643ISO/IEC TR 8802-1ITU-T V.41IEC 61158-6-12IETF RFC 1213IEEE Std 802.1DIEC 61784-2IEC 61131-2IEC 61131-3IEC 61158-1ISO/IEC 15802-1ANSI X3.66 (R1990)BS EN IEC 61158-5-12:2019IEC 61158-5-12:2019IEC 61158-2:2014Replaces BS EN 61158-4-12:2014 which remains currentAll current amendments available at time of purchase are included with the purchase of this document.