现场总线应用层（FAL）为用户程序提供访问现场总线通信环境的方法。在这方面，FAL可以被视为相应应用程序之间的窗口。 IEC 61158的本部分提供了基本时间关键型和非时间关键型的通用元素 自动化环境中应用程序之间的消息传递通信以及21型协议特定的材料。术语“时间关键”用于表示存在一个时间窗口，在该时间窗口内，一个或多个指定的行动需要以某种确定的水平完成。如果未能在时间窗口内完成指定的操作，则可能会导致请求操作的应用程序失败，并对设备、工厂以及可能的人身安全造成风险。交叉引用:ISO/IEC 7498-3ISO/IEC 10731ISO/IEC 8822IEC 61158-6-21:2019ISO/IEC 8824ISO/IEC 7498-1ISO/IEC 9545ISO/IEC 8824（所有部分）IEC 61158- 4-21:2019BS EN IEC 61158-3-21:2019IEC 61158-3-21:2019ISO/IEC/IEEE 60559:2011IEC 61158-2ISO/IEC/IEEE 8802-3IEC 61158-1:2019替换BS EN 61158-5-21:2012，其保持当前状态。购买本文件时，包括所有当前可用的修改件。

The Fieldbus Application Layer (FAL) provides user programs with a means to access the fieldbus communication environment. In this respect, the FAL can be considered a window between corresponding application programs. This part of IEC 61158 provides the common elements for basic time-critical and non-timecritical messaging communications between application programs in an automation environment as well as material specific to the Type 21 protocol. The term "time-critical" is used to represent the presence of a time-window within which one or more specified actions are required to be completed with some defined level of certainty. Failure to complete specified actions within the time window risks failure of the applications requesting the actions, with attendant risk to equipment, plant, and possibly human life.Cross References:ISO/IEC 7498-3ISO/IEC 10731ISO/IEC 8822IEC 61158-6-21:2019ISO/IEC 8824ISO/IEC 7498-1ISO/IEC 9545ISO/IEC 8824 (all parts)IEC 61158-4-21:2019BS EN IEC 61158-3-21:2019IEC 61158-3-21:2019ISO/IEC/IEEE 60559:2011IEC 61158-2ISO/IEC/IEEE 8802-3IEC 61158-1:2019Replaces BS EN 61158-5-21:2012 which remains currentAll current amendments available at time of purchase are included with the purchase of this document.