ČSN EN IEC 61158-6-21 ed. 2

(18 4020) This document provides common elements for basic time-critical and non-time-critical messaging communications between application programs in an automation environment, as well as material specific to Type 21. The term "time-critical" is used to represent the presence of a time-window, within which one or more specified actions must be completed with some defined level of certainty. Failure to complete specified actions within the required time risks the failure of the applications requesting the actions, with attendant risk to equipment, plant, and possibly human life.