IEC 61158-4-24 Ed. 3.0 b:2023

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 entities:
• in a synchronously-starting cyclic manner, according to a pre-established schedule, or
• in an acyclic manner, as requested 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.