EVS-EN IEC 61158-4-12:2019

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
a) in a synchronously-starting cyclic manner, and
b) 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.