BS EN 61158-4-12:2014 PDF

1   Scope

1.1   General 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.