BS EN 61158-4-24:2014 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, according to a pre-established schedule, or

b) 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.