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,
and
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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.