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
-
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.