BS EN 61158-4-7:2008 - Page 4

Industrial communication networks. Fieldbus specifications. Data-link layer protocol specification. Type 7 elements | Status: Current, Under review

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

1. a) in a synchronously-starting cyclic manner, according to a pre-established schedule, and
2. 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.