IEC 61158-4-21 Ed. 3.0 b:2023

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IEC 61158-4-21 Ed. 3.0 b:2023 Industrial communication networks - Fieldbus specifications - Part 4-21: Data-link layer protocol specification - Type 21 elements
standard byInternational Electrotechnical Commission , 03/01/2023

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The DLL provides basic time-critical data communications between devices in an automated environment. Type 21 provides priority-based cyclic and acyclic data communication using an internal collision-free, full-duplex dual-port Ethernet switch technology. For wide application in various automation applications, Type 21 does not restrict the cyclic/acyclic scheduling policy in the DLL.