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Introduction to Sercos interface
Applications Types
Sercos III
General Architecture
Sercos III Profiles
Blended Infrastructure
Additional Resources
Sercos I and II
Sercos Standardization
  General Architecture
  Physical and Data Link Layers
  Network Topologies
Sercos III Cycle
Identification Numbers (IDNs)
Sercos III Stack
Data Consistency
Infrastructure Hardware

Sercos III Cycle
Communication on a Sercos III network occurs in strict cyclic intervals. A cycle time from 31.25 Ásec. to 65 msec is chosen by the user for a given application. Longer cycle times mean that more slaves can be serviced in one cycle. Within each cycle, data is exchanged between Sercos III nodes using two types of telegrams: MDTs (Master Data Telegrams) and ATs (Acknowledge Telegrams). After all MDTs and ATs are transmitted, Sercos III nodes allow the remaining time in the cycle to be used as a Unified Communication Channel (UCC), which can be used to exchange data using other formats, such as EtherNet/IP and TCP/IP. Thus, each cycle handles both hard-real-time Sercos traffic and standard Ethernet traffic, with the portion allowed for each fully configurable.

The network remains available to UCC traffic until the next cycle begins, at which time Sercos III closes the nodes to UCC traffic again. This is an important distinction. Sercos was designed to provide open access at all ports for other protocols between Sercos cyclic real time messages. No tunneling is required. This provides the advantage that any Sercos III node is available to use other protocols, such as EtherNet/IP or TCP/IP, without any additional hardware to process tunneling whether Sercos III is in cyclic mode or not. Sercos nodes are specified to provide a store and forward method of buffering non-Sercos messages should they be received at a node while cyclic communication is active.
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