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How it works

SERCOS III requires dedicated hardware on both the Master and the Slave side. Such SERCOS III hardware relieves the host CPU of all communication tasks, and ensures quick real-time data processing and hardware-based synchronization. The SERCOS user organization provides a SERCOS III IP core to support FPGA-based SERCOS III hardware development. SERCOS III uses a summation frame method. Daisy chain or closed ring cabling is required for the network nodes. Data is processed while passing through a device, using different types of telegrams for different communication types. Due to the full-duplex capability of the Ethernet connection, a daisy chain will actually yield a single ring already, whereas a proper ring topology will produce a double ring, allowing for redundant data transfer. Cross-traffic is enabled by the two communication interfaces every node is equipped with: in a daisy chain as well as a ring network, the real-time telegrams pass through every node on their way back and forth, i.e. they are processed twice per cycle. Hence, devices are capable of communicating with each other within a communication cycle, with no need to route their data through the Master.

Besides the real-time channel, which uses time slots with reserved bandwidths to ensure collision-free data transfer, SERCOS III also provides for an optional non-real-time channel. Nodes are synchronized on the hardware level, with a cue taken straight from the first real-time telegram at the beginning of a communication cycle. The Master Synchronization Telegram (MST) is embedded into the first telegram for that purpose. Keeping synchronization offsets below 100 nanoseconds, a hardware-based procedure compensates for runtimes and runtime variations resulting from the Ethernet hardware. Various network segments may use different cycle clocks and still achieve fully synchronized operation.

 

Layer model for openSAFETY over SERCOS III