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How it works

POWERLINK uses a mixture of timeslot and polling procedures to achieve isochronous data transfer. In order to ensure coordination, a PLC or an industrial PC is designated to be the so-called Managing Node (MN). This manager enforces the cycle timing that serves to synchronize all devices, and controls cyclical data communication. All other devices operate as Controlled Nodes (CN). In the course of one clock cycle, the MN sends so-called “Poll Requests” to one CN after another in a fixed sequence. Every CN replies immediately to this request with a “PollResponse,” which all other nodes can listen in on. A POWERLINK cycle consists of three periods: during the “Start Period,” the MN sends a “Start of Cycle Frame” (SoC) to all CNs to synchronize the devices. Jitter, i.e. clock rate inaccuracy due to fluctuations in the cycle, is as lows as about 100 nanoseconds. Cyclic isochronous data exchange takes place during the second period (“Cyclic Period”). Multiplexing allows for optimized bandwidth use in this phase. The third period of a cycle marks the start of the asynchronous phase, which enables the transfer of large, non-time-critical data packets. Such data, e.g. user data, is spread out over the asynchronous phases of several cycles. POWERLINK distinguishes between real-time and non-real-time domains. Since data transfer in the asynchronous period supports standard IP frames, routers separate data safely and transparently from the real-time domains.

 

Layer model for openSAFETY over POWERLINK