How POWERLINK works
Unlike other real-time industrial Ethernet systems, POWERLINK is a completely software-based solution that is 100% compliant with the IEEE 802.3 Ethernet standard. Such close conformity with the standard and the absence of proprietary hardware allows POWERLINK to ensure that all of the benefits and flexibility of Ethernet technology have carried over to this real-time protocol as well. As a result, users can continue to use the same standardized hardware components and diagnostics tools they've been using. In order to achieve its real-time capabilities, POWERLINK relies on a mixed polling and time-slot procedure that allows only one node at a time to transmit data. Generally speaking, communication takes place as it does in an organized roundtable discussion where a moderator prompts panelists to make their statements. In this scenario, the moderator sees to it that everyone gets a turn to get a word in by explicitly inviting one participant after the other to speak at a specific time. In contrast to standard Ethernet, this procedure ensures that nodes cannot "speak" at the same time, eliminating the need for arbitration as well. POWERLINK networks use the following communication structure: one node – a PLC, motion controller or industrial PC, for example – is arbitrarily designated to function as the managing node (MN), which serves as the "moderator of the discussion". All other devices operate as controlled nodes (CN). The MN defines the clock pulse for synchronizing all devices and also manages the data communication cycle. Over the course of one cycle, the MN successively polls each CN using PollRequest messages that also convey additional data from the MN to each polled CN. Each CN then transmits its own data to all other nodes, this time via PollResponse messages. A POWERLINK cycle consists of three phases. In the first phase, the MN sends a "Start of Cycle" (SoC) frame to all CNs to synchronize the devices. Payload data is then exchanged in the second, or isochronous, phase. The third phase of a cycle is the asynchronous phase, which is where non-time-critical data such as TCP/IP data or parameter configuration data is transferred.