Data link layer
Deterministic timing is achieved by applying a cyclic timing schedule for all connected nodes to access the physical layer. The schedule is divided into an isochronous phase and an asynchronous phase. During the isochronous phase, time-critical data is transferred; the asynchronous phase reserves bandwidth for non time-critical data. The cycle sequence is controlled by the managing node (MN) in the real-time domain. The MN frees access to the physical medium via explicit messages. As a result, just one single node has access to the network at all times, thereby preventing collisions. The CSMA/CD mechanism, which causes non-deterministic Ethernet behavior, has no effect when POWERLINK operates free of disturbances.
POWERLINK uses MAC addressing in accordance with IEEE 802.3. Each device has a unique MAC address. In addition, nodes in the real-time domain are assigned an EPL node ID. The respective node ID of a device can be selected by a node switch on the front side of the device. Ethernet POWERLINK also offers standard IP addressing when needed. This means that real-time devices can be accessed from anywhere in the world via the Internet. Local IP addresses are assigned to devices in a real-time domain. The local IP address for a particular device is derived from the respective node ID. The transition to the Internet is made via Network Address Translation (NAT), as when connecting to an Internet service provider.