EPL is a hard RT protocol based on Fast Ethernet. Like EtherCAT, it uses the Ethernet II Frame type field. EPL devices use standard Ethernet hardware with no special ASICs. EPL can deliver a cycle time of 200 µs with jitter under 1 µs. Its frame is encapsulated as illustrated in Figure 4.

Figure 4 — Powerlink Encapsulation

EPL uses cyclic communication with time-slot division and the master/slave model. One master (manager) is allowed per network. The master schedules all transmissions and is the only active station—slaves transmitting on request.

The four EPL cycle subdivisions are illustrated in Figure 5.

Figure 5 — Powerlink Cycle

During the Start Period, the EPL master broadcasts the "Start-of-Cyclic" (SoC) frame which synchronizes the slaves. The timing of this frame provides the only time base for the network synchronisation: all other frames are purely event-driven.

After transmitting the SoC frame, the Cyclic Period occurs as the manager polls each station with a "Poll Request" frame. Only then does the slave respond with a "Poll Response" frame containing data—hence, collisions are avoided. The slave broadcasts its response to all devices—thus, inter-slave communication can occur.

After successful polling of all slaves, the master broadcasts the "End-of-Cyclic" (EoC) frame, informing each slave that the cyclic traffic progressed correctly.

The Asynchronous Period allows non- cyclic data transfers under master control. To transmit during this period, a slave must have informed the master in its "Poll Response" during the Cyclic Period. The master builds a list of waiting slaves and employs a scheduler to guarantee that no send request will be delayed indefinitely. During the Asynchronous Period, standard IP datagrams can be transferred.

Unlike PROFInet, EPL does not employ switches to avoid collisions or to provide the network synchronization; the master controls this. EPL networks can be built with standard hubs, and it is proposed that each device incorporate a hub for ease of bus implementation. Switches, although not prohibited, are not recommended for EPL because they add jitter and reduce determinism. Since the EPL network avoids collisions via time-controlled bus access, up to 10 hubs can be cascaded (an allowable exception to the 5-4-3 Ethernet rule).

Currently, EPL devices demanding RT communication cannot co-exist on the same segment as non-RT Ethernet devices. However, EPL devices can operate as normal Ethernet devices. In Protected Mode, the RT segment must be separated from normal traffic by a bridge or router. In Open Mode, RT traffic shares the segment with normal traffic, but RT communication is compromised. In the next Powerlink version (V3), IEEE 1588 will be used to synchronize traffic across multiple RT segments—providing a more distributed EPL implementation, but true RT segments will still contain only EPL devices. Unlike PROFInet where normal Ethernet and RT devices can co-exist and not affect RT traffic, EPL must be protected from non-RT communication through bridges or routers. Unlike PROFInet or EtherCAT, which need special ASICs, EPL employs standard Ethernet hardware.