PROFInet is a plant-wide fieldbus standard for distributed automation systems. It uses object-orientation and available IT standards (TCP/IP, Ethernet, XML, COM). PROFInet is also built on IEEE 802.3 and is interoperable with TCP/IP—allowing it to be implemented on existing Ethernets. It is compatible with PROFIBUS-DP.

PROFInet V1, has a response time of 10-100 ms. PROFInet-SRT (Soft Real-Time) allowed PROFInet to work with a factory automation cycle time of 5-10 ms, achieving RT solely in software. It uses TCP/IP and a dedicated software channel for RT communications. PROFInet-IRT brings a hard-RT element to the PROFInet protocols. The three PROFInet protocols allow differing degrees of RT. PROFInet for hard RT is PROFInet-IRT.

PROFInet-IRT

PROFInet IRT (Isochronous RT) was developed for systems requiring sub-microsecond synchronization, typically high- performance motion control systems. The benchmark for such a system is 1 ms cycle time, 1 µs jitter accuracy, and guaranteed determinism [6]—which IRT fulfils.

Since software introduces jitter above 1 µs, IRT (unlike SRT) is a hardware solution with highly synchronized Ethernet nodes. Using full-duplex switched Fast Ethernet, it divides the communication cycle into a standard TCP/IP open channel and a deterministic RT channel (see Figure 6). The channel ratio is system-dependent and is chosen by the system engineer.

Figure 6 — PROFInet-IRT Channel Division

Each PROFInet-IRT device has a special ASIC for handling node synchronisation and cycle subdivision and incorporates an intelligent 2- or 4-port switch.

The PROFInet switch in every node is highly synchronized, contains a schedule of bus access and can deal with RT and non-RT traffic. It prioritizes RT traffic and provides full-duplex links for all ports. Contemporary switches (even cut-through) add jitter that would impact on determinism. PROFInet switches minimize jitter to where it has a negligible effect. The PROFInet communication model allows both RT and non-RT traffic to co-exist on one network without additional precautions.

By 2005, PROFInet-IRT and SRT will incorporate PROFISafe, the PROFIbus safety solution for manufacturing and processing industries.

PROFInet, of all the solutions discussed here, offers the greatest determinism—and since this is built into the PROFInet-IRT device, the systems engineer is spared from the burden of configuration to guarantee RT communication.