PROFINET has class, in fact, many classes: Conformance Classes, Real Time classes, media redundancy classes, and even more class(es). This article covers Real Time Classes and how PROFINET works.

Real Time Classes

The Real Time classes determine:

1. The services used and how the real time protocol operates “under the hood”
2. If the communication is (clock) synchronized or unsynchronized among devices
3. Which path is taken in the device or through the network
4. And if there might need to be hardware support (like a PROFINET ASIC or FPGA)

All PROFINET devices must support RT (Real Time) which usually means around 250 microseconds-10 milliseconds update time with <100 microseconds of jitter and the data is sent unsynchronized.

Here is a 20-second video explanation of synchronized and unsynchronized:

RT uses a special EtherType (0x8892) for PROFINET which streamlines the Real Time message when using standard Ethernet components, and allows skipping the TCP/IP layers for the real time application’s process data. In the vast majority of applications this will work just fine. This also applies to Conformance Classes CC-A, CC-B.

IRT

It is optional to support IRT (Isochronous Real Time) which is usually <1 millisecond with <1 microsecond of jitter. It is used in motion control applications where multiple devices need clock synchronization and a higher level of determinism using bandwidth reservation and scheduling. A Conformance Class C (CC-C) device would support both IRT and RT for example.

Another optional service is RT over UDP/IP (user datagram protocol/ internet protocol). We haven’t seen many vendors using this due to RT/IRT being much more robust and deterministic in the field and simpler to use. With UDP/IP, you can run into more delays and overhead and have additional commissioning, however you can transfer the RT frame via a router. Most PROFINET traffic uses switches only on the PROFINET part of the system since RT/IRT use layer 2 communications. Of course some services with PROFINET always use UDP or TCP so they can still be used through a router and switch (for example, configuration and diagnostics). For developers the Real Time class is set in the GSD file of the device (tag = SupportedRT_Classes) or in the controller configuration tool. In PROFINET terminology this is mentioned in the specs and shown below:

• RT_CLASS_UDP = RT over UDP/IP (with this RT can be sent via a router, not used much up to now)
• RT_CLASS_1 = RT (for CC-A and CC-B)
• RT_CLASS_2 = IRT (depreciated and not used today)
• RT_CLASS_3 = IRT (used when motion or high speed applications are necessary, CC-C)

For further study, you can read about Conformance and Media Redundancy classes in our other articles or have a look at the PROFINET system description here.