Tech Tip: PROFINET’s Got Class – in Real Time

  • Post category:TECH TIPS
  • Reading time:3 mins read

PROFINET has class, in fact, many classes: Conformance Classes, Real Time classes, media redundancy classes, and even more class(es). Last issue covered Conformance Classes; this Tech Tip covers Real Time classes.

The Real Time classes determine the services used and how the protocol operates “under the hood,” if the communication is synchronized or unsynchronized among devices, which path is taken in the device or through the network, 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 for PROFINET which streamlines the Real Time message when using standard Ethernet components. In the vast majority of applications this is just fine. This also applies to Conformance Classes CC-A, CC-B. 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 just 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 just 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 (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)

Next class in the next issue: media redundancy classes.

–from the PROFI Interface Center in Tennessee where all these classes are explained in class – the PROFINET Certified Network Engineer class.