Transfer Mechanism of \ PROFINET

The PROFINET IO communication is not a typical master-slave-communication. In fact there is a parameter in the controller that sets the communication basic cycle (“SendClockFactor”). For each IO Device it is possible to set a reduction rate (“ReductionRatio”). Due to this reduction rate the cycle time between IO Controller and each IO Device can be set separately in dependence of the performance requests System Performance.

The IO Controller sends the output data to each planned IO Device (following the device specific parameterized cycle time). Each IO Device sends its input data with the same cycle time to the IO Controller.

Communication Channels

PROFINET IO is building up on standard ETHERNET and uses two different communication channels:

For non-time-critical processes PROFINET IO uses standard ETHERNET communication over UDP/IP (NRT communication). These non-time-critical processes are:

  • “Start up” of the bus communication
  • Allocation of the IP-addresses to the PROFINET IO Devices (DHCP)
  • Parameterization of the PROFINET IO Devices

For time-critical data exchange PROFINET IO uses the RT communication channel. The time-critical processes are:

  • Reading and writing of process data of the PROFINET IO Devices
  • Alarm messages of a certain PROFINET IO Device
  • Allocation of names and addresses to the PROFINET IO Devices
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PROFINET NRT and RT

Frame structure

The PROFINET RT frames consist of MAC header, process data and a checksum. The frames have the following structure:

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PROFINET RT telegram

The individual parts of the frame have the following sense:

  • Destination address: MAC address of the receiver
  • Source address: MAC address of the sender
  • VLAN-Tag: prioritizing of the data exchange
  • EtherType: Identifier of the following data (PROFINET RT uses 0x8892)
  • Data: process data
  • Checksum: checksum of the transferred data

The PROFINET NRT frames also contain IP/UDP, RPC NDR. In this case the VLAN tag is not used.