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
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:
Figure: 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.