Data exchange

PROFINET uses a simple consumer/provider model for data exchange. The provider sends data to the consumer cyclically. In the case of input signals, the provider is the I/O module and the consumer is the PLC; in the case of output signals, these roles are reversed. This efficient method of data exchange has two different variants of the PROFINET protocol, RT and IRT, to thank for its real-time properties.



Real-time communication with PROFINET RT

In PROFINET RT (real-time) communication, data communication runs parallel to the TCP/IP stack via an optimized software stack. The connection from the controller to the device is established via the TCP/IP stack. The TCP/IP stack ensures that every device has an IP address and can also be addressed at any time using other protocols (such as HTTP for web servers).

To comply with real-time requirements, PROFINET uses the VLAN tag in the header, where it sets the highest freely available priority level 6, ensuring that PROFINET telegrams are then forwarded in preference in switches.

PROFINET RT defines update rates of between 250 µs and 500 ms for data exchange. Separate update rates can be set for the input and output data of a module. The possible update rate is determined by the performance of the module.



Real-time communication with telegram jitter under 1 µs

The same properties apply for PROFINET IRT as for PROFINET RT, although the real-time class has been supplemented with specific hardware in order to reduce telegram jitter to less than 1 µs.

The IRT phase supports synchronous real-time data transmission that is independent of Ethernet communication. This is achieved using a time-slice procedure, which regulates every device, controlling when real-time data, alarms, and TCP/IP communication are forwarded in the network precisely to the µs.

PROFINET IRT-capable devices can also communicate in RT mode.


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