Siemens SIMATIC PROFINET Function Manual page 158

Functions
5.5 Real-time communication
Fields of application of PROFINET with IRT
PROFINET with IRT is particularly suitable for:
● Considerable deterministics with large quantity structures concerning the I/O user data
communication (productive data)
● Considerable performance also with many devices concerning the I/O user data
communication (productive data)
● Parallel transfer of productive and TCP/IP data via a cable, even with considerable data
traffic whilst ensuring the forwarding of productive data by reserving the transmission
bandwidth.
International standard IEC 61158
The communication procedures RT and IRT are standardized in the international standard
IEC 61158.
5.5.2 RT
PROFINET IO with real-time communication (RT) is the ideal solution for integrating
IO systems. This is a solution that also uses standard Ethernet in the devices and
commercially available industrial switches as infrastructure components. A special hardware
support is not required.
If you want to use PROFINET functions that provide an additional value, such as topology
detection, diagnostics, device exchange without exchangeable medium/PG, you have to use
switches that support the PROFINET standard according to the IEC 61158 standard. In the
integrated switches of PROFINET devices and PROFINET switches (e.g., the SCALANCE
product family), PROFINET functions in accordance with the PROFINET standard are
implemented and can be used without restriction for integration in the PROFINET IO system
(see also section Active network components (Page 28).
Real-time communication (RT)
PROFINET IO frames have priority over standard frames in accordance with IEEE802.1Q.
This ensures the required determinism in the automation technology. The data is transferred
via prioritized Ethernet frames. With RT, you can achieve update times ≥ 250 μs with RT.
Switching mechanisms
Switches in SIMATIC meet the real-time requirements with two mechanisms on PROFINET:
"Store and forward" and "Cut through".
Store and forward
In the Store and forward method, the switch stores the frames completely and places them in
a queue. If the switch supports the international standard IEEE 802.1Q, then the data is
sorted according to its priority in the queue. The frames are then forwarded selectively to the
specific port that can access the addressed node (Store and forward).
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PROFINET with STEP 7 V14
Function Manual, 09/2016, A5E03444486-AG