Siemens SIMATIC PROFINET Function Manual page 160

Functions
5.5 Real-time communication
Properties of isochronous real-time
Topology configuration is a prerequisite for IRT.
In addition to the reserved bandwidth, the frames from defined transmission paths are
exchanged for the further optimization of data transfers. For this, the topological information
from the configuration is used for planning the communication. Transmission and reception
points of every individual data frame at every communication node are thus guaranteed. This
allows you to achieve optimal usage of the bandwidth and reach the best possible
performance in the PROFINET IO system.
Use of IRT allows you to achieve update times with very high deterministics ≥ 125 µs and a
jitter accuracy of the send clock < 1 µs. Isochronous applications are possible with IRT (see
section Isochronous mode (Page 182)).
* The minimum update time depends on the CPU used. The update times supported by a
CPU are listed in the technical specifications of the CPU manual.
Note
IWLAN and IRT
PROFINET devices that are connected to PROFINET IO via access points, do not support
IRT.
Synchronization
The prerequisite for the IRTcommunication is a synchronization cycle for all PROFINET
devices in a sync domain for distributing a mutual time basis. A synchronism of the
transmission cycle of the PROFINET devices within a sync domain is achieved with this
basic synchronization.
A sync master specifies the clock to which sync slaves are synchronized. An IO controller or
an IO device can have the role of sync master. If the sync master fails, all IO devices with
IRT will fail.
Sync master and sync slaves are always devices in a sync domain. Bandwidth is reserved
within the sync domain for IRT communication. Real-time and non-real-time communication
(TCP/IP communication) is possible outside of the reserved bandwidth.
Default sync domain
If you have created a PROFINET subnet, a special sync domain is automatically created, the
default sync domain. All PROFINET devices configured for the PROFINET subnet
automatically belong to this sync domain.
For additional information on creating sync domains, refer to the STEP 7 online help.
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PROFINET with STEP 7 V14
Function Manual, 09/2016, A5E03444486-AG