Siemens SIMATIC PROFINET Function Manual page 226

Functions
5.9 Standard machine projects
For configuration-controlled IO systems, the following additional rules apply:
● When configuring MRP (Media Redundancy Protocol):
The ports configured as ring ports must not be interlinked via user program.
However, devices with ring ports (devices of an MRP domain) can be optional IO devices.
● When configuring docking stations (= IO devices changing during operation):
Neither the docking station nor the first IO device of a docking unit may be optional IO
devices.
The ports of the docking units must not be interlinked via user program.
● When configuring IRT:
The order of synchronized IO devices ("IRT devices") must be defined by the
configuration and must not be changed in the different variants of a standard machine.
For this reason, the ports of the IRT devices must not be interlinked via user program.
However, you have the possibility to configure IRT devices primarily as optional IO
devices.
You also have the option to interconnect, by user program, RT devices that are, for
example, separated from this line by a switchport (see figure).
Figure 5-91
226
Example: Configuration control for IO systems with separated RT device
PROFINET with STEP 7 V14
Function Manual, 09/2016, A5E03444486-AG