Siemens SIMATIC ET 200AL, System Manual

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Functions
5.4 Media redundancy (ring topologies)
PROFINET with STEP 7
Function Manual, 05/2021, A5E03444486-AL
175
How media redundancy works in a ring topology
The data paths between the individual devices are automatically reconfigured if the ring is
interrupted at any point. The devices are available again after reconfiguration.
In the redundancy manager, one of the two ring ports is blocked in uninterrupted network
operation for normal communication so that no data frames are circulated. In terms of data
transmission, the ring topology is a linear bus topology. The redundancy manager monitors
the ring for interruptions. For this purpose, it sends test frames not only from ring port 1 but
also from ring port 2. The test frames pass through the ring in both directions until they
arrive at the other ring port of the redundancy manager.
An interruption of the ring can be caused by loss of the connection between two devices or
by failure of a device in the ring.
If the test frames of the redundancy manager no longer arrive at the other ring port during an
interruption of the ring, the redundancy manager connects its two ring ports. This substitute
path once again restores a functioning connection between all remaining devices in the form
of a linear bus topology.
The time between the ring interruption and restoration of a functional linear topology is
known as the reconfiguration time.
As soon as the interruption has been eliminated, one of the two ring ports is blocked again in
the redundancy manager. The redundancy clients are informed about the change and use the
original paths to the other devices again.
Media redundancy method
The standard method of media redundancy in SIMATIC is MRP (Media Redundancy Protocol)
with a typical reconfiguration time of 200 ms. Up to 50 devices can participate per ring.
In addition, the real-time capable media redundancy process MRPD (Media Redundancy with
Planned Duplication of frames) is also available.
5.4.1 Media Redundancy Protocol (MRP)
Media Redundancy Protocol (MRP)
The "MRP" process works in conformity with the Media Redundancy Protocol (MRP) that is
specified in the standard IEC 62439-2.
Requirements
• All devices in the ring support MRP.
• You have complied with the rules for topology set out below. STEP 7 monitors compliance
with the rules during compilation and outputs corresponding alarms.