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3.3.2
Parallel and redundant communication via PRP
To prevent the formation of a loop on the Ethernet level in PDS applications at the time of
the segment change, PRP switches are required (e.g. SCALANCE X204RNA). These
switches ensure redundant communication over parallel Ethernet paths. To break up the
loop, one PRP switch is required on each vehicle and a central switch at the stationary end.
The stationary masters must be connected alternately to "PRP Section A" and "PRP Section
B" so that a PLB segment with "PRP Section B" always follows a PLB segment with "PRP
Section A". If required, the stationary subdistribution of the PRP sections can be achieved
with standard switches, such as SCALANCE X208.
Observe the following points when using PRP:
Increased network load with non-PROFINET communication
Use of the "Parallel Redundancy Protocol" (PRP) in the higher-level network results in an
increased network load in the Power Line Booster communication system. This is because
all frames are transmitted redundantly here. These frames can only be delivered in the PRP
section in which the receiver is located. In the other PRP sections, this data is flooded and
thus sent to all PLB masters of the PRP section. This can lead to a high network load or a
reduction of the broadcast bandwidth with a large number of stations.
Because the PLB master knows the Ethernet stations in its segment, it can block frames to
unknown stations to prevent flooding. The process for handling frames to unknown stations
can be configured via the Buffer configuration of the Traffic Manager (Page 96).
Jumbo frames with MTU
The standard MTU (Maximum Transmission Unit) for user data is 1500 bytes for Ethernet.
All stations in an Ethernet network can thus, by default, transmit only frames with a
maximum data packet size of 1500 bytes of user data. The redundancy required by using
PRP provides the frames in the PRP section with an additional 6-byte PRP trailer. In the
extreme case, the maximum data packet size can thus be exceeded in the PRP section. If
switches are used in the PRP section, these packets may be rejected, depending on the
switch configuration.
In the case of Siemens SCALANCE switches, for example, this can be prevented by
activating "Oversize mode".
PROFINET devices in the PRP section
Direct PROFINET communication between a PROFINET controller outside a PRP section
and a PROFINET device within a PRP section is not possible (see also the SCALANCE
X204RNA manual). For example, it is not possible to configure a switch in the PRP section
as a PROFINET device in the hardware configuration of the PLC.
Special adaptations were made for this on the Power Line Booster so that, in spite of this
general PRP limitation, the PLBs can be configured as a PROFINET device in the PRP
section.
Power Line Booster
Operating Instructions, 11/2019, A5E36203125A-AG
Plant planning
3.3 Double slider solution (PDS)
39
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