wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

PROFINET 40-13
40.4.5 redundancy
The goal of redundancy in automation systems is to increase the system availability significantly, thus a
redundancy manager (RM) and several clients should be used to assure the correct operation in this cases.
The media redundancy protocol (MRP), according to IEC 61158 and IEC 61784, describes PROFINET
redundancy (for TCP/IP and RT frames) with a typical reconfiguring time of communication paths after
a fault of less than 200.ms. The MRP operates with a physical ring structure where the RM is the coordinator.
The task of an RM is to check the functional capability of the configured ring structure. This is done by
sending out cyclic test frames: as long as the ring structure is intact, it receives all the test frames again after
one circulation through the ring. The architecture allows an RM to prevent standard frames from circulating
and to convert a physical ring structure into a “logical” line structure. The RM must communicate changes
in the ring to all clients involved (switches as so-called passers) through special “change in topology” frames.
In principle, even in PROFINET IO redundant installations, the same previously described data transmission
mechanisms are used. The only difference is the communication path used to transmit the frames
(UDP/IP and RT frames). These frames are only transmitted via the “healthy” channel (single channel)
that looks like a line structure. A redundancy client (RC) is a switch that acts only as a “passer” of frames
and generally does not assume an active role. It must have two switch ports in order to connect to other clients
or the RM in a single ring. In PROFINET IO, only “managed switches” that support MRP are used for
implementing media redundancy. Redundant switches can be configured, e.g., via SNMP or Web services.
The media redundancy for real-time (MRRT) protocol defined in IEC 61158 describes the handling of
RT frames of RT_CLASS_1 and RT_CLASS_2 for redundancy operation. Operation of MRP is always
a prerequisite for operation of MRRT. IEC 61784 describes the procedure for using the MRRT protocol.
With RT communication, the MRRT protocol enables a virtually smooth switchover of communication
paths if a fault occurs. This is accomplished by redundant transmission of RT frames (i.e., via two
channels) if the destination port is designed as a redundant port (see Figure 40.5). On the receiving side,
two RT frames always arrive, provided the redundant transmission is error-free. Only the first frame to
arrive is forwarded to the application. Also in this case, the RM must check the functional capability of
the system by means of test frames.
Redundant ports
Station 1
Source
Managed switch
Station
2
Process data
Station
8
Station
3
Test frame
Station
7
Station
4
Comparison
old/new data
Station
5
Destination
Station
6
FIGURE 40.5
Media redundancy with MRP increases the plant availability.
© 2011 by Taylor and Francis Group, LLC