F 8627X (0650)
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6.4.2 Redundant MODBUS communication
To ensure a redundant MODBUS communication, the MODBUS master must be redundantly
connected to the MODBUS slave (see Chapter 5.2.5.3).
To ensure the redundant MODBUS communication between a H41q/H51q PES and a MOD-
BUS master, the following two methods are possible:
Cable redundancy
Under all circumstances, MODBUS communication only takes place via a single ethernet
channel. If the MODBUS master no longer receives a responding telegram on the active chan-
nel, it can switch to the other channel and continue exchanging data. The MODBUS master
can thus switch to the redundant channel if a network segment fails (e.g. broken ethernet cable
or a faulty switch).
Redundancy with two "Peer to Peer" connections
In this case, the MODBUS master in use must possess the function to establish two indepen-
dent MODBUS "Peer to Peer" connections to the MODBUS slave's two F 8627X.
The same data are then transmitted over both ethernet connections to the two F 8627X simul-
taniosly.
The user must ensure that separate BUSCOM address ranges are used for each Ethernet
channel transmitting the redundant BUSCOM variables (see figure below).
Figure 10: Partitioning of the BUSCOM Import address range for the redundant
BUSCOM variables
The logic of the user program must ensure, that the user program always processes the most
current data record of the channel.
A monotonically increasing sequence number, incremented by the MODBUS TCP master, can
serve e.g. as a criterion for determining how up-to-date the BUSCOM variables in the separate
address ranges are.
Figure 10 shows an example, in which the sequence number is registered in the BUSCOM va-
riables Channel1 and Channel2, respectively.
Note In case of port 8896, the BUSCOM variables are mapped into the
F 8627X process data image. The MODBUS TCP master must there-
fore access the BUSCOM variables using the identity numbers
(see
Chapter 7).
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