Siemens SIMATIC S7-400H Manual page 100

Communications
When fault-tolerant systems and standard systems are interconnected, the
availability of communications cannot be enhanced by means of a twin electrical
bus system. In order to be in a position to use the second bus system as a
redundant bus, you have to use a second S7 connection, which has to be
managed accordingly in the user program (refer to Figure 7-7).
Redundancy
block diagram
Figure 7-7
Redundancy
block diagram
Figure 7-8
Failure behavior
Two-fiber ring and bus systems
Since standard S7 connections are used in this particular instance (the connection
terminates at the CPU of the subsystem, in this instance CPUa1), an error on the
fault-tolerant system – for example, CPUa1 or CPa1 – and an error on system b –
for example, CP b – both result in total failure of communications between the
systems involved (refer to Figures 7-7 and 7-8).
There are no differences specific to the bus system for failure behavior.
7-14
Fault-tolerant system a
CPU CP
a1
a1
OLM
OLM
Fault-tolerant
system a
OLM/
CPUa1 CPa1
Bus1
OLM/
CPUa2 CPa2
Bus2
Example of Interconnected Standard and Fault-Tolerant Systems on a
Redundant Ring
Fault-tolerant
system a
CPU CP
a1 a1
Fault-tolerant
system a
CPUa1 CPa1
CPUa2 CPa2
Example of Interconnected Standard and Fault-Tolerant Systems on a
Redundant Bus System
S7-400H Programmable Controller Fault-Tolerant Systems
System b
CPU
CP
b
b
OLM
System bus as opti-
cal two-fiber ring
Connection
System b
CPb
CPUb
System b
CPU CP CP
b2
b1 b1
System b
Connection
Bus1
CPb1
CPUb1
CPb2
Bus2
Bus1
Bus2
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