Siemens SIMATIC S7-400H Manual page 195

Differences between Fault-Tolerant
Systems and Standard Systems
Certain differences from standard S7-400 CPUs have to be taken into account
when you configure and program a fault-tolerant programmable logic controller
containing a CPU 417-4H. On the one hand, compared to a standard S7-400 CPU,
a CPU 417-4H has additional functions, while on the other hand a CPU 417-4 does
not support certain functions. This has to be taken in account in particular when
you wish to run a program that was created for a standard S7-400 CPU on a
CPU417-4H.
The items in which the programming of fault-tolerant systems differs from that for
standard systems are summarized below.
If you use one of the calls concerned (OBs and SFCs) in your user program, you
will need to adapt your program accordingly.
Additional functions of fault-systems
Function
Redundancy error OBs
Additional information in OB
start information and in
diagnostic buffer entries
SFC for fault-tolerant
systems
Fault-tolerant
communication connections
Self-test
Switched I/O
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
Additional Programming
I/O redundancy error OB (OB 70)
CPU redundancy error OB (OB 72)
You will find detailed information in the Reference Manual
System and Standard Functions
The rack number and the CPU (master/standby) are
specified. You can evaluate this additional information in the
program.
You can use SFC 90 "H_CTRL" to influence the sequences
of fault-tolerant systems.
Fault-tolerant connections are configured, no further
programming is required.
You can use the SFBs for configured connections when
you are using fault-tolerant connections.
The self-test is performed automatically, no further
programming is required,
No additional programming is required, refer to Section 6.3
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