Using I/O on the S7-400H
7-4
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Single-channel, one-way I/Os and user program
Information read in on one side – for example, from digital inputs – is transferred
automatically to the second subsystem via the synchronization link in redundant
system mode.
After the information has been transferred, both subsystems have the data from
the single-channel, one-way I/O and evaluate them in the two identical user
programs that are present. It is therefore not decisive whether the I/O is connected
to the master CPU or to the standby CPU as far as processing of the information in
redundant system mode is concerned.
In single mode, the one-way I/O assigned to the cooperating subsystem cannot be
accessed. This has to be taken into account in your programming as follows: you
have to assign functions to the single-channel, one-way I/O that can only be
performed conditionally. In this way you make sure that certain functions for I/O
accesses are triggered only in redundant system mode and in single mode of the
subsystem concerned.
Notice
The user program has to update the process image for single-channel, one-way
output modules in single mode too (e.g. direct accesses). If subprocess images
are used, the user program must update the subprocess images in OB 72
(redundancy return) accordingly (SFC 27 “UPDAT_PO”). If it did not, old values
would initially be read out to the single-channel, one-way output modules of the
standby CPU following transition to redundant system mode.
Failure of the single-channel, one-way I/O
In the event of a malfunction the S7-400H with a single-channel, one-way I/O
behaves like a standard S7-400 system, in other words:
• When the I/O fails, the defective I/O is no longer available.
• When a subsystem fails, the entire process I/O of that subsystem is not
available any more.
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