Siemens SIMATIC S7-400H System Manual page 175

Rule
A single-channel switched I/O configuration must always be symmetrical.
● This means, the fault-tolerant CPU and other DP masters must be installed in the same
● or the DP masters must be connected to the same integrated interface in both
Single-channel switched I/O and user program
In redundant mode, in principle any subsystem can access single-channel switched I/O. The
data is automatically transferred via the synchronization link and compared. An identical
value is available to the two subsystems at all times owing to the synchronized access.
The fault-tolerant system uses only one of the DP interfaces or PROFINET interface at any
given time. The active DP interface is indicated by the ACT LED on the corresponding IM
153–2 or IM 157.
The path via the currently active DP interface or PROFINET interface is called the active
channel, while the path via the other interface is called the passive channel. The DP or PNIO
cycle is always active on both channels. However, only the input and output values of the
active channel are processed in the user program or output to the I/O. The same applies to
asynchronous activities, such as interrupt processing and the exchange of data records.
Failure of the single-channel switched I/O
The fault-tolerant system with single-channel switched I/O responds to errors as follows:
● The I/O is no longer available after it fails.
● In certain failure situations (such as the failure of a subsystem, DP master system or DP
The following applies to each station affected by a failure: If both DP slave interfaces or
PNIO connections are currently functional and the active channel fails, the previously
passive channel automatically becomes active. A redundancy loss is reported to the user
program when OB 70 starts (event W#16#73A3).
Once the problem is eliminated, redundant mode is restored. This also starts OB 70 (event
W#16#72A3). In this situation, there is no changeover between the active and passive
channel.
S7-400H
System Manual, 03/2012, A5E00267695-11
slots in both subsystems (e.g. slot 4 in both subsystems)
subsystems (e.g. to the PROFIBUS DP interfaces of both fault-tolerant CPUs).
slave interface module IM153-2 or IM 157; see section Communication (Page 213)), the
single-channel switched I/O remains available to the process.
This is achieved by a switchover between the active and passive channel. This
switchover takes place separately for each DP or PNIO station. A distinction is made
between the following two types of failure:
– Failures affecting only one station (such as failure of the DP slave interface of the
currently active channel)
– Failures affecting all stations of a DP master system or PNIO system.
This includes unplugging of the connector at the DP master interface or at the PNIO
interface, shutdown of the DP master system (e.g. RUN-STOP transition on a CP 443-
5), and a short-circuit at the cable harness of a DP master system or PNIO system.
Using I/Os in S7–400H
13.3 Using single-channel switched I/O
175