Single-Channel, Switched I/O And User Program; Failure Of The Single-Channel, Switched I/O - Siemens SIMATIC S7-400H Manual

Single-channel, switched I/O and user program

In Redundant mode, in principle each subsystem may access single-channel
switched I/O. The information is automatically transferred over the synchronization
link and compared. An identical value is available to the two subsystems at all
times owing to the synchronized access.
The S7-400H only ever uses one of the interfaces at any given time. The active
interface is indicated by the ACT LED on the corresponding IM 153-2 or IM 157.
The path via the currently active interface (IM 153-2 or IM 157) is described as the
active channel and the path via the other interface as the passive channel. The
DP cycle always runs via 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

In the event of a malfunction the S7-400H with a single-channel, switched I/O
behaves as follows:
• When the I/O fails, the defective I/O is no longer available.
• In certain failure situations, e.g. failure of a subsystem, a DP master system or
a DP slave IM153-2 or IM 157 interface module (refer to Chapter 8), the
single-channel, switched I/O continues to be available to the process.
This is achieved by switch-over between the active and slave channel. This
switch-over takes place separately for each DP station. On a failure a
distinction is made between
– failures affecting one station only (failure of the DP slave interface module of
the currently active channel)
– failures affecting all the stations of a DP master system.
These include unplugging the DP master interface module, shutdown of the
DP master system (e.g. on RUN-STOP transition on a CP 443-5) and a
short circuit in the cable loom of a DP master system.
The following applies to each station affected by a failure: if both DP slave
interface modules are currently functional and the active channel fails, the previous
slave channel automatically becomes the active channel. A redundancy loss is
reported to the user program via the start of OB 70 (event W#16#73A3).
Once the fault has been remedied the redundancy is restored. This again results in
the start of OB 70 (event W#16#72A3). In this instance there is no switch-over
between the active and slave channel.
If one channel has already failed and the remaining (active) channel also fails, then
there is a complete station failure. This results in the start of OB 86 (event
W#16#39C4).
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Using I/O on the S7-400H
7-7