System And Operating States Of The S7-400H; Introduction - Siemens SIMATIC S7-400H System Manual

System and operating states of the S7–400H
This chapter features an introduction to the subject of S7-400H fault-tolerant systems.
You will learn the basic terms that are used in describing how fault-tolerant systems operate.
Following that, you will receive information on fault-tolerant system states. They depend on
the operating states of the different fault-tolerant CPUs, which will be described in the next
section.
In describing these operating states, this section concentrates on the behavior that differs
from a standard CPU. You will find a description of the standard behavior of a CPU in the
corresponding operating mode in the
The final section provides details on the modified time response of fault-tolerant CPUs.
11.1

Introduction

The S7-400H consists of two redundantly configured subsystems that are synchronized via
fiber-optic cables.
Both subsystems create a fault-tolerant automation system operating with a two-channel (1-
out-of-2) structure based on the "active redundancy" principle.
What does active redundancy mean?
Active redundancy means that all redundant resources are constantly in operation and
simultaneously involved in the execution of the control task.
For the S7-400H this means that the user programs in both CPUs are identical and executed
synchronously by the CPUs.
Convention
To identify the two subsystems, we use the traditional expressions of "master" and "reserve"
for dual-channel fault-tolerant systems in this description. The reserve always processes
events in synchronism with the master, and does not explicitly wait for any errors before
doing so.
The distinction made between the master and reserve CPUs is primarily important for
ensuring reproducible error reactions. For example, the reserve CPU may go into STOP
when the redundant link fails, while the master CPU remains in RUN.
S7-400H
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Programming with STEP 7
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