Introduction - Siemens SIMATIC S7-400H System Manual

System and operating states of the S7-400H

8.2 Introduction

8.2 Introduction
The S7-400H consists of two redundant configured subsystems that are synchronized via
fiber-optic cables.
The two subsystems create a redundant automation system operating with a two-channel
(1-of-2) structure based on the "active redundancy" principle.
What does active redundancy mean?
Active redundancy, commonly also referred to as functional 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.
Conventions
To distinguish between the two units, we use the traditional expressions of "master" and
"standby" for dual-channel fault-tolerant systems in this description. The standby 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 standby CPUs is primarily important for
ensuring reproducible error reactions. So the standby CPU may go into STOP when the
redundant link fails, while the master CPU remains in RUN.
Master/standby assignment
When the S7-400H is initially switched on, the first CPU to be started assumes master mode,
and the partner CPU assumes standby mode.
The preset master/standby assignment is retained when both CPUs simultaneously POWER
ON.
The master/standby setting changes when:
1. the standby CPU starts up before the master CPU (interval of at least 3 s)
2. the redundant master CPU fails or goes into STOP
3. No error was found in TROUBLESHOOTING mode
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(see also section TROUBLESHOOTING operating state (Page 87))
System Manual, 09/2007, A5E00267695-03
S7-400H