What Does Active Redundancy Mean?; Declaration; Master/Standby Assignment - Siemens SIMATIC S7-400H Manual

System and Operating Modes of the S7-400H
5.1 Introduction
The S7-400H consists of two redundant configured subsystems that are
synchronized via fiber-optic cables.
The two subsystems create a fault-tolerant programmable logic controller operating
with a two-channel (1-out-of-2) structure on the "active redundancy" principle.

What does active redundancy mean?

Active redundancy, frequently referred to as functional redundancy too, means that
all redundant resources are constantly in operation and are simultaneously
involved in the execution of the control task.
This means for the S7-400H that the user program in the two CPUs is completely
identical and is executed simultaneously (synchronously) by the two CPUs.

Declaration

To identify the two subsystems, we use the traditional expressions of "master" and
"standby" for two-channel fault-tolerant systems in this description. The standby
always operates so that it is synchronized with the events on the master, meaning
that it does not wait for an error instance.
A distinction between the master CPU and the standby CPU is primarily important
for ensuring reproducible error reactions. Thus, for example, the standby CPU
switches to STOP mode in the event of the redundant link failing, whereas the
master CPU remains in RUN mode.

Master/standby assignment

When the S7-400H is turned on for the first time, the first CPU to be stated up
becomes the master CPU; the other CPU becomes the standby CPU.
Once the master/standby assignment has been established, it remains like that
upon simultaneous POWER ON.
The master/standby assignment is modified by:
1. The standby CPU starting before the master CPU (interval of at least 3 s)
2. Failure or STOP of the master CPU in redundant system mode
3. No fault was found in TROUBLESHOOTING mode (refer also to Section 5.3.6)
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Automation System S7-400H Fault-tolerant Systems
A5E00068197-07