Siemens SIMATIC S7-400H Manual page 202

Glossary
I/O, redundant
We speak of a redundant I/O when there is more than one input/output module
available for a process signal. It can be connected as a one-sided or switched
I/O. Terminology: "redundant one-sided I/O" or "redundant switched I/O"
I/O, single-channel
We speak of a single-channel I/O when – in contrast to a redundant I/O – there is
only one input/output module for a process signal. It can be connected as a
one-sided or switched I/O.
I/O, switched
We speak of a switched I/O when an input/output module can be accessed by all
of the redundant central processing units on a fault-tolerant system. It may be
single-channel or multi-channel (redundant).
Link-up
In the Link-up system mode of a fault-tolerant system the master CPU and the
standby CPU compare the memory configuration and the contents of the load
memory. If they establish differences in the user program, the master CPU
updates the user program of the standby CPU.
Master CPU
The central processing unit that is the first redundant central processing unit to
start up. It continues to operate as the master when the redundant link is lost.
The user program is processed in an identical manner in both the master and
standby CPUs.
Mean time between failures (MTBF)
The average time between two failures and, consequently, a criterion for the relia-
bility of a module or a system.
Mean down time (MDT)
The mean down time essentially consists of the time until error detection and the
time required to repair or replace defective modules.
Mean time to repair (MTTR)
Mean time to repair denotes the average repair time of a module or a system – in
other words, the time between the occurrence of an error until the error has been
rectified.
Glossary-2
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04