Siemens SIMATIC S7-400H System Manual page 496

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 may be connected as one-sided or switched module. Terminology:
"Redundant one-sided I/O" or "Redundant switched I/O"
I/O, single-channel
When there is only one input/output module for a process signal, in contrast to a redundant
I/O, this is known as a single-channel I/O. It may be connected as one-sided or switched
module.
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 of a fault-tolerant system. It may be single-channel or
multi-channel (redundant) module.
Link-up
In the link-up system mode of a fault-tolerant system, the master CPU and the reserve 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 reserve
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 redundancy connection is lost . The user
program is processed identically in both the master and reserve CPUs.
Mean Down Time (MDT)
The mean down time MDT essentially consists of the time until error detection and the time
required to repair or replace defective modules.
Mean Time Between Failures (MTBF)
The average time between two failures and, consequently, a criterion for the reliability of a
module or a system.
Mean Time to Repair (MTTR)
The mean time to repair MTTR denotes the average repair time of a module or a system, in
other words, the time between the occurrence of an error and the time when the error has
been rectified .
496
System Manual, 03/2012, A5E00267695-11
S7-400H